How big a heat pump do you need? A simple Rule of Thumb.

[@danielholloway1054]

This is very interesting and a great rule of thumb but should only be used as a guide. Heat pump installers are required to meet specific temperatures within the house. This rule of thumb is based upon the present occupant consumption, this method will work very well for that occupant. But what happens if they sell their house and it turns out, that they liked their house cool, or were very frugal, as a lot of people are. This would result in a very low consumption and hence a low heat pump calculation. The new occupants however, perhaps elderly or people in need of a warm house will have an undersized heat pump. I’m all up for simplicity so thanks for this guide.

[@michaeldepodesta001]

Daniel, yes, I think I agree with all your comments. The role of the rule of thumb is really to act as a check on heat pump surveys with routinely overestimate heat pump size requirements. This leads to oversizing - in 50% of cases the oversizing is by more than 50%. The gas consumption is in general well known, and someone carrying out a heat loss survey should check that the results are roughly consistent with prior gas consumption. Best wishes: Michael

[@terryjimfletcher]

The title of this video should be "How to check that your installer has specified the right size of Heat Pump". It's only when you hit 14minutes into the video does Michael start saying that his rule of thumb has other variables - this 57.3 figure needs to be adjusted for where you live, and then later "you need to add 10% for every 300m your home is above sea level". Yes, it should be used as a guide to check that your installer isn't UNDER, or more likely UNDER rating your heat pump (which is one way to kill the efficiency) - like Michael states @ 16m20s, but should not be used to buy a heat pump.

[@michaelduvwiama1370]

This is very interesting! My final year dissertation is on heat pump retrofitting for energy efficiency and I definitely need some of these experiences you have 😅👏

[@karencarpenter8275]

Excellent. Thank you. Simple rule (but the curious will want to know why!). Very good. This all stems from 57.3! Who found that?!!!

[@eddyd8745]

I've just had a heat pump survey carried out and your figure came out pretty much bang on.

[@michaeldepodesta001]

Eddy, thank you for taking the time to drop me a line. It's always heartening to know the rule of thumb still works! Good luck with your endeavours: If you choose to go ahead with it, I hope the installation goes well. Best wishes Michael

[@eddyd8745]

@@michaeldepodesta001 Hi Michael, I'm waiting for the new Octopus Cosy to come out, unfortunately that will be a 6kW device so I might be waiting a while for a larger version to be released.

[@michaeldepodesta001]

​@@eddyd8745 Yes: don't be in a hurry. This is a great time to start monitoring weekly gas and electricity usage - very little now so the gas consumption is just from cooking and hot water preparation. Maybe you will get a chance to measure over winter and see how the gas consumption goes. A 6 kW heat pump will produce 6 x 24 = 144 kWh/day of heat. You can check whether that's enough for you. Best wishes Michael

[@mortenchristensrn2487]

Thank you for making this video. It kept me from buying an greatly oversized heat pump.

[@michaeldepodesta001]

Thank you for taking the time to let me know. I'm working on a new project to now to improve the rule of thumb to take account of people who use variable amounts of domestic hot water. It should be ready in teh next couple of weeks.

[@kingsleyimo8012]

That was simply wonderful. Your presentation was simple and straight to the point. It highlighted all the very important points I needed to make an informed decision about selecting a heat pump. Thank you so very much as this came at a crucial time of making a decision on what size of heat pump I might need.

[@michaeldepodesta001]

Thanks Kingsley. But please remember it's a rule of thumb. I made the video because friends had been told they required heat pumps 3 times too large - and installers just said "That's the calculation". My hope was that if an installer suggests an anomalously large heat pump, then you would be able to ask them to justify that in detail - there is a lot of oversizing going on. Anyway: Best wishes for your endeavours. M

[@symonchester]

Awesome video, glad to be reassured that the calculations that Octopus quoted me are correct, can't wait to get mine installed

[@michaeldepodesta001]

Symon: best wishes for your installation. Keep warm! M

[@peteglass3496]

Gosh, it was back in the late 70s when I was given a review copy of a self-published 1976 book from a chap who dug up his garden and put in his own ground source heat pump. I always been interested but never quite had the confidence to do anything. With my difficult 3-storey 1860 terrace which I did partial internal insulation 30+yrs ago [50mm PU foam] it'll be an outlier for most installers to understand. Your rule of thumb will give a first order idea of where I stand before getting out my R-value and U-value tables. I think I'll have to do my own spreadsheet before letting any installer through the door ... meanwhile the ancient boiler staggers on ... 😁 Thanks for the help, Michael, I'll try and find time to read more detail in your blogs.

[@michaeldepodesta001]

Pete Glass, good evening. Yes both air-source and ground-source heat pumps have come on a long way since the 1970s If you are thinking about a heat pump then here are my two top tips. 1. On one of these cold days, read the gas meter at midday one day, and then last midday the next day. This will tell you how much gas you have used in 24 h on the coldest days of the year. Typically it might be (say) 100 kWh of gas. This tells you that you could replace your boiler with a heat pump that could deliver 100 kWh of heat in a day i.e. 100/24 = 4.2 kW peak power. This is a really reliable measurement. I wrote about this here protonsforbreakfast.wordpress.com/2022/12/11/cold-weather-measurements-of-heat-transfer-coefficient/ and here: protonsforbreakfast.wordpress.com/2022/11/07/what-to-do-on-the-coldest-day-of-the-year/ 2. Try John Cantor's web site and his book "Heat Pumps for the Home" heatpumps.co.uk/book-heat-pumps-home/ Do get back to me if you have any specific questions. M

[@jeremyallen2813]

Really useful explanation , I think focusing on real usage is the way forward ( if you have the history ) You have helped me make a decision on my heat pump selection.

[@michaeldepodesta001]

Jeremy, thank you for your kind words. Good luck with your installation! M

[@neilafram7886]

Excellent to-the-point presentation. It should be the bases of any selection and types of dwelling. The calculation is simple but most importantly easy to understand and apply especially to design and build companies. MCS requirement is extremely complicated, especially for installers. The method needs to be adapted for domestic application. Thank you

[@DrMatthewhannah]

Thanks Michael, great presentation, sent here from BetaTalk Podcast which was also excellent 👍

[@michaeldepodesta001]

Thank you. I am working on a more detailed rule that will take account of more things. But it is hard!

[@gavthegas547]

Michael what a fantastic video. I have been doing a lot of monitoring on my own home and the MCS heat calculations are always far too high. Your rule of thumb is spot on. Thanks

[@michaeldepodesta001]

Thank you. I am working a slightly more general rule that will include more 'odd cases' i.e. where people only the heat the house for an hour or two per day. M

[@michaeldepodesta001]

Thank you. I am glad it makes sense. I am working on a follow up to take account of some of the causes of variability. But it is taking time!. All the best: M

[@mikekelly5869]

I opened the video expecting some kind of guesstimate like 100W/sq.m but I got a pleasant surprise: this is actually a nice condensed sensible way to calculate equivalence that makes a lot of sense for end users who can't or don't want to do the more complex load calculations. well done! I'd suggest leaving a bit of margin to account for differences in Winter temperatures, unless the reference year was unusually cold. Choosing the closest capacity above the calculated load might be enough.

[@michaeldepodesta001]

Mike, Thanks for your kind words. There is a 'bit of margin' in the calculations with regard to boiler efficiency and assumptions about use of gas for Domestic Hot Water. But in the end it is a 'rule of thumb'. The aim is really to be able to apply a sanity check to some of the very large heat pump sizes being recommended by the MCS analysis. Best wishes Michael

[@Gary-ts6dh]

5:24 - You have obviously put in a great deal of thought an effort into this problem. This looks quite a simple way of determining need; but, I'm curious as to how you arrived at the constant used in the denomenator of your 'formula'. Thanks for sharing your thoughts. 10:43 - Thank you for answering my question later in the video. A little calculus never hurt anybody! I could have just watched until the end and dropped the original post.

[@aprobin1]

Thank you very much for the video, really useful. Can I offer a couple of suggestions for the heat pump sizing: 1. I believe your calculation assumes a heat pump running 24hours/day. I appreciate that’s probably the most efficient, but practically It might be better to assume a lower figure than this. 2. Instead of using the annual degree days, how about using the degree days for the coldest month (or week) of the year. The degree days web site will generate monthly or weekly figures and that way you’ll be assured your heat pump will cope with the very coldest parts of the year.

[@aprobin1]

Obviously, some of the factors used in the formulas will need adjusting if using degree days per month (or week) instead of degree days per year

[@michaeldepodesta001]

aprobin1: Thank you very much. Both excellent suggestions. 1. The calculation is based upon the average temperature in the dwelling. The rule of thumb will give misleading results if someone only operates the heating intermittently - it will look they have a very well-insulated house and their actual gas consumption will not be a good guide to average heat demand. 2. Yes, that's a great idea. I wrote about a similar idea of making measurements on the coldest day in these articles protonsforbreakfast.wordpress.com/2022/11/07/what-to-do-on-the-coldest-day-of-the-year/ protonsforbreakfast.wordpress.com/2022/12/11/cold-weather-measurements-of-heat-transfer-coefficient/ Thanks for your insightful comments and good luck with your endeavours. M

[@aprobin1]

@@michaeldepodesta001 Thank you very much for taking the time to reply. I see what you’re saying regarding avg temperature in the house over a 24hr period. Makes sense. One of the problems I find, having a heating system just large enough to balance heat output with heat loss in steady state is that if I go away for a week or so, which I frequently do, when I get back it takes literally days to get the house back up to normal temperature. If sized so the avg temperature in the property can be maintained by running the heat pump 12 or 16 hours a day (well insulated house so still not too much temperature variation), then a bit of extra capacity to blast this up to 24hrs/day as required wouldn’t come amiss.

[@chemicalandy3]

Thanks for this video. I've just had an indicative quote for a 16kW heat pump with the size based on a basic description of my house. They did say that this may come down to 12kW after a survey. Your formula gets me to about 4kW!

[@michaeldepodesta001]

Thanks Andrew: That's the point of the 'rule of thumb'. The survey based approach is very thorough but makes some assumptions that bias the calculations to oversizing. A Twitter call the other day asked installers if anyone had ever come across an undersized heat pump: no one had come across such an installation ever! Anyway: good luck with your endeavours! M

[@joewentworth7856]

Same here. I did these calculations and got 3kw and all quotes were 10kw or higher. We have settled on 7kw as a bit of a buffer. But the msc calcs are quite lazy about the building material used.

[@michaeldepodesta001]

@@joewentworth7856 Joe. Good Morning. Thanks for the feedback: I hope te rule of thumb helped. M

[@joewentworth7856]

@@michaeldepodesta001 fantastic set of videos wish I had found earlier . Heat pump commissioning this week...

[@bhut1571]

Nicely done. I think here in Canada 18 C rather than 16.5 C is used as the standard, so our HDD values might be higher. What is surprising is that during the 30 years we have lived in this home the HDD has dropped from 5800 down to 5200...Sadly it's Global Toasting. Cheers

[@mickdownes9265]

Really interesting, explained really simply as well. I was doing my calcs as you were speaking. Even downloaded the degree days for my location.

[@michaeldepodesta001]

Mick: Thank you. Remember this is a 'rule of thumb' - but I hope it helps. All the best: M

[@kasparbradshaw8592]

Hi Michael It's a great rule of thumb with the exception of well insulated properties, those which don't heat 24/7 and those with a bigger hot water demand. If hot water is being circulated around the house this could skew the results alot too as that can add a big chunk of energy consumption. A few comments. 1. I don't think average gas boiler efficiency is 90% but closer to 75 or 80% as few people run their Heating system with a return temp below 55deg. Also running a boiler with a return temp at 55deg is not 90% efficient. Condensing efficiency only starts at a return temp of 55 and increases the colder you run the system. If you have a gas boiler on weather compensation then hopefully it will also run down to 35 and 45deg and you will get the 90% efficiency. The rule of thumb using 80% would decrease the output of the rule of thumb so reduce contingency a little which is not critical. 2. More importantly there is an easy way to know the heat loss by experimentation. If you know your daily space heating demand in kWh you are almost there. A heat pump is generally designed to take 1-2 hours to heat the hot water tank. Assume 2 hours worst case. A hot water tank should have your daily useable volume but assume it is heated fully twice a day that is 4 hours of DHW operation per day. That leaves you with 20 hours to work in space heating mode, assuming you continuously heat the house. Divide your daily demand during the coldest day by 20 and that is the peak power heating required. If you have gas consumption for both space heating and hot water you can remove 2kWh per person for hot water. In your case If it's 50kWh purely for space heating per day so divided by 20 hours gives 2.5kW. Bear in mind that this only works on the actual coldest day that the heat pump is expected to bring the house to temperature. If it's only 5 deg outside and you are sizing to -2 (standard for London) then you would need to compensate for this. If you don't heat 24/7 then the operating time is less. If you have the heating on for only 8 hours per day then the heat pump would need to provide the winter daily space heating demand in 8 hours (or a few more If you start heating in advance). 50kWh/8h = 6.25kW. I don't agree that continuous heating uses the same energy consumption as intermittent heating. I guess it all depends on the thermal mass of the property and how long it takes to get to temperature. Masonry buildings with EWI have alot of thermal mass but timber frame which is lighter will have less. The heat emitter will also affect intermittent vs continuous. If you are using UFH then it will take a long time but radiators are quicker to respond. If you benefit alot from solar gain then a fast responding heat emitter will be more efficient. I would say generally, the same way you can lower your heating consumption by reducing from 24 to 21deg target room temp you can also lower energy consumption by reducing night time temp to 15deg and reducing daytime temperature when you aren't home. In terms of calculating your own heat loss based on U values it doesn't have to be so hard. For a quick rough estimate I calculate the whole house heat loss rather than individual rooms. I calculate the total floor, wall and roof areas and multiply each by relevant U values. For window and door area I use an approximation of 30% of total wall area. I also do the room by room calculation but only after having a rough idea first. It's a shame that alot of installers oversize but to be fair there are lots of mistakes you can make as an installer if you aren't interested in how the heat pump actually performs after its installed. Radiator or UFH sizing, pipework size, pump speed setting (where required), pipe insulation, distance from heat pump to tank to name just a few. If you are interested in installing a heat pump in London and want an installer who cares, you can contact the company where I work, www.Nextstepheating.co.uk

[@michaeldepodesta001]

Kaspar, Thanks for that considered response. You raise so many interesting points that I will have to think about them for a while. And thank you for the pointer to Next Step Heating: I will add them to my list of installers that I share. Thanks again Michael

[@ashtontechhelp]

Thanks very much, i have long thought that it makes much more sense to look at how much gas you're actually burning and use that as your source information. Provided you know how efficient your boiler is, you have a good chance of arriving at the correct value.

[@michaeldepodesta001]

Exactly so. And as someone suggested, when a really cold day is forecast, just measure the gas consumption on that day and that will tell you EXACTLY how much heat is required to keep your home warm on the coldest days of the year! M

[@pmac798]

Excellent presentation Michael very useful, thank you 😊

[@michaeldepodesta001]

Phil. Thank you for your kind words. Good luck with your endeavours. M

[@AmosMoses777]

Very Interesting. I suppose you could take an average summer time gas usage (no heating) to estimate the amount of gas used for DHW and cooking and deduct from the total usage. We live in a 2017 flat and the insulation is good. DHW represents a good 20-25% of our annual usage and I don't think we'd want to oversize a heat pump by that much.

[@michaeldepodesta001]

Yes, exactly. And yes, for your well insulated house, the rule-of-thumb would definitely overestimate heat pump size by about a third. Best wishes: Michael

[@theengineer1910]

This is one of the best videos on this topic.

[@michaeldepodesta001]

Thank you: I am blushing.

[@hvacdesignsolutions]

Michael, Could have saved yourself a lot of time by simply using the BRE Group Domestic Heat Pump Energy Efficiency Calculator. It uses historical UK weather data (Leeds), and includes domestic hot water efficiencies in its sizing and annual efficiency calculation. It's been developed in conjunction with CIBSE UK, and tailored for the UK market. Also has a product database, which includes Vaillant and many more.

[@michaeldepodesta001]

Thank you. That looks very useful.

[@fredgray2376]

Thank you Michael, that was an excellent presentation and very helpful.

[@michaeldepodesta001]

Thank you :-)

[@peterdoyle6819]

Outstanding presentation - thank you very much.

[@Umski]

Thank you - first time I've found a "easy" way to make this calculation based on actual usage rather than heat loss calcs - the Heat Loss Coefficient was the missing link for me as I have over 10 years data on gas use - varying from 10000 to 13000kWh per year (with PV that contributes to the DHW since 2014) - by this rule of thumb I would need a 5kW or lower HP which is way lower than the 9kW I was thinking - the current gas boiler is still massively over-sized at 10-20kW modulation. I may get a proper heat loss calc done too but this gives me hope that the insulation I've added is actually pretty good considering it's a 20 year old 4 bed bungalow with a 200m2 floor area which had been previously insulated at rafter level (not ceiling) by the previous owners that built it. I added another 170mm to the 100mm and was thinking of adding external insulation too. I've also replaced all the glazing with much lower u-value glass. Current boiler runs at 55C as a maximum so I need to work out if running a lower flow temp will need bigger rads. The other consideration is we only heat the house to 18-19C rather than 20-21C though we have a wood burner for the living space...all useful info!

[@michaeldepodesta001]

@Umski, Good Evening. I trust you are well. If your consumption is between 10,000 kWh/year and 13,000 kWh then you are not using a lot of energy to heat your home. Assuming you live somewhere in the UK then, Yes, a 5 kW heat pump (ish) sounds about right. I would encourage you to get a heat loss calculation done separately from a MCS heat pump installation because then you are in charge of the information. Depending on your location it might cost £200 ish. For a bungalow heat loss through the ceiling/roof is relatively more important so you have definitely done the right thing adding insulation there. I would think twice about the external wall insulation. I have had it done and I like it, but it is expensive (~£100/m^2) and it sounds like it may not be necessary. If you have money for insulation, per haps underfloor insulation or underfloor heating might be worth considering. There are two issues with heat pumps which I wrote about here protonsforbreakfast.wordpress.com/2023/08/02/hydronics/ The first is the diameter of the pipes in the installation - and a bungalow necessarily has longer pipes than a two storey home. And the second is the radiators. The article has a spreadsheet you can download which will give you a rough idea of whether you need new radiators. Your heating requirement is low for such a large house ~50 kWh/year per square meter which is in line with good modern building practice. A Passivhaus is 15 kWh/year per square meter. If you save money on the external wall insulation, I recommend that you buy a battery - and use it to run the heat pump from cheap rate electricity. Anyway - congratulations on your warm home: do drop me a line if you think I can help at all. Michael

[@Umski]

@@michaeldepodesta001 thank Michael for the very detailed reply, yes am located in Cambridgeshire - yes the external insulation is pricey, it was a toss up last winter between that and battery storage which won 😁 We didn’t import any electricity between March an October as a result so it’s paying itself off quickly and I didn’t massively oversize it (just 4.8kWh to start) - I don’t currently have an EV either so there’s scope for expansion though an HP would quickly suck up any spare PV generation. The logic behind the external insulation was that part of the bungalow has superficial cladding and is the coldest being smaller and exposed on 3 sides so it would have been simpler to DIY to see if it warranted the rest being done. The internal plumbing is a bit of an unknown though I suspect largely uninsulated pipes hidden under limited insulated flooring annoyingly as it’s MDF I believe. It would be nice to add UFH but would be far too disruptive to do it so I think sticking with the existing rads and upgrading those that need it to double convectors would be a good starting point 👍 It’s an interesting journey but for me driven by cost and the principle of moving away from gas but without throwing £££ at the problem - that also means being savvy with the PV and using electricity as and when it’s available and using gas as a last resort - the boiler is 20+ years old now so it would make sense to go HP and I think it will be a relatively straight swap - I just need someone without an agenda to verify my calcs 😉

[@michaeldepodesta001]

@@Umski Good Morning Sounds like you are proceeding sensibly. Regarding PV and heat pumps, their operation is (unfortunately but obviously) almost entirely out of phase. In the summer we operate the DHW cycle (and tumble dryer, dishwasher etc) during the day to use the excess PV, but in autumn/winter - switching soon - we switch to running these in the cheap hours at night. The PV takes care of about 10% of the electricity in winter. Regarding EWI over. limited portion of the house. Why? Because the insulation is very effective, but heat flows 'sideways' through the walls from the uninsulated exposed walls to the insulated interior walls. I discovered this very well known effect because I am an idiot and I wrote about it here. protonsforbreakfast.wordpress.com/2020/06/23/intuition-and-experience/ Regarding costs, a heat pump will only make financial sense if you have a battery to time-shift your consumption of electricity to the cheap hours in winter. It makes carbon sense with or without battery. Best wishes for your endeavours. Michael

[@Umski]

@@michaeldepodesta001 thank you again - the EWI experiment you did was fascinating - I guess it's like the analogy of having holes in a bucket - that said, I am wondering whether the benefits are still valid in that the section of the house that I was thinking of insulating can be essentially looked at as an extension or annex even though it was all built together - insulating all 3 outer walls would reduce the surface area for the "sideways" heat to escape i.e. it would hit the insulated roof space or the narrow join to the main part of the house - much like a cover on a hot water bottle - this is just speculation mind 😁 Likewise, I maybe hadn't appreciated how well insulated it might be so the other step was to go for MHRV as I know there are some cold spots in a couple of rooms...the HP sizing has been the eye opener for me - I'll need to find someone trustworthy to run some official calcs as you suggest 👍

[@michaeldepodesta001]

@@Umski It seems you are proceeding very sensibly. Regarding EWI, my point is to be aware that it might not be quite as good as you hoped. I would encourage you to make some kind of measurement *before* you do the work so you can see whether or not what you did made a difference. This might be measuring the difference in temperature between two locations in your home. Let me know if you would like advice about how to measure the temperature. Regarding MVHR, you can estimate the actual level of air changes per hour in your home by measuring the CO2 concentration protonsforbreakfast.wordpress.com/2021/06/06/estimating-rates-of-air-change-in-homes/ All the best: M

[@PeterJFlower]

I am an anorak like you and a consultant heating engineer, I worked out my heat loss really acurately to 5.7kW but using your rule of thumb I worked out 3.56kW so the answer seemed a little low. Then I realised that I live alone and go out to work, the heating is off all day and I keep the house cool in any case. A heat pump should work roughly 24 hours per day so it may not be safe to rely on rule of thumb alone if the operation times change. Over all I think the video is very helpful as any cross check or rule of thumb is a great idea, but please I advise everyone to work out the correct heat loss answer even if it is hard work to do that as the operating times make a huge difference.

[@michaeldepodesta001]

Excellent points. I am surprised because the formula has a couple of factors that should tend to slightly overestimate the required heating power. So you calculated your heating power to be 5.7 kW at what internal and external temperature? And using what method?

[@PeterJFlower]

@@michaeldepodesta001 My heat loss calculation used an outside temp of -3 and individual room temperatures of between 18 and 21. The house is a three bedroom 1930s semi with cavity wall insulation downstairs, new pvc double glazing, 50mm internal insulation upstairs and loft insulation so it is fairly well insulated. The calculation used CIBSE figures and carefully worked out U values for all of the building fabric. I've checked the calculation 4 different ways, so I remain confident that 5.7kW is reasonably correct although the infiltration rate always seems like an educated guess.

[@michaeldepodesta001]

@@PeterJFlower Thank you. I will reflect on your results. Meanwhile, you can estimate actual infiltration rates (Air Changes per hour) by using a CO2 meter. It's much easier than using a pressure change technique. I wrote about it here protonsforbreakfast.wordpress.com/2021/06/06/estimating-rates-of-air-change-in-homes/ Best wishes Michael

[@brackcycle9056]

Agree. & there is the rub .. With gas if you heat morning & evening only, you save 1000s of Kwhs of heat loss while the house is cool, Switching to a heat pump, What to do ? Commit to 24 hour a day warmth, even if you are out & miss it, or oversize the pump to give good rate of warming up as you come home ?

[@michaeldepodesta001]

@@brackcycle9056 Yes. But I don't think the difference is so great in most cases. The actual saving by switching off heating during an absence depends on the length of the absence compared to the time constant with which the house temperature falls. I agree that for longer absences - probably a day or more - just switching off the heating will save energy. But for shorter absences I think it is quite finely balanced. If a boiler works flat out (say 20 kW) for 2 hours to restore comfortable temperatures then that is 40 kWh of heating. If the absence was for 10 hours (say) then a heat pump could supply (say) 4 kW of heating to achieve the same result, but using only 13 kWh of electricity. I need to do some calculations on this!

[@Gastell0]

Thanks for this information! I can get a rough guestimate at least to my current heat loss, which is enormous as I have predicted due to it being large and ancient building xD Now I probably can get some estimations on how much I would be able to reduce it with various insulation materials and techniques

[@jchidley]

I am so glad that I found your channel - I hope that you’ve got more content. I also read your articles and they answered all my additional questions (how valid is HDD? What is the relationship between the set temp and the overall numbers? Etc)

[@michaeldepodesta001]

Thanks for the kind comments. I am working on more stuff but I am a bit slow! Best wishes Michael

[@frankjohn1408]

Thanks, the hp therefore should not be greater than 40 percent of the general load. If the unit is running at a slower rate would this have a negative effect on the COP? I guess the manufacturers need to answer this one. Thanks for the great video.

[@pfischer4664]

Brilliant. The best ideas are often the simplest. Thank you!

[@johnh9449]

Excellent thank you. I've used a detailed room by room heat loss calculation spreadsheet for my property and calculated about 12kWh loss for the coldest day with 23°C internal and with hot water on top it's covered by a 16kWh heat pump. I was a bit suspicious it was too big but doing the rule of thumb calculation comes to just over 14kWh without DHW which is about right - so that's reassuring! Thanks 😀👍

[@michaeldepodesta001]

John, Excellent news. That's exactly why I put the 'rule of thumb' out there: as a kind of sanity check because those heat loss calculations can easily overestimate heat pump power. 12 kW or 14 kW is a very hefty heat pump, but then 23 °C internal temperature is a very significantly warmer home than many. It's your home and your money and hence your choice, but if you lower the thermostat by 2 °C you will save quite a bit and lower the maximum power required by around 10%. Regarding hot water, I am not aware of guidance as to how to add that into the sizing. Personally I didn't add it in at all! Typically, the heat pump switches from space heating to water heating for an hour or so, and then switches back to space heating. In summer our heat pump does this during the day when there is plenty of solar and the air is warm. In winter it does this at night using cheap-rate electricity. Either way, we didn't add it into the heating requirement, I just noted that a bigger heat pump will reheat a DHW cylinder faster than a smaller one. A 200 l cylinder at 50 °C contains around 10 kWh of stored heat so you can calculate the re-heat time from cold. The articles below describe some measurements I made of this cycle protonsforbreakfast.wordpress.com/2021/08/14/heat-pump-first-operational-data/ protonsforbreakfast.wordpress.com/2021/09/07/assessment-of-heat-pump-heating-water-to-50-c-and-70-c/ So my inclination would be to size your heat pump just for the space heating. In any case: good luck with your adventures! Michael

[@johnh9449]

@@michaeldepodesta001 Thanks Michael. I've tended to find I wake up with a sore throat too iften with lower temperatures so I have it set warmer than most. To offset the cost I'm to supplement the power requirement with an 8kW solar array and 15kWh battery, making use of a solar diverter and EV tariff. I graphed and factored all this in month by month which predicts I'll be off grid between mid April and October with overall 75% savings in energy before the prices went up and the best thing of course being getting rid of gas. With inflation devaluing savings it's also a good investment with solar alone paying back 20% per year and hedging against further price rises. Without being political the government should be making and facilitating similar investments and getting everybody off fossil fuel dependency.

[@michaeldepodesta001]

@@johnh9449 They all sound like very smart moves.We have an 4 kWp array, 13.5 kWh battery and are off-grid for 4 to 5 months - and I have just ordered more solar to extends that protonsforbreakfast.wordpress.com/2022/09/01/its-been-a-sunny-summer/ We also found that the solar covers 20% of demand even in November. I think you have made some very smart choices. Good luck!

[@davidjoelsen3399]

Thanks for a great presentation that is clear and logical - I am a subscriber now.

[@imranmajid1978]

Very well put together and presented video.

[@michaeldepodesta001]

Imran: Thank you.

[@brucemacneil]

I have been reading your blog posts and email blasts for quite a while - Happy to see the videos - a larger audience potential. I appreciate the detail available in the written form but the vids are more universally accessible.

[@brucemacneil]

BTW - likely replacing our primary residence boiler in Canada with and ASHP. Ambient temps often below -20C. And, our secondary residence ( Ambient often at -30C with -40C nights) is 100% heat pump.

[@davidreece1642]

@@brucemacneil Bruce, which brand of ASHP do you have and does it keep you warm in those harsh Canadian tempertures?

[@brucemacneil]

@@davidreece1642 I have 2 houses. One is in Toronto (winter temps touch -20C but rarely) - it is now very well insulated and heated with a boiler. It has been heated via a boiler since forever - we are replacing the boiler with ASHP and air handler "heads" inside. We presently have air conditioning via a superficially similar system. The second house (a full size house) is in Quebec - winter temps are -20C normally with prolonged stretches at -35C. This house is heated with a 20 year old ASHP that looks like a standard brand. This heat pump is connected to an air handler with ductwork on the inside. Brands - I take my cues from Southeast Asia where there are millions of "split air conditioning" units installed. These generally have no brand ID and are very inexpensive. AND - have been functioning perfectly for decades. I am frugal, a mechanical engineer with loads of experience building new and renovating older buildings. IF you are in an area where the Ambient temps are usually above -20C, almost any unit works as well as any other. Claims of reliability should be suspect and the best plan is to have a good relationship with a trusted installer. If you are looking for ASHP to work well to -45C (really) - a Canadian company called Arctic Heat Pumps would be my recommendation. If you are in an older home with hot water radiators do not discount the idea of a mini spit ASHP - Some regions of the world are experiencing super hot summers for the first time and the houses do not have any cooling. The mini-split ASHP does heating and cooling. Also - insulation and windows are key. It is routine to reduce the heat loss/gain in an older home by 90% through a remodel with insulation.

[@davidreece1642]

@@brucemacneil Thank you for such a detailed reply. We don't have many days below -5C in the UK so your real life situation is very reassuring for people like me who are thinking about ASHP.

[@brucemacneil]

@@davidreece1642 The Coefficient of Performance of almost any ASHP will be greater than 3 at -5C. Cape Breton Island - colder climate - almost every house has added an ASHP over the past 10 years.

[@mfwwhelan7561]

At last: a comprehensible concise explanation !

[@cvplumbersandheating]

Just did the rule of thumb calculation using your method, 13500kwh gas used over the year, my draughty Honley requires 4.6kw heat pump

[@michaeldepodesta001]

Dear CV Plumbers and Heating. That sounds about right. But please remember this is a rule of thumb, if you want to install a heat pump you will need to get a heat loss survey. Sadly these surveys seem to overestimate heat pump sizes and so the Rule of Thumb is there to help you ask critical questions of the heat loss survey comes out with an unrealistic heat pump size. If you want to see whether your home is really draughty, take your annual gas consumption figure and estimate how much heat is used for space heating. Take your consumption annual 13,500 kWh and multiply by (say) 85% for efficiency and then subtract (say) 5% for DHW ~11,000 kWh. Then divide this by the living area of your home in square metres. So if your home is 80 m^2 then the figure is 11,000/80 = 137.5 kWh per year per square metre (kW/y/m^2). The best possible result is ~ 15 for a PassivHaus: 25 is an excellent refurb. 50 is modern building specification. 100 is a reasonable UK house. Above 100 and you would probably benefit from some draught proofing and insulation. All the best Michael

[@afrocraft1]

Great analysis but the biggest flaw, if one can call it that, is that you're assuming heat load changes in a linear way with outside temperature. So you use HTC, an _average,_ to model heating amount required to raise temperatures. In many older, leakier homes, though, linearity doesn't hold, especially at the extremes: You need more energy to raise the temperature than your HTC assumes. Probably still a useful guide, though!

[@michaeldepodesta001]

Dear afrocraft, Good afternoon. You say that in older leakier homes that linearity doesn't hold. But I don't know why you state that. The linearity arises from pretty basic physics. I have measured this in my own home and as you say, the results are not completely linear. This is because - I think - of solar gain in spring and autumn which I don't account for explicitly. But the winter data is convincingly linear and extrapolate to zero at zero heating demand. You can read about the measurements here. protonsforbreakfast.wordpress.com/2021/02/15/understanding-heat-flow-in-my-home-the-key-graph/ Best wishes Michael

[@afrocraft1]

@@michaeldepodesta001 Thanks for the response! Here's what I am finding: When I use your HTC method for my 3000 SQFT home in NE USA, I get 6 kwh heating capacity. But when I use actual runtimes on cold days (~17 F or -8 Celsius, which is our design temperature), I get 14 kwh. I think both are correct: My average heating demand over a year is 6 kwh, but my demand on a cold day more than doubles! But I'm not an HVAC engineer...

[@michaeldepodesta001]

@@afrocraft1 I'm a little confused by your use of kWh (energy) when from the context I would expect you to be talking about kw (power). So taking your annual gas usage (kWh) and dividing by 2,900 you get an estimate of a heat pump power as 6 kW (Power) *if your home is the southern half of the UK.* You home is in the NE US which is a big place. I worked out the degree days for Albany HDD16.5 ~ 3,200 compared with 2,100 for UK town of Oxford. So a home in the NE US would require about 50% more heating than the identical home in the UK. Additionally your peak heating demand is 28 °C rather than 20 °C. I think these two factors are the origin of this discrepancy. Best wishes Michael

[@afrocraft1]

@@michaeldepodesta001 Thanks so much for this. I reviewed the HDD I used and found wide variation in the estimate (1600+ to 3625) provided by the website you recommended, depending on which station you use! My HVAC contractor advised to use Reagan National (DCA), which turns out to be more like UK weather (~2035 HDD). Now recalculating, I have better agreement between load estimate using actual furnace run times and the HTC method. It works!

[@dellybeanhead]

Excellent presentation. Thank you. Did it work for you over the last year. My concern with calculations based on 1 year is that it may not work on the coldest day. A small pump may work based on averages but will it be man enough on the coldest day of the year. I had an issue like this when calculating PV required for my off grid house where average radiation is so much lower in December which meant that the actual requirement for solar panels was around 4 to 5X what had been calculated by other companies. And it does only just work! Is there a similar issue with the rule of thumb for heat pumps?

[@michaeldepodesta001]

Good Evening. Yes, thanks for your comment which is very important. Roughly speaking, you can use the heating degree days and average gas consumption to estimate the Heat Transfer Coefficient for the house. You can estimate this using daily, weekly or seasonal averages as long as the days weeks or seasons include some cold weather. Suppose the HTC is 300 W/°C. As you say , you want to estimate the heating required on the coldest likely temperature - in my locale that's -3 °C but we actually had -5 °C this year. This is called the design temperature and so if you want to maintain 20 °C inside the house you can estimate that you will require say 23 °C x 300 W/°C = 6,900 W or 6.9 kW. Most people get the heat pump which is the 'next size up' from their estimated maximum demand. Does that makes sense? Best wishes Michael

[@dellybeanhead]

@@michaeldepodesta001 Thank you for taking to time to reply. Yes absolute perfect sense. Thank you

[@alanbennett3812]

Hi. Thanks for the info. Amazing! My house is 306m². 9" solid brick. New roof (fully insulated) and windows in 2013. I'm in the process of insulating the ground floor crawl space. 116m² Using my average of 32000kwh of gas and a 80% efficiency for the boiler and using my heating degree day. A heat pump size of 10kw is being suggested ! Amazing. I heat the house to 19.5c 2 hours in the morning and 7 hours in the evening. I also use a 8kw stove on the colder days I'll get a heat geek heat loss survey done in the new year. £500 though. Ouch! They'll monitor the house over a few days. A local ASHP installer estimating an heat loss of 20kw! It was an educated guess. Am I right in thinking a 22kw system (Two 11kw HP mitsubishi Cascade system) could produce 528 kwh of heat per day? Looking back at my octopus data (only from March 2023) my highest gas heat load was 165kwh. (at 19.5c) To be honest, the house could be a little warmer sometimes. It's finding the balance between cost and comfort. Fingers crossed, both will improve with a ASHP. I have 9.24kwp 15 panels south 7 west 6kw solaredge Homehub inverter 10kw solaredge battery. 100amp. Single phase. System predicted to produce 8.4Mwh per year. I've exported 2MWh this year. I wonder how much of the heating demand will be covered by solar? Thanks again.

[@michaeldepodesta001]

Lots to comment on here! “Hi. Thanks for the info. Amazing! My house is 306m². 9" solid brick. New roof (fully insulated) and windows in 2013. I'm in the process of insulating the ground floor crawl space. 116m² Using my average of 32,000kwh of gas and a 80% efficiency for the boiler and using my heating degree day. A heat pump size of 10kw is being suggested ! Amazing. I heat the house to 19.5c 2 hours in the morning and 7 hours in the evening. I also use a 8kw stove on the colder days.” The Rule of Thumb should work if during the heating season your home stays at roughly the same temperature during the day and night. “I'll get a heat geek heat loss survey done in the new year. £500 though. Ouch! They'll monitor the house over a few days.” Good idea. It’s real money, but it’s real work and expertise and genuinely valuable insights. “A local ASHP installer estimating an heat loss of 20 kW! It was an educated guess. Am I right in thinking a 22kw system (Two 11kw HP Mitsubishi Cascade system) could produce 528 kWh of heat per day? “ Yes “Looking back at my Octopus data (only from March 2023) my highest gas heat load was 165 kWh (at 19.5 °C).” 165 kWh/day is equivalent to just under 7 kW. Near to me in west London the average temperature in March 2023 was 8.5 °C and teh coldest temperature was - 2°C on teh 11th March 2023. I would guess your peak demand would not be much more than that www.wunderground.com/dashboard/pws/ITEDDING4/graph/2023-03-8/2023-03-8/monthly Do you have data from January - there were some very cold days this January 2024 (16th to 19th in the London Region). This would give you a very good idea about your real heat demand. 10 kW corresponds to 240 kWh/day “To be honest, the house could be a little warmer sometimes. It's finding the balance between cost and comfort. Fingers crossed, both will improve with an ASHP.” Each extra degree Celsius increases heat load by around 12%. I have 9.24kWp 15 panels south 7 west 6kw Solaredge Homehub inverter 10kw Solaredge battery. 100amp. Single phase. System predicted to produce 8.4Mwh per year. I've exported 2MWh this year. I wonder how much of the heating demand will be covered by solar? Not much of your heating demand will be covered by solar because obviously the solar peaks in summer and heating peaks in winter. But it should not be zero. On my system - smaller than yours - in December and January I get ~ 3 kWh/day, which corresponds to about 10 kWh of heating. It gets better in November and February and is quite substantial - maybe 50%? In March and October. "Thanks again. " You’re welcome. Good luck with your endeavours

[@MrDavegeo]

Brilliant video, very useful. Couple of points though - if I use less than average because I don't mind it a bit cooler, should I allow for more average use for people who live in the house in the future? Also, the impression I got from Octopus is that if they want to stay MCS accredited then they're not allowed to go under the MCS calculations, so we can tell them till we're blue in the face to install a smaller heat pump and they can't.

[@michaeldepodesta001]

Thank you for your kind words. Three things. 1. The 'Rule of Thumb' works well when the temperature of dwelling doesn't fall too much either at night or during the day. Two or three degrees Celsius is fine, but larger variations - e.g. letting the dwelling cool right down (may 10 °C) overnight or during a long working day - can lead to underestimates often true heating power required. 2. MCS surveys make assumptions and they are often ridiculous. They have surveys about (a) Air Change Rates are often overestimated by factors of 2 or 3 (b) The temperature of party walls is assumed to be very low - perhaps 10 °C (c) the temperature of teh ground underneath a house is assumed to be very low - close to outside air temperatures and (d) And there is even a factor 2 uncertainty on the U-value of solid brick walls. Making poor assumptions can lead to oversizing by a factor two. In one case I am involved with, a family measured daily gas consumption during etc coldest days and found that their home used on average 8 kW during the coldest weather. The MCS survey suggested the heat loss was 16 kW to 18 kW and they recommended 2 x 12 kW heat pumps! A decent surveyor in teh face of appropriate evidence will adjust the assumptions to reach the right conclusion. 3. Yes, reasonably I think, MCS base their calculations on an internal temperature of 21 °C, because the assumption is that 19 °C may be fine for you, but for someone else who buys the home, or you when you grow older, 21 °C is reasonable. 4. Yes, Octopus do everything 'by the book' even when they have teh data which shows them exactly what your gas consumption was on teh coldest days! But there are better installers out there using better heat pumps and more insight. Do you have any Heat Geeks near you? I wish you good luck with your endeavours in the face of these bonkers rules. M

[@MrDavegeo]

@@michaeldepodesta001 so zoning the heating where a living room can be 19C in an evening then drop to 13C overnight would throw the rule of thumb out the window?

[@michaeldepodesta001]

@@MrDavegeo Yes. The rule of thumb is based on the idea that the property is at (roughly) the same temperature all the time. If the temperature dropped from 19 °C (16 hours a day) to 16 °C overnight the average temperature falls to 18 °C - which won't make much difference. But if the temperature dropped from 19 °C (16 hours a day) to 11 °C overnight the average temperature falls to 16 °C - which is beginning to be quite a different average temperature. The rule of thumb would then underestimate the size of heat pump required. Why? Because while heat pumps can maintain a property at a stable temperature, they are not as powerful as boilers (typically 6 kW versus 25 kW) and so the rate at which they can heat (or re-heat) a dwelling is limited. So when heating with a heat pump the heat pump operates (more or less) all the time, and set back temperatures are typically just a couple of degrees lower than the normal temperature.

[@AlistairBrugsch]

7:20 if you have 3600 on the top and bottom of the division they equate to 1 and cancel each other out. So they don't need to be in the equation at all (it's fine to keep them in for illustrative purposes but un-necessary for the actual calc!

[@michaeldepodesta001]

Indeed. As you say, it was for the purpose of being explicit.

[@SeanQuinn-v5i]

Thank You Very Much Yourself

[@peterkis5442]

Excellent presentation!

[@michaeldepodesta001]

Thank you :-)

[@SRMV19]

WHAT A LEGEND! Thank you SIR !

[@kenbone4535]

Well done Michael

[@AnalogueKid2112]

As an American in the Midwestern US, this is all foreign to me but still interesting. Here, most homes are heated by a natural gas fired forced air furnace. Since we need air conditioning in the summer, it’s more economical to combine the delivery system. I do have a thought around calculating heat pump size though. I have a Nest thermostat which keeps track of how many hours the system runs each day. I monitor this for the coldest days of the year, and then I take the highest daily usage, multiply by the output of the furnace (we use British Thermal Units!) and divide by 24. That gives you the approximate size of heat pump needed, provided it can output that much at the balance point temperature.

[@michaeldepodesta001]

Good Afternoon. That is an excellent and robust way to estimate the heat pump size required. I have three comments which you are probably already aware of but I will says them any way. 1. Heat pump sizes. The badge on a heat pump (say 5 kW) specifies its thermal output at a standard condition - which may to be the coldest that it becomes at your location. So check that the heat pump can output the required amount of heat AT THE COLDEST TEMPERATURE YOU CAN REASONABLY EXPECT. 2. Gas burners vary in efficiency - can be as low as 75% if they are not condensing. 3. Gas burners can easily heat water to 70 °C but heat pumps will struggle to do that. Using the lowest possible flow temperature that will heat your house will result in better heat pump operation (higher COP) Anyway, thank you for stopping by and commenting. Best wishes: Michael

[@charlesstewart2304]

Great video and very informative - but on the rare occurrence when it is -10C outside this will only raise the temperature to 10C inside the house? And I assume the kW require is output not consumption of electricity.

[@michaeldepodesta001]

Charles, Good Evening. First, yes, the 'kilowatt nameplate power' is thermal power at a specified condition typically with an outdoor temperature of 3 °C if I remember correctly. Some heat pumps have lower power at lower temperatures - so a 5 kW nameplate heat pump might only have 4 kW at (say) -10 °C. For other heat pumps - notably Vaillant - their output exceeds their nameplate output down to exceedingly low temperatures. The electrical power for a 5 kW heat pump is typically between 1 kW and 2 kW. Second, yes, if your heat pump is sized exactly to be at maximum power with a design temperature of -3 °C, then on occasions where it's - 10 °C it will not heat your house to 20 °C. For those occasions - 1 day in 20 years? - and becoming less likely as the climate warms - it will be necessary to plug in an extra heater. For the other 7,299 days, the heat pump will heat your home with more than 100% efficiency, and reduce emissions of carbon dioxide. If you are particularly concerned about this, then you should deliberately oversize your heat pump - they will all operate happily down to about -20 °C. Best wishes Michael

[@josemanuelhda]

great video, thanks for sharing, but I must correct something it is about oil (gasoil) conversion to gas kwh, here in spain we use plenty of oil boilers ... yes, so expensive, but canalized gas does not reach everywhere unfortunately the conversion used in saunier and vaillant calculators for oil boilers is 7,5 kwh of actual heat per liter, quite less than the 10 kwh you propose, as an older oil user I can tell you than 7.5 kwh of heating is quite more acurate in real world situations than the value of 10, it seems oil boilers do have a lot of energy loss so a better estimate wold be as follows 10 kwh gas less 15% loss equals 8,5 kwh heating power, 1 liter of oil gives you the equivalent to 8,8 kwh of gas, so, if you divide 2900 by 8.8 it gives you rounding 330 liters of oil equivalence to your 2900 kwh of gas my saunier genia air max 8 in three winters on the row has given me 6.5 kw power per hour of use, I used to burn 2200 liters of oil per year, that should give me a power needed of 6,67 kw ... so close both results I have done the maths with other heat pump and old oil users in a telegram channel we use for sharing data and information, and I can tell you this 7.5 kwh per liter is quite a precise estimate so for oil users your formula should be liters per year/330 and that would give the average heat pump power needed to obtain that same energy hope it helps, and if you agree or want to check my data you or course are free to do it here in spain our winter season is shorter than yours, so a 10% plus over the calculation maybe a little short, in my case with maximum power with low temperatures outside (all night below -2) and the house cold to 12 degrees, I measured in my case a steady production of 8.5 kwh heating from my machine, although it says 8 kw in its comercial name, it can actually go up easily to 9,5 and in ocassions to 10 kw power output in this circunstances I think it is better to apply maybe for a 15-20 percent excess from your formula anyway thanks for your shared knowledge, and I will follow with attention your experiment this year after removing your 50 liters bottle of water (LLH), I will give you a tip that you already assume, several people here have done it already, some of them making a reversible bypass, no one has used again the LLH ... with no LLH you gain control, confort and efficiency, I am sure your data will prove it

[@michaeldepodesta001]

Jose Manuel, Good Morning. Thank you very much for your input. Chemically and Thermodynamically I think that 10 kWh/litre of oil is correct, but you are saying that in practice oil boilers are only 75% efficient. That's very depressing but quite believable. When I next give this talk I will amend that slide. Regarding the LLH, the coming winter will see what difference it has been making! BEst wishes: keep warm! Michael

[@stefanheitmannds]

Hi, very good video, can you please explain me, if im on a milder climate, with less degree days, the formula generates larger heat pumps, as this number divides the kwh total consumption. In my case have a house with 110000kwh, and only 2000 degree days, if i had 2500 degree days the pump will relut in a smaller one that with the 2000 degree days. Hope u can clarify that to me Thanks!!

[@michaeldepodesta001]

Stefan, Good Morning. Yes, this is a thing which has confused others too. I think I've answered it somewhere in the comments, but I can't see that answer now. Let's use your example - and guess that you have made a typo and your gas usage is 11,000 kWh (not 110,000 kWh as you have typed). The formula works by estimating the Heat Transfer Coefficient (HTC) for your house - a measure of how 'leaky' it is. If you house uses 11,000 kWh/year and is in a climate which requires 2,000 heating °C-days then this is equivalent to a HTC of 11,000 kWh/year/2,000 which around 5.5 kWh/day/°C. This means your dwelling required 5.5 kWh/day to raise it 1 °C above the outside temperature. If you wanted to raise it (say) 20 °C above teh outside temperature then this would require 20 x 5.5 = 110 kWh/day - this is equivalent to a heating power (continuously) of 110/24 = 4.6 kW. Now suppose that A DIFFERENT HOUSE also consumed 11,000 kWh/year but it was in a COLDER CLIMATE which requires 2,500 heating °C-days then this is equivalent to a HTC of 11,000 kWh/year/2,500 which around 4.4 kWh/day/°C i.e this different house from yours would be less leaky. So to raise this less leaky house (say) 20 °C above the outside temperature would require 20 x 4.4 = 88 kWh/day - this is equivalent to a heating power (continuously) of 88/24 = 3.7 kW. Does that make sense? I do hope so. Best wishes Michael

[@stefanheitmannds]

@@michaeldepodesta001 It does make total sense!! ! as for the 110 thousend kwh we calculated a 44 kw HP, with the 60 to 75w per scuare meter formula in a 600m2 house. It seems it does work for our mild climate this unrecomende formula. Much is to say that I´m very thankfull for your time and explanation. Very very usefull!

[@peterkis5442]

Dear Michael, I have tested the equation but something is not clear for me. In my opinion dividing the annual energy usage with the annual HDD is only correct if the base temperature matches what is already set at the boiler controls (if its possible to set at all). The HTC is inversly proportional to the base temperature therefore it produces incorrect results when there is a mismatch in the base temp. For example: EDDS (Stuttgart, 2023) Base temp#1 10°: HDDannual#1 = 746,9°C; Base temp#2 20°C: HDDannual#2 = 3008,7°C So far so good, because if the base temp is set lower, there are less days in a year at hand that count. Energy expenditure for example: 25kWh, boiler efficiency: 80% HTC#1: 25/746,9*0,8*(1000/24)=1,12W/°C HTC#2: 25/3008,7*0,8*(1000/24)=0,27W/°C now If I multiply the number with 20 in case #1 i get a heat pump of 22.3kW and in case#2 I get 5.5kW. So it would appear that my house has bigger heat losses, and needs a bigger heat pump. Maybe I understood the formula incorrectly, I would like to have your opinion.

[@michaeldepodesta001]

@peterkis5442 Good Evening, In your comment you states that: "The HTC is inversely proportional to the base temperature". No! The Heat Transfer Coefficient describes the way heat flows into or out of the dwelling. It describes the way the dwelling loses heat. You calculate two examples of HTCs, HTC#1 and HTC#2: ***THESE DESCRIBE TWO DIFFERENT DWELLINGS*** In the first dwelling 25,000 kWh are used per year to maintain it at 10 °C: In the second dwelling 25,000 kWh are used per year to maintain it at 20 °C: Clearly, the second dwelling is much better insulated and so it needs a smaller heat pump. OK? Best wishes Michael

[@adblocker276]

Great video. What I don’t understand is octopus have sent an engineer around and they calculated the heat loss for my house at 12kw. My gas usage is about 16000 kWh - per your formula I need a heat pump around 5.5kw but they said there couldn’t install it because it didn’t fit with in their financials I suppose

[@michaeldepodesta001]

Dear @adblocker. I'm sorry but that's all too common. If not used by experienced people, the software which calculates the heatloss tends to overestimate heat loss quite badly. I wish I could say something to practically help, but the only thing I can think of is that the situation is improving slowly. I know it doesn't help you. Sorry. Best wishes Michael

[@adblocker276]

@@michaeldepodesta001 hi thank you for replying. Money is not an issue for me (as long as I can get the boiler upgrade grant) I’m happy to get a heat pump installed. I’m also getting a 12kw PV array installed with Tesla Powerwall3. As soon as that is installed in the next couple of months I will get the heat pump. My concern is I want the house to be entirely self sufficient in regards to energy use. If the heat pump needs to be massive my solar panels may not be enough. Should I get another company to do the heat loss calculations? Any recommendations?

[@michaeldepodesta001]

@@adblocker276 Good Evening. First of all congratulations! That sounds like a super system I am sure the combination of panels and a Powerwall 3 will give you splendid results. I don't think that you will be able to become self sufficient all year round. I don't know where you are, but in the UK for a south-facing system, a 12kW-peak array (which is MASSIVE! 36 panels!) will yield ~50 kWh/day on average in summer but it's winter yield will average 10 kWh/day *at best*. But winter generation depends strongly on shading low to the horizon. 10 kWh/day is enough to cover normal household usage, but if you are running a heat pump your winter usage will rise considerably. In my home it peaks at about 25 kWh/day. I would not worry about this: you system will keep you off grid for many months of the year (perhaps 6 months) and you will have very small bills in all but the coldest months. And you will export a lot of electricity. Regarding your heat pump, I would recommend you stabilise your house at a comfortable temperature, and take weekly measurements of gas consumption over the winter: this will tell you the actual amount of heating you use to keep your home as want it. If you use 700 kWh of gas in a week then with an 85% efficient boiler that will deliver ~595 kWh of heat which corresponds to about 595/7 = 85 kWh/day which is an average of 3.5 kW. You can also make the same measurements on a very cold day to work out the biggest daily usage. It may feel frustrating to wait but when you know for sure the size of heat pump you require, you will feel much more comfortable about the installation. Best wishes with your endeavours. Drop me a line if you would like advice about the measurements. Michael

[@alibro7512]

Thanks for such a useful tool but I have a question about the oil calculations, You said multiply oil usage by ten to get the same result as the gas kWh usage but does that take into account the difference in efficiency between oil boilers and gas. My oil boiler is 19 years old and not condensing so is probably 70% to 75% efficient whereas you mentioned a gas boiler should be around 90% efficient. The other thing I have heard is the comfort level goes up with a heat pump so I'm assuming they heat the house more than they did before. Does this calculation take that into account? We use approximately 2000 litres of oil per year so I am trying to get my head around how large a heat pump we would need and how many kWh we would use in a year to have similar heat in the house. Thanks again.

[@michaeldepodesta001]

Thank you for your kind words. Regarding your particular questions. First of all, please remember that this is a rule of thumb. In calorific terms 2,000 litres of fuel oil consumption is roughly equivalent to around 20,000 kWh of gas. So if this is your annual usage, then this would suggest very roughly 20,000/2,900=6.9 ~7 kW maximum heating demand. This assumes that (a) the dwelling is kept at a roughly uniform temperature throughout the heating season and (b) you hot water usage is not crazy. If you switch heating off all day nd then blast it on for a couple of hours in the evening then this may cause the formula to underestimate your heating requirement. If you do use a lot of hot water then the formula will overestimate your heating demand. However you are right that many oil burners are not as efficient as modern gas boilers. The 2,900 number in the formula assumes 90% efficient burning so if the oil boiler is only 75% efficient, then the heat load will be less. 20,000 kWh of gas burned at 90% efficiency yields approximately 18,000 kWh of heating in your home. 2,000 litres of oil is equivalent to 20,000 kWh of gas burned but only at (say) 75% efficiency and so yields approximately 15,000 kWh of heating in your home. So the estimate of a 7 kW heat pump would be an overestimate. A better estimate would be around 75%/90%= 0.833 * 6.9 = 5.7 kW. In practice a 7 kW might well be a good fit giving you some headroom and faster re-heating of domestic hot water. The 'comfort' issue is difficult to assess qualitatively. In my house, all the rooms are at a very similar temperature all the time. This is a pleasant environment to be in. Now at the moment - and through previous winters - our internal temperature is a smidgeon over 21 °C which is hotter than teh 20 °C set point, but we have not tweaked teh system because we just enjoy it! Every extra degree Celsius causes around 12% extra heating demand. Best wishes in your endeavours. Michael

[@alibro7512]

@@michaeldepodesta001 Thank you for such a comprehensive reply. We are probably not typical in that we tend to keep our house cooler than most but use a multi fuel stove in the living room for local heat. Using two digital thermostats one upstairs and one downstairs we have downstairs at 18C in the morning for 2 hours and 6 hours in the evening. We also have upstairs at 18C in the morning for 2 hours but only 3 hours in the evening. In between these times the temp is set to 14 or 15C so basically off as the temps rarely fall so low unless it is very cold outside. I understand with a heat pump I would need to have more even temps throughout the day, that would be OK as it would give more comfort but I guess might confuse the calculations. Hot water wise we probably use a tank of water most days which I believe will be around 5 or 6kWh based on what our solar Iboost uses to heat the tank. So from these calculations we could estimate a 7kW HP would be sufficient but if I decided I would quite like the house a bit warmer then maybe a 9kW HP would be a safer bet. We might also be able to retire our multi fuel stove. Or are we better to keep to the 7kW HP and use the stove on the coldest days? I hear conflicting advice on whether over sizing a HP is a problem. Obviously I need to get any installer to do their own calculations but at least I now have a useful estimate and if they say we need 15kW HP I can tell them they need to redo their calculations. Thanks again for all your help.

[@bazcurtis178]

A very useful video. I have my quote from Octopus for a 4kWh heat pump. What I am still no clear on is, how do I work out the running cost. I assume it doesn’t need to run at full power all day.

[@michaeldepodesta001]

@bazcurtis178, Good Afternoon. I looked at the issue of running costs in this article on my blog. protonsforbreakfast.wordpress.com/2023/12/17/variability-of-heating-demand-throughout-a-year/ My analysis was complicated - working out how hard the heat pump worked on each day of the year from the last couple of years. But it ended up with a simple conclusion: the cost is likely to be "about the same as heating with gas". If the installation is great - with heating being delivered at low temperatures, then the average seasonal COP (or SCOP) is likely to be high and running costs low. You can get an idea of what's possible by looking at heatpumpmonitor.org . If you click on the headings you can sort by supplier and look at other Octopus installations. They are not the top-performing installations, but they do seem to generally have a SCOP greater than 3. Try also clicking on the icons over on the right of the screen to see live monitoring of the heating system. Do get back to me if I can help more. M

[@bazcurtis178]

@@michaeldepodesta001 Thanks Michael. I will read that. The SCOP on my quote is 3.2. We are in a 7 year new build with solar (8x 400 watt panels, east, 6 400 watt panels, west) and battery (9.6kWh)

[@michaeldepodesta001]

@@bazcurtis178 OK. Now I understand. It's a modern house with reasonable thermal properties. My own maximum thermal load at - 5 °C is 3.5 kW so we are not too different. But the solar panels and battery will make a BIG difference. The solar panels will ( I guess) generate around 4,000 kWh/year - but really very little in winter - perhaps just 1 or 2 kWh/day. But you have a useful size of battery. So my guess is that you will have very good savings over using gas for heating and hot water. We operate separate summer and winter schedules. In summer we heat hot water and operate the Legionella cycle in the afternoon, after the battery has filled up. We all also time other heavy loads for afternoon when the excess solar would just be exported. We then run overnight on the battery (we have a Powerwall 13.5 kWh but see this article on degradation of the battery capacity protonsforbreakfast.wordpress.com/2023/12/30/solar-pv-review-of-2023/). In summer we go off-grid for around 4 months. If you still have young children you won't manage that! In winter, we heat hot water at night with cheap electricity and charge up the battery. We then run for as long as we can from teh battery. In teh darkest coldest days of winter we only get to midday before we need to buy full price electricity, but in December and January we pay an average price of just over 10 p/kWh for electricity. SO our bills are around £500/year for everything. I did try to write a spreadsheet to calculate the optimum tariff with real solar data and a battery protonsforbreakfast.wordpress.com/2023/03/05/tariff-calculation-spreadsheet/ But I don't recommend it. It sort of works, but I think it is at best indicative. But if you have spare time you might have fun. But in short, if you have solar and decent battery, you will save 'a packet'! I do hope things go well. All teh best Michael

[@afrocraft1]

Great stuff. Does this estimation also apply to cooling with a heat pump?

[@michaeldepodesta001]

Afrocraft, good evening. No, this doesn't apply to cooling. To calculate an equivalent formula for cooling one could use the 'degree-days' web site but instead look up 'Cooling Degree Days' rather than 'Heating Degree Days'. It is thankfully not such a strong requirement in the UK at the moment... Best wishes: Michael

[@KevinWorth-z3p]

Hi Micheal, thanks for your video, its very interesting but has left me a little confused. You base your calculations on current gas useage however the amount of gas used to heat any home depends on the boiler they have. For example, last year's gas usage was approximately 18000kW (including hot water & cooking) from a 40 year old heat only boiler. If I had a modern heat only boiler my gas usage would likely to be significantly less and would significantly affect the outcome. I don't really know what the efficiency of my old boiler is but i suspect less than 80%. Using 80% as an approximation and a 20⁰ "lift" it would suggest a 4.2kW ASHP which in my 170yr old detached house in North of England with solid walls, no DPC and very little loft insulation seems very unlikely. If I substitute assumed gas usage from a modern 90%+ efficient boiler then the calculated Heat Pump requirement would be nearer 3.2kW (and this still takes into consideration my hot water (cylinder in uninsulated loft) and cooking. Ok, the cooking use is negligible so barely affects the calcs which you rightly point out but selecting the correct ASHP output is critical when it comes to comfort v economy. Undersizing will result in not being warm/comfortable enough and therefore using additional heating on colder days whereas oversizing will will result in greater electricity useage (and presumably higher cost of appliance). Im sure your calculations work pretty well assuming a modern, well insulated home with a modern boiler but for older houses and equipment I just dont see how this works given the significant variation of the potential gas usage to heat the same home.

[@michaeldepodesta001]

Dear @user-rs5ce5yj9x When one considers the problem of heat pump sizing, the true quantity we would like to know is generally not available to us. We would like to know the heat transfer coefficient - the heating power required to raise the internal temperature 1 °C above the background. As the video explained, one estimate this quantity from a heat loss survey, but these are notoriously inaccurate because (in general) the construction of a particular property is unknown. The idea of the rule of thumb is to act asa check on a heat loss survey. I don't understand your calculations: boiler efficiency is already included in teh rule of thumb. For 18,000 kWh of gas consumption the rule of thumb recommends a 6.2 kW peak output heat pump. This doesn't seem like a crazy figure to me - it's close to the UK average. And no, the rule of thumb makes no assumptions about the type of dwelling: a house, old or new, or an apartment. Best wishes Michael

[@calumcadwallader2127]

Amazing video thank you sir

[@kevinw7419]

Knowledge, du! College Lecturer for over 25years!

[@michaeldepodesta001]

Mmm. Yes, I was a lecturer in Physics for a bit under 20 years, at Birkbeck College and University College London. There I literally wrote a text book on the properties of materials, which is very useful if you want to understand the phase changes in either boilers or heat pumps. Then I moved to the National Physical Laboratory where I worked for 20 years as an expert in temperature measurement. At NPL I made the most accurate temperature measurements ever made and carried out work leading to the re-definition of what we mean by "one degree Celsius". I was also honoured to be awarded an MBE for my services to science. After retirement I used all my skills to monitor my home and publish the results so that others could learn from both my mistakes and successes. From your comment: "Knowledge, du! :College Lecturer for over 25years!" you seem to show contempt for people who actually know things. This ties in with your previous comments which seem to be along the lines of "I know a bloke who said...". Perhaps if you spent a few years at University you too might learn a thing or two. Best wishes Michael

[@nattyco]

That's very helpful. I only have a 3 bedroom flat and from your formula will need 10KW /day to raise the temp by 20 degrees . Mind you, I like the temperature between 21 and 22 degrees during the day and off at night and live in Scotland which is colder than Manchester. A heat pump will cost me a fortune to run.

[@michaeldepodesta001]

Dear @nattyco, good evening, Did you mean 10 kilowatt *hours* per day? That doesn't seem a lot to me: most houses require closer to 100 kWh/day. But you haven't given me enough data to really make a judgement. In any case, I wish you all the best in your endeavours. M

[@williamprophet]

Amazing…. Thank you

[@LCD72]

If I understand you correctly (unlikely) you have given 3 formulae for calculating heat pump size required: 1. Heat pump power (kW) = annual gas usage (kWh)/ heating degree days (HDD) 2. HTC/20 x 1000, where HTC = (annual gas usage (kWh)/HDD for the location) x boiler efficiency (say, 90% = 0.9) x (1000/24) 3. Annual gas usage/2900 If I input my values, I end up with 3 quite different heat pump sizes: 1. 14,870kWh/2048 (HDD for Exeter) = 7.3kW 2. 14,870/2048 x 0.9 x 1000/24 = HTC of 272. Heat pump size = HTC x 20 = 272 x 20 = 5,440W = 5.4kW 3. 14,870/2900 = 5.1kW

[@michaeldepodesta001]

JG, Good Morning. Looking at these in reverse order. 3. 14,870/2900 = 5.1kW This is a rule of thumb which I devised to apply anywhere in England south of Manchester and assumes a 20 °C internal temperature i.e. it uses HDD 16.5 data. If the house is occupied normally, I expect that this should be within about 10% of the right answer. 2. 14,870/2048 x 0.9 x 1000/24 = HTC of 272 W/°C Heat pump size = HTC x 20 = 272 x 20 = 5,440W = 5.4kW This is the specific application of the rule of thumb for Exeter data , and 5.4 kW is indeed within about 10% of Calculation #1. 3. 14,870kWh/2048 (HDD for Exeter) = 7.3kW This is not quite the calculation you think it is. If you look at the units of the calculation kWh divided by °C-days the final units are kWh/°C-day i.e. this is another measure of the HTC. To convert to W/°C multiply by 1000 (to convert kWh to Wh) and divide by 24 (to convert days to hours - which then cancel) 7.3 x 1000/24 = 303 W/°C. This is bigger than the answer in calculation#2 because it does not take account of the boiler efficiency. Overall, the answer comes out at about 5.5 kW and that seems very reasonable. I’m sorry it’s so complicated, but it is! Best wishes Michael

[@LCD72]

@@michaeldepodesta001 Thanks for elucidating all that. As I looked through my scribbles made while watching your video, it seems that you might have outlined a fourth way of coming up with the required heat-pump size - actually the calculations you went through at the beginning: 4. Heating power required to raise temp. by 1C above background (external) temp i.e. heat transfer coefficient (HTC) (=W/degrees C) = annual gas usage (kWh)/57.3 = 14,870.05/57.3 => 259.5 watts per degree C Per day: (24 hrs x 3600 seconds x 259.5)/3600 seconds x 1000W = 6.228 kWh per day per degree C. So, to raise temp by, say 20 degrees C, requires 20 x 259.5 = 5190W or 5.19 kW (5.19kW for 24 hours = 5.2 x 24 = 124kWh of heating per day) Heat pump size = 5.2kW Is that right? The heat pump size comes out very similar to the other rule of thumb methods.

[@borismartens5209]

Thank YOU very much :)

[@misenko856]

Nice video! What do you think the size of unit I need if my heat loss is 5KW/-7C and temperatures here in winter barely reaching -15C ? The lowest average temperature is around 0C in January. I think 8KW unit would be enough but was also thinking about 12K but I fear it would be oversized and cycle a lot. Thanks!

[@michaeldepodesta001]

Misenko856, Good Morning, Nothing is simple! Here is one thing to bear in mind and two thoughts! The thing to bear in mind is that the nameplate capacity (e.g. 5 kW) is the heating power under specified conditions - in Europe often an outside temperature of 7 °C and and a flow temperature of 35 °C. The heating capacity falls at lower temperatures and with higher heating flows. So a nominal 5 kW heat pump might have an output of only 4 kW at =5 °C. So you need to assess the heating power *at the lowest temperature you plan for the heat pump to operate*. This requires looking at data sheets. Thought#1: It's expensive building for extremes. If you only have two or three days at - 15 °C, then it may be worth installing a heat pump that meets your requirements for (say) 360 days out of 365 - and just using additional electrical/gas/wood heaters on the cold days. Thought#2: If you are designing for these temperatures, then maximising radiator area, or using underfloor heating will help keep your flow temperature low and improve your COP - and this is a gift which makes all other design aspects easier and makes the system cheaper to operate. Good luck with your endeavours. M

[@misenko856]

​@@michaeldepodesta001 Hey, many thanks for quick response. I've tested my radiators with 45C water temperature (with electric heating) and it maintains 23C indoors when there is -5C outside so this is pretty good and fine for me (yes in my country we are used to such high indoor temps lol, not like in UK 20 and less :)). More over I want to buy Samsung HTQuiet which is suitable more than enough and it will run at 50 - 55 only in -15 or -20 degrees. I plan to run it at 45C most of the winter (is capable up to 70C of heating water, insane). Also this pump should be able to maintain full 8KW power up to -25C! without additional heating so this question is also out of scope. And since my heat loss is 5KW at -7C, then at -15 it could be around 6.5KW which still keep some reserves until 8KW. So even at -20C I should be able to maintain comfortable temperature inside. This is the reason why I think 8KW unit should be plenty enough. BTW, I've tested with my electric heating which has 99% effectivity and my daily consumption was 100KW with -5C outside. 8KW heat pump should be able to produce 192KW/day so for my house this should be good enough to cover the winter. If my calculations are correct of course, I'm not expert here :) And as you said it "Thought#1" i can still use my electric heating or my air-conditions units to help with heating if disaster happen and we get -40C like in Russia :D Now I only debating if to wait for new LG Therma V with new R290 coolant or not lol. Can't decide since this Samsung unit looks very good.

[@michaeldepodesta001]

@@misenko856 It sounds to me like you are already pretty well prepared. Regarding when to purchase a unit, if you act now it will not be installed until mid-winter at best (I guess?) and so it may be wise to just make observations of the 'before' situation this winter, and then install in the spring or summer when heating is not so critical. In any case: keep warm! All the best Michael

[@misenko856]

@@michaeldepodesta001 You right, however, the last winter I was heating with 2x5KW split wall AC units already so there is no hurry with installation of the heat pump. Savings were great but it's a bit annoying way of heating because of noise from fans you know and the temperature in rooms is different a bit :) So this will be more elegant solution and with better effectivity and comfort hopefully. Before I had pure electric heating and this is insanely expensive these days. Thanks for your hints.

[@billycan8852]

Very interesting stuff

[@andrewludwig9251]

I find it very ironic that British people use kilowatts to size their heaters and we Americans use British Thermal Units BTU's to size ours! You would think it would be the other way around! 11Kilowatts is essentially 36,000 BTU's. To make it even more complicated, we convert 12,000 BTU's to a ton, so 36,000 BTU's is equal to 3 tons of cooling capacity, (based off of using actual ice to cool buildings 100 years ago.)

[@michaeldepodesta001]

Andrew, Good Evening again. Yes, ironic and irritating. And I have never understood why the US is so fond of what we called 'Imperial Units'. - why didn't you throw off the units when you went independent? I used to work with people from NIST and they were perpetually embarrassed by this. But hey ho - that's the way of world! Best wishes: M

[@simonwalker6825]

This is really useful

[@michaeldepodesta001]

Simon, I am glad you feel that way! All the best: Michael

[@LeiChat]

Thanks for the great video. Is sizing a hot water tank a rule of thumb thing too, or is there a minimum size required to comply with regulations/certification? Currently have a 150L tank for our gas boiler and a couple of quotes we have had for ASHP install are recommending replacing it with a 210L tank. I feel like a 180L would suffice for our consumption.

[@michaeldepodesta001]

Lei Chat Good Afternoon, Yes. There are many rules of thumb for the size of domestic hot water cylinders. But here is my experience with a 200 litre cylinder. The system is set to re-heat the cylinder once a day to 55 °C - it typically puts in around 3 kWh of heat. My wife and I live modestly and shower (between us) around once a day and we never run out of hot water. When children are home taking multiple longer showers, we need to have two heating cycles per day. So my advice is that the cylinder size depends upon the size of your family and their habits. This guide shows that DHW consumption's very variable assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/48188/3147-measure-domestic-hot-water-consump.pdf This site ... www.hotwater.org.uk/sizing-a-hot-water-cylinder/ suggest a 200 litre tank based on DHW use of • Low Consumption = 20 - 30 litres/day • Average Consumption = 30 - 50 litres/day • High Consumption = 50 - 70 litres/day So like I said it's variable. But if that DHW is not all needed at the same time, then a smaller cylinder may suffice. Sorry to sound so vague. M

[@LeiChat]

@@michaeldepodesta001 No need to apologise. Really appreciate your insight. Thank you.

[@baronsilverbaron757]

So just checked my gas consumption for 2 years for heating and hot water and it comes to 42354 divided by 2 = 21177 divided by 2900 which means a 7.3 kWh heat pump. Cheers.

[@michaeldepodesta001]

If you live in the UK south of Manchester and keep you home at around 20 °C, yes, that's a good estimate for the size of heat pump required. A heat loss survey might come up with 8 or 9 kW, If it comes up with much more then your should ask the person doing teh estimate to explain the difference. Best wishes. M

[@baronsilverbaron757]

@@michaeldepodesta001 What size do I need if I want to keep my home at 22 C? I just use gas for hot water and underfloor heating so would I be needing a low heat system?

[@michaeldepodesta001]

@@baronsilverbaron757 kzbin.info/www/bejne/nnTImJuBlpifn68 Every °C above 20 °C adds about 12% to household consumption. SO 22 °C adds about 24% to consumption. Did the 21,177 kWh per year corresponded to your average household temperature being 22 °C? (a) If so, i.e. the temperature of your home would stay the same, that means the heat transfer coefficient of you home is about 24% lower than estimated. But you will need to warm it to 22 °C when the outside temperature is (say) - 3 °C so that's about 10% extra heating. So overall the estimate of heat pump size would be around 7.3 x 0.76 x1.1 which is about 6.1 kW. (b) If not, i.e. you want your home warmer than it was before, then teh heat pump would need to be larger around 7.3 x 1.24 x 1.1 which is about 10 kW. Best wishes Michael

[@baronsilverbaron757]

@@michaeldepodesta001 Thanks. That's very helpful.

[@goudhaantje3569]

Where I live the gas consumption is measured in m3. I have it measured per day with a so called smart meter. I will check how to convert m3 to kw.

[@michaeldepodesta001]

Good Morning. This is also true in the UK. If I remember correctly (please check) 1 cubic metre is about 11.4 kWh of energy. The number varies through the year due to small changes in the composition of the gas. In the article below I explain how to raise energy consciousness by converting meter readings to energy readings in units of kWh. protonsforbreakfast.wordpress.com/2022/11/14/energy-consciousness-raising/ Best wishes Michael

[@frankjohn1408]

With regard to the HPump size; if the hp has an inverter goes this not match the demand to the load and therefore if an oversize hp is used it should not be an issue?

[@michaeldepodesta001]

Dear Frank John. Yes, to a degree. But not wholly because the inverter can modulate only down to some fraction of the maximum power - on my 5 kW heat pump about 40% or 2 kW thermal output. In fact my peak thermal load is around 3.6 kW which happened for about 3 days this year. But thermal surveys can easily suggest installing a heat pump twice the correct size. For my installation I might have had an 8 kW heat pump installed which would modulate down to 3.2 kW thermal output - but then cycle on and off below that. So such an oversized heat pump would basically be switching on and off all the time. This may not be harmful, but I hear people say that it could shorten the compressor life - but I genuinely don't know. Best wishes Michael

[@Nikoo033]

Interesting video. However, word of caution, the calculation you show at the bottom of the screen at 06:49 is mathematically-speaking wrong, because of the units you display. Your result 6.3 would basically be 6.3 hours. There’s no need to have 3600s as denominator and numerator and the 1000 factor is just to convert W to kW so it is “unit-less”🙂. Your calculation should simply be: 262W used for 24hours: so 262W x 24hours = 6288Wh or ~6.3kWh

[@michaeldepodesta001]

Thanks for paying attention. It's a pleasure to 'meet' someone who pays attention to dimensional analysis. Such analyses were more relevant to my previous employment at the UK's National Physical Laboratory. If you are interested in units and measurement, you may enjoy this talk I gave in 2017 at the Royal Institution in London. kzbin.info/www/bejne/mpjYi395ndCFjJo Best wishes Michael

[@Hallochen12]

Hello, to begin with very nicely expained. Thankyou. I have a few questions : Will the HP sizing be same if we use ground source HP for a city in Germany ?

[@michaeldepodesta001]

Aditi Rawool, Good Morning, No, the sizing will be different because it depends on ethnic local climate. The figures I give apply to the UK south of Manchester. To work out the equivalent number for Germany, take a look at the video from 17 minutes and 39 seconds kzbin.info/www/bejne/nnTImJuBlpifn68 This explains how I arrived at the rule of thumb. You can then look up your local climate at www.degreedays.net and work out the equivalent figure for your locale in Germany. Best wishes Michael

[@oaklejant]

Amazing video, I just share it to some friends. Question: if the required Heat Pump is 5.2kW (as in your example) are they thermal? Therefore, it means that those 5.2kW need to be divided by COP 3, then the electrical power of the HP should be 1.7kW?

[@michaeldepodesta001]

Oaklejant, Good Morning. Yes, exactly that. The heating power required is ridiculously low. protonsforbreakfast.wordpress.com/2022/04/12/could-you-heat-your-house-with-a-hairdryer/ If the COP is around 3 when it is around 0 °C outside and around 4 when it is 10 °C outside then: For the 0 °C case, the thermal power will be 5.2 kW and the electrical power will be 5.2 kW/3 or about 1.7 kW. For the 10 °C case, the thermal power will be reduced to (say) 2.6 kW and the electrical power will be 2.6 kW/4 or about 0.65 kW. I hope that help: best wishes M

[@marklewisduncombe]

very interesting. Just had a heat pump survery done and it came back recomending a 16kW heat pump. our house is old and most likely not the most heat pump friendly. However, to heat the house with our current oil boiler we use on avergae 900 litres of oil a year, worst case is 1500 litres and that was with a poorly insulated loft. I wonder if we are an edge case which the heat pump survey doesnt cater well for. Just 2 of us in the house, and thermostat set to 17 during the day in winter and usually off all summer and overnight in winter. I also wonder if the survey calculation needs to take into account the criteria to meet the bouiler upgrade scheme grant. The rule of thumb is certainly making me think about if a heat pump is the way to go for us or if other zero carbon solutions can minimise the oil use instead.

[@michaeldepodesta001]

Mark, Good Morning, Thank you for stopping by. Obviously I don't know your house, but I can't see how 1,500 of litres - 15,000 kWh of delivered heat per year - can translate to a 16 kW heat pump. As you say, the live in your home may lead to some mismatch, and yes it is quite possible that the result of the MCS process (required for BUS subsidy) has resulted in an overestimate of the heat pump size required. In honesty, the rule-of-thumb is designed to do exactly what it is making you do i.e. think "Does this make sense?" It doesn't really deal with complex cases, but my guess your unusual day-time/seasonal temperature profile may not be well-captured by the MCS process. This whole issue is very lively on Twitter at the moment - follow eg @betateach, @TrystanLea, @kenbone44 @suburbanpirate and friends and you may learn something. They are very helpful people. My guess is a heat pump is the answer - it's hard to beat 300% efficiency! - But in any case: best wishes with adventures. Michael

[@marklewisduncombe]

@@michaeldepodesta001 thanks Michael, the rule of thumb has done its job, queried it with the surveyor waiting to hear back. I've actually met Trystan an absolute mine of information and experience.

[@davidreece1642]

@@marklewisduncombe Would the installer be allowing for the poor loft insulation. Better to spend money on loft insulation before installing ASHP.

[@marklewisduncombe]

@@davidreece1642 no, we were having loft insulation upgraded to 300mm the week of the survey. The guy doing the survey asked to see the insulation quote and did the numbers using what was on the quote.

[@davidreece1642]

Thanks for your reply. I'm getting the impression that ASHP as being over specified.

[@stevebannell2948]

Hi Michael, really informative thanks. Sorry if this has been addressed previously, but is there a simple calculation to convert annual oil consumption to kwh so that I can arrive at a heat pump size? I estimated the oil consumption for 1 year as 3,200 litres. (If that quite sounds high, it is a 5 bed detached house.)

[@michaeldepodesta001]

Good afternoon. Yes, the heating effect of 1 litre of oil is equivalent to roughly 10 kWh of gas consumption. The video covers that point here: kzbin.info/www/bejne/nnTImJuBlpifn68 So 3,200 litres is around 32,000 kWh - which is quite high but not crazy. The rule of thumb suggests an 11 kW heat pump, but that's just in the south of England and assumes the oil is used just for heating and hot water - not for example using an AGA. If you divide the 32,000 kWh/year by the floor area of your property in square metres you will get an estimate for the general quality of the insulation. For example, if the living area of your home is 200 m^2, then the figure of merit is 32,000/200 = 160 kWh/year/m^2 which is a typical English home. I would recommend trying to get that down if you can with draught proofing and insulation. The best possible retrofit is ~ 25 kWh/year/m^2, modern building standards are less than 50 kWh/year/m^2 and figures between 50 and 100 kWh/year/m^2 should probably be possible and will help the heat pump run very efficiently. Best wishes Michael

[@stevebannell2948]

@@michaeldepodesta001 Wow, that’s gone above and beyond! Thanks so much. Lots of food for thought. I’m more interested in the environmental impact than saving lots of money but I hope that installing a heat pump with battery and solar later will at least break even.

[@alansailing1387]

Thanks for this information and the formulas. Do these calculations work in Australia? We only have our heating on for a lower portion of our year, but it is also a warmer winter. When I do the first calculation, or the graphs, I get a 6.5kW heating required. When I use the heating degree days formula, I get a 8.5kW answer. (using HDD of 1600 / 18C, gas consumption of 19,075 kWh pa, Melbourne Australia)

[@michaeldepodesta001]

@alansailing1387 Good afternoon, The method might possibly work. If your gas usage is mainly for winter heating then it should give an indicative value of heat pump size, but if you are using a lot of your gas on something else (a hot tub?) or if you use more than 5% of your gas on hot water heating, then the formula may be inaccurate. I downloaded the monthly HDDs for Melbourne Airport in Victoria. Taking them in 12 month groups, it seems the HDDs are around 1200 °C-days per year. The base temperature of 16.5 °C corresponds to keeping a home at roughly 20 °C. Typically, people don't put the heating on until the outside temperature falls about 3.5 °C below their desired internal temperature. Using the formula at 13m07s in the video the HTC for your dwelling appears to be 593 W/°C. i.e. it takes 593 watts to warm your home by 1 °C. In UK terms this is a very large dwelling or a very leaky dwelling. In terms of heat pump size, If the minimum temperature in Melbourne falls to around 0 °C then you would want a heat pump with a power of 20 x 593 watts which is about 12 kW. Does any of that make sense? Best wishes Michael Description: Celsius-based heating degree days with a base temperature of 16.5 C Source: www.degreedays.net Accuracy: Estimates were made to account for missing data: the "% Estimated" column shows how much each figure was affected (0% is best, 100% is worst) Station: Melbourne Airport Comparison, VIC, AU (144.91E,37.73S) Station ID: 95866 Month starting HDD 16.5 °C days/year 2021-07-01 206.5 1138.9 2021-08-01 163.3 1163.6 2021-09-01 130.8 1171.9 2021-10-01 110.7 1187.9 2021-11-01 79.6 1173.8 2021-12-01 36.7 1180.9 2022-01-01 1.7 1194.3 2022-02-01 10.9 1202.0 2022-03-01 15.2 1208.3 2022-04-01 49.8 1215.9 2022-05-01 140.1 1237.2 2022-06-01 193.6 1245.4 2022-07-01 231.2 1216.6 2022-08-01 171.6 1156.9 2022-09-01 146.8 1148.8 2022-10-01 96.6 1106.2 2022-11-01 86.7 1115.0 2022-12-01 50.1 1070.5 2023-01-01 9.4 1039.8 2023-02-01 17.2 1038.4 2023-03-01 22.8 1032.7 2023-04-01 71.1 1033.8 2023-05-01 148.3 1042.4 2023-06-01 164.8 1027.4 2023-07-01 171.5 1063.7

[@davidreece1642]

Hi Michael, I've had the following supplied by an ASHP installer: "Indoor - Outdoor delta T (23.85 ºC) Peak heat loss at design temperature (11.85 kW) Space heating energy requirement (29678 kWh/pa) Space heating electrical energy consumption (7789 kW/pa) Based upon the information that has been inputted into the calculation tool, the heat loss of the building has been calculated to be 11.88kW @ -2.85°C." My comment back to the installer is: "Four of our bedrooms are not used and neither is a bathroom so the temps in those would be perhaps 15C not 20C. Realistically, the heating season will be 1st Oct to 30th April or about 180 days, with the coldest months being Dec/Jan/Feb in which exterior temps may get down to -2.85C for perhaps 30 days in total. So a 17kW ASHP for those 180 days (17 * 24 hrs* 180 days)would provide 73440 kWh but a 8kW ASHP would provide 34560 kWh, which is still almost 20% above your 29678 annual heat loss figure." My 8kW power ASHP example is based on your rule of thumb calcs. I wondered what your views of the installers figures are or have I got something wrong? Edit: if you do answer I will not quote anything you say in your answer.

[@dpjdpj]

Thank you for the very helpful presentation, Michael. I just did a quick check against a recent quote. The expected heat demand was 9.2kW and they are suggesting a 12kW ASHP as a result. Do you think something like a 10kW pump would suffice instead?

[@michaeldepodesta001]

@dpjdpj Good Afternoon. A couple of points. First, regarding the names of the heat pump e.g. "a 12 kW heat pump". This typically means that the heat output will be 12 kW under a specified condition, often when the outside temperature is 7 °C and the water is heated to 35 °C. This is call 7/35 specification. Typically the heat pump output will fall as the outside temperature gets lower and the temperature of the water in teh radiators increases. If you are in the UK then the heat pump design is likely based on an outside temperature of -3 °C and a water temperature that might be 45 °C. It could be that the heat pump with a label "12 kW" might have an output of 10 kW under these conditions. Ask the person who quoted you, or look up the specification for the heat pump they recommended. Or let me know the recommended heat pump and I will see if I can find out. Secondly. What type of house is it? How many people live there? And what was the gas consumption last year? Best wishes Michael

[@dpjdpj]

@@michaeldepodesta001 yes we are UK based and it’s been designed for 45°C flow rate at -2°C. It’s a 300sqm new build so don’t have any historical data to fall back on. They estimated energy required for space heating and hot water to be 24,500 kWh. They specified a 12kW Vaillant aroTHERM ASHP, 400l Joule DHW Cylinder and a 100l OSO Buffer tank. I did try to look up the ASHP but couldn’t find the output figures you mention. I’ve also seen from other videos that buffer tanks are rarely required.. I’m very new to this area so my knowledge is limited but have seen plenty of videos where consumers have been specified the wrong set up (usually overspecified), hence my questions. thanks again!

[@michaeldepodesta001]

OK so it’s large house and a new build. Here are some typical heating requirements for SPACE HEATING for different types of building. 1. A Passivhaus has a heating requirement of less than 15 kWh/year/square metre. So for 300 m^2 that’s just 4,500 kWh/year. 2. A top-notch Enerphit refurbishment has a heating requirement of less than 25 kWh/year/square metre. So for 300 m^2 that’s 7,500 kWh/year. 3. My refurbished house (with EWI but without underfloor insulation) has a heating requirement of ~ 50 kWh/year/square metre. So for 300 m^2 that’s just 15,000 kWh/year. This is also a good modern ‘normal’ building standard. 4. Most UK homes have heating requirements in the range of ~ 100 - 150 kWh/year/square metre. So for 300 m^2 that’s just 30,000 - 45,000 kWh/year. So for a new build, an expected heating demand (with DHW) of 24,500 kWh/year looks (IMO) to be on the high side. 24,500 kWh/year of heating would correspond to ~28,800 kWh/year of gas consumption which (using my rule of thumb) would correspond to about 10 kW of peak heat pump demand. The basic Vaillant Specifications can be found here: professional.vaillant.co.uk/downloads/aproducts/renewables-1/arotherm-plus/arotherm-plus-spec-sheet-1892564.pdf So if the heat calculation is correct, then the heat pump size is about right - 20% oversize is fine. But if - as I suspect - the heat calculation is an overestimate, then the heat pump might be on the large side. From my perspective at the end of a KZbin comment, that’s just about all I can say with the data I have. Except to wish you every happiness in your new home. Michael

[@dpjdpj]

@@michaeldepodesta001 many thanks again, very helpful. I look forward to more videos!

[@MrSchnomen]

Thank you

[@farhada147]

No Michael, thank you very much 🙂

[@gregvet2008]

Very interesting. I have an electric boiler which should translate to 100% efficiency theoretically. I also record daily heating kWh usage separate from house kWh use and when I turn the boiler on/off during the heating season. Likewise, I have a weather station and can obtain heating degree days (in both c and F although I have not found a calculation with F.) for the period of time that I keep the boiler turned on/off. If I use the heating degree days vs the rule of thumb, I get a 4.6 kW heat pump for the former but 5.7 kW for the latter. Which is correct? and does your calculation assume the heat pump is turned on 24/7 during the heating period at full power?

[@michaeldepodesta001]

Gregory, Good Afternoon, Firstly, regarding degree days, you can generate them yourself from your own data, or use data from degree days.net www.degreedays.net/ . Also to convert from °C-days to °F days just multiply by 1.8 If I have read your comment correctly, you find that * Heating degree day calculation suggests a 4.6 kW maximum heating demand. * Rule of thumb suggests a 5.7 kW maximum heating demand. I don't know why these differ but there are several possible reasons. As you suggest, the rule of thumb assumes that the whole dwelling is at the thermostat temperature 24/7. If your house is not heated for a substantial fraction of the time then the gas usage may be lower than it would be for heating 24/7 and this will lower the recommended size of heat pump. The role of the 'rule of thumb' is to get something in etc correct ballpark. So if your installer on the basis of a survey suggests 8 kw or 10 kW, you will be in a position to ask how that estimate can make sense. In any case: good luck with your installation. M

[@rosemarymetz617]

Thanks for the video. My post relates to the values you’ve used in your calculations for a heat pump. I've realised that in working out the Heat Transfer Coeffiecient (W/° C) with your formula you use kWh for annual gas usage whereas my gas bill (in Australia) presents data in MJ rather than kWh. As I’m innumerate I’d be very grateful if you could let me know if substituting annual gas as MJ (37069) would work? My previous 12 month kWh consumption from my electricity bill is 1499kWh which yields a very small .5kW heat pump. Can’t be correct! As mentioned, I can’t do maths to save myself but I’m really keen to understand what size heat pump I’ll need to get off gas. Currently, the output load from my 13 radiators is calculated to be 20kW but don’t know if this figure is relevant. Hope you can steer me in the right direction for the rule of thumb calculation. Thanks!

[@michaeldepodesta001]

Rosemary Good Afternoon. First of all you are in Australia! Take care - the formula were devised for typical temperatures in the southern half of the UK. So the heating demand where you are will almost certainly be different from the UK. Regarding conversions, take your gas consumption in MWh and multiply by 0.278 to give the gas consumption in kWh. e.g. 37,069 MJ = 10,300 kWh As you comment the electrical consumption - which does heat your house slightly - is not the main source of heating so teh heat pump size estimate is incorrect. Gas boilers are typically rated at between 20 kW and 30 kW but this massively exceeds the heating power required in most houses. To reduce output, the boilers switch on and off, heating water in the radiators to (say) 75°C for 10 minutes - outputting maybe 20 kW - but then switch off for the next 20 minutes. Overall this gives an output power of around 20/3 = 6.7 kW Heat pumps instead produce heat at a steady rate - that increases as the outside gets colder. This keeps the house at a steady temperature throughout the heating season. Because I don't know about your climate, I would suggest you contact a local professional for advice. Best wishes Michael

[@rosemarymetz617]

@@michaeldepodesta001 Thanks for your detailed response Michael. Your video really helped clarifying lots of questions while also raising others as there’s a dearth of technical information available here (outer Melbourne). While mindful that our temperature zone is coastal, south eastern Australia, with similar although milder conditions than Tasmania were not comparable to southern England, I was hoping the science was still applicable. Yes, my gas boiler output is 31kW although I’m running the water temp’s to radiators at 60°C which is comfortable given the level of insulation. So far I’ve had several quotes where one company has suggested 2 x 10.75kW heat pumps and another a 16kW HP plus buffer tank. Confusing. Given the information in your video, albeit without direct application to my location, I feel more confident to go with a moderate rather than an oversized/supersized HP. Thank you!

[@michaeldepodesta001]

@@rosemarymetz617 Both those heat pump installations are likely at least twice what you need. I looked up the Heating Degree Days for Melbourne Airport (1184, 1132, 1264 °C-days per year for the last 3 years). So for the last year you used 10,300 kWh of gas to heat against 1264 °C-days of demand - assuming 20 °C indoor temperature. This gives a rough estimate of the HTC of about 305 W/°C. It looks like the coldest it typically gets in Melbourne is around 0 °C so that's 20 °C of demand so you need around 305 x 20 = 6,111 W of heating at most i.e. a heat pump which outputs 6 kW is enough. Note 1: The heat transfer coefficient is not very good! My guess is some insulation - particularly loft insulation would help - and would also help with keeping cooling in summer. Note 2: You need to check that your radiators can deliver 6 kW of heat to the house with a 50 °C flow temperature. I can't quite think how anyone could come up with such large sizes of heat pump except that maybe they know how to install them and feel comfortable that they will be able to provide enough heat. Best wishes Michael

[@rosemarymetz617]

@@michaeldepodesta001 Many, many thanks Michael. Aside from the info on heat pumps it seems I have to beef up the insulation. On all counts, great information. Thank you.

[@dr.gregkettering5835]

Regarding Heat Pump size: Many manufacturers state various kW heat outputs depending on the ambient temps, (7C, -7C, -20C) and have different values (8.2 kW to 5.5 kw in this instance.) for that particular ASHP. Once I calculate the HP size (kW) with your recommendations, Should I pick the kW size based on the coldest (-20c) rating?

[@michaeldepodesta001]

Dr. Greg, Good Evening. First of all well done for spotting that the heat pump thermal output power changes with external temperature, and also for noting that the 'labelled power' e.g. 5 kW, 7 kW etc is only the labelled power under one specified condition - generally *not* the coldest temperature. The industry recommendation - which I agree with - is to pick a so-called 'design temperature'. This is the coldest likely temperature in your locale. In southern England this is generally around -3 °C. You should choose a heat pump which can deliver the required heating load *at this temperature*. One could design for - 20 °C, but this would be overpowered for 99.99% of the time and would underperform for all this time. If you design for the recommended design temperature in your locale, the heat pump size will be 'just right' for most of the year. If one experiences a -20 °C day, the heat pump will still work, but you will need to supplement it with normal electrical heating for the one day or so per two or three decades on which that cold-weather event occurs. Does that make sense? All the best Michael

[@gregvet2008]

@@michaeldepodesta001 The ASHRAE design temp of my region is -14F(-25.6C). This temp is for Leadville which is typically colder than Buena Vista (my town). The coldest recorded by my station was -29.5C but this temp is uniquely low. The average low temps here is -5C to -10C

[@michaeldepodesta001]

@@gregvet2008 First of all - WOW! - I know it's the internet and I should be used to it at my age - but how wonderful to communicate with someone in Colorado! Secondly. I don't know your climate so I looked up a simplified version of your local climate data... www.timeanddate.com/weather/@7174247/climate According to that, a temperature of -25.6 °C seems extreme as a design temperature, but obviously it still gets pretty cold in your winter. As I understand it, the design temperature should be a temperature which will *likely* be the coldest temperature of the winter. This is as opposed to the coldest possible temperature. I can't make those choices for you: if you have access to local weather records then you can likely make a rational choice based on those records. It's always comforting to design for the most extreme weather, but you pay a price in less efficient operation through the rest of teh year. Depending on your circumstances, it might be an idea to use two heat pumps, with the second heat pump kicking in only in extreme weather. Good luck - and keep warm. M

[@markegg262]

@@michaeldepodesta001indeed, one can add many kW of on demand capacity for a few £ in the form of convectors or oil-filled. Many lose sight that total costs are capital+running and let the 2nd term run away from them.

[@mapryan]

I think is more likely sometime will recommend a heat pump that’s too small, as anything larger than 7kWH requires planning permission

[@michaeldepodesta001]

Mike Good Afternoon. I don't know where you are based, but that is not my understanding. In the borough of Richmond where I live, air source heat pumps are considered permitted development except in conservation areas. And even if it were true, the fact on teh ground reported by installers and surveyors is that no-one has ever installed an under-sized heat pump! And 10% or 20% oversize is fine. But it is important to avoid 300% oversize - which has been proposed in my experience. Best wishes. Michael;

[@rogertrant2101]

Hello Michael, I have been looking for an alternative method to determine heat loss and came across your video. We use our central heating usually from October through until about end of March ( 6 months ) each year, the house is insulated reasonably well and have an annual gas usage of 4200 kwh, using your rule of thumb it would be 4200/57.3 * 20 = 1466 which seems rather low. would i have to find the degree heating days just for the six months the heating is on. Also i wandered if there might be a way to heat up a room to a certain temperature and then time how long it takes to drop a certain level to calculate the heat loss.

[@davidreece1642]

I use 20000kWh gas , my calculation for HP power is 8.4kW, which is about 5 times yours (in terms of gas usage and HP power) so your figures are correct using Michael's methodology.

[@michaeldepodesta001]

Roger, Good Evening. Heating for 6 months is pretty normal, but an annual gas usage of 4,200 kW/year is very low. Is your dwelling particularly small? For reference my extended 3-bed semi (floor area 162 m^2) used 15,000 kWh of gas a year in 2018, and around 7,500 kWh/year after triple glazing and external wall insulation. In the winter of 2020/21 peak heating demand was 50 kWh/day which corresponds to an average heating power of just over 2 kW average power. Currently we heat the house with a 5 kW heat pump, but it is becoming clear that it is probably about two times too large! Yes there are systems and services out there that will do the heating experiment for you. Basically they put a thermostatted heater in each room and measure the power dissipated to maintain the room - and by adding the numbers up the whole dwelling - a certain number of degrees above the outside. Obviously this needs to be done in winter. I'm sorry that I can't remember the name of the service. Good luck with your adventures! M

[@davidreece1642]

@@michaeldepodesta001 "Currently we heat the house with a 5 kW heat pump, but it is becoming clear that it is probably about two times too large!" So there is a significant element of over-specing even using your own experimental data. Interesting. Looks like there could be huge over-specing by installation companies leading to ASHP getting bad publicity "The Ugly Truth" . Do you find that your 5kW HP is cycling ie switching itself on and off throughout the day, rather than running at lower power continuously? If it is cycling could this be used as an indicator that your ASHP is over-speced? (Just a thought as an interested householder not a heating engineer.)

[@michaeldepodesta001]

@@davidreece1642 Yes. The system cycles on and off when the outside temperature is above roughly 5 °C (if I recall correctly). You can see that in the data on this blog article: protonsforbreakfast.wordpress.com/2021/11/15/a-weekend-away/ Someone on Twitter asked installers if they had *ever* come across an undersized heat pump: none of them had. As you say, it seems the entire industry is biased towards systems that are - in engineering terms - over specified. The cycling has no down side to me but I am told it may be bad for the pumps longevity.

[@rogertrant2101]

@@michaeldepodesta001 Hello Michael, its not that small its a 3 bedroom 130m^2 bungalow built in the 80's, its timber framed with about 25mm pir insulation in the walls and block outer walls, floors are concrete base with chipboard floating on 25mm of polystyrene and attic has 250mm fiberglass insulation, the extension i built in 1998 is exactly the same type of structure but with 75mm pir board and outside walls are rendered 4" Thermalite blocks, and all windows and doors are upvc double glazed. I had a new gas boiler fitted 5 years ago and it has still got 2 years warranty left yet so wont be looking for anything different for a while, but this winter will be experimenting turning down the flow rate temperature to see if that makes any difference in gas usage. I wouldn't employ anyone to do a heat experiment i would either try to find out the formula's to do it my self or more than likely just use your rule of thumb, i am a bit worried about the amount of electricity a heat pump uses 50 kwh is more than i use in a week now, of coarse if the price of gas ends up the same as electricity or i can get on a time of day tariff with battery storage then that would make a big difference.

[@quilmore]

Very interesting video, I have something that in my head doesn't make sense, when @12:45 you shown a function for HTC that uses the inverse of heating degree days for the calculation, that will give a smaller heat pump size the LARGEST the number of HDD you get, that calculation would give a HP's size of less than 4kw for Aberdeen and 5.8kw for Newquay, that doesn't make sense to me, please explain where I've got it wrong

[@michaeldepodesta001]

quilmore, good afternoon. I think it is correct, but the reasoning is subtle. Suppose identical dwellings in Aberdeen and Newquay had identical gas consumption. Because the heating demand in Newquay is lower than in Aberdeen one would conclude that the Newquay HTC was much higher i.e. the Newquay dwelling would be much 'leakier' than the Aberdeen dwelling. Heat pump size is determined by HTC and the maximum demand. In this video I didn't take much account of how the maximum demand varies across the UK. o assuming the maximum demand was the same in both places, the increased HTC in the Newquay dwelling would need to be larger. Days below 0 °C are rarer in Newquay than in Aberdeen, but they do occur. Does that make any sense? Best wishes. Michael

[@quilmore]

@@michaeldepodesta001 I would have guessed a like for like dwelling in Aberdeen would need a larger size heat pump than one in Torquay to keep the same level of warmth inside, I'll have to inform myself more, it's fascinating, greetings from Dublin

[@michaeldepodesta001]

@@quilmore Your intuition is correct. But your intuition would also tell you that the dwelling Aberdeen would have a bigger gas consumption. If the gas consumption is the same in the two dwellings, this implies the dwelling in Aberdeen has a lower HTC. Does it make sense yet? M

[@ManChickin]

I gather heat pumps work well with under floor heating, would it be more efficent just to leave the underfloor heating on a lower temperature and let the heat pump tick over? rather then switching it on and off again?

[@michaeldepodesta001]

Dear @ManChicken In short yes, it would be more efficient. Two points. 1. Heat pumps already do this: it's called weather compensation. When it's -5 °C outside my heat pump circulates water at 45 °C. When it's 10 °C outside the water is only heated to 30 °C. 2. The problem comes because of a technical issue with the compressors that drive the working fluid around the circuit internal to the heat pump. In the 'olden days' the compressors could operate at 100% or 0%. Then electronic motor controllers became a thing and the compressor motor could be "modulated" to run anywhere between 100% and 40% (this lower level varies from manufacturer to manufacturer). So this copes well in cold weather, but in slightly warmer temperatures i.e. with the lowest levels of heating demand - which corresponds to roughly half the year - when demand is less than 40% of maximum, the heat pump has to still cycle on or off. Does that make sense? This is why it's important to get the heat pump size about right. If you install a 10 kW heat pump in a house then it will cycle on and off when it delivers less than 4 kW. If the house has a maximum heating requirement of 5 kW then the for most of the year the heat pump will be cycling on and off. Best wishes Michael

[@umarrehman7514]

hi @michael many thanks for your video. I can see that you have included the boiler efficiency in the calculation, while you have not included the Heat Pump efficiency in sizing. Heat Pump efficiency (COP) should be between 3.5 and 2. If we consider SPF of even 2, we are almost sizing the system double the size. Please suggest on this. How to include heat pump COP in the calculation, if we don't include that, why so? Thanks

[@michaeldepodesta001]

Umar, Good Evening. When people specify a heat pump size, for example 5 kW, this describes the maximum THERMAL power of the heat pump: the maximum amount of HEAT it can move into your house per second. All these calculations are concerned with heat. When using a heat pump to heat a house, the ELECTRICAL power is much less than the THERMAL power. As you say, for a 5 kW heat pump the maximum ELECTRICAL power will probably be only 2 kW. So once you have an estimate of the maximum THERMAL power - using the rule of thumb or by having a survey - you can then work out the maximum ELECTRICAL power by dividing by the COP. Does that make sense? Best wishes Michael

[@umarrehman7514]

@@michaeldepodesta001 thank you.

[@umarrehman7514]

@@michaeldepodesta001 the two formulas 1 HP power = Annual Gas Consumption/2900 and Heat Transfer Coefficient = Annual Gas Usage/ 57.3, are they effective for all building types and building sizes?

[@michaeldepodesta001]

@@umarrehman7514 Umar, yes. These rules apply to any dwelling in the UK south of Manchester. But they are just 'rules of thumb'. The idea is that after you have had a survey, then if the results are very different from the rule of thumb, ask your surveyor why that might be. Things that can lead to odd results are if the dwelling has big extremes of temperature change during the day, or just heating one part of the home. This article by Nicola Terry discusses several shortcomings of the Rule of The Thumb energy-surprises.blogspot.com/2022/06/how-big-should-your-heat-pump-be.html Best wishes Michael

[@eddiephillips6740]

I live on my own how can I work out you usage if I was out all day working now I'm retired and home all day so I have no record of normal usage

[@michaeldepodesta001]

Eddie, good evening. If you heat your house with gas I would recommend that you try reading your gas meter before and after a very cold day. If you're in the UK, the very cold weather has just passed, but we might have another spell. On the very cold day: 1. Read your meter at midday 2. Keep the house at the temperature you would like it to be - normally around 20 °C all night 3. Read your meter at midday the next day. If you need help reading the meter drop me a line. Assuming your boiler is 85% efficient - that's normal - and you have used (say) 150 kWh to keep your home warm on a cold day then you probably need a heat pump with a power of 150 x 85%/24 = 5.3 kW. Does that help? All the best Michael

[@pumpkinhead456]

Last year I used 43000 kws of gas, in the same period there were 2080 HDDs, plus 5% for elevation gives me a heat pump size of 16.3kw. My very thorough heat loss calc, where the only unknown was air changes (best guess stuff) suggests a heat loss of 15.5kw. That's pretty close in my book. I've read all your articles, the only bit I didn't get is if 3.5 below thermostat should be a variable or just read it as a constant? Great work!

[@michaeldepodesta001]

Dear Pumpkinhead456, thanks for your comment. Indeed, that level of agreement is pleasing. Two points: Firstly, that's a very high gas consumption so you might like to consider whether insulation of any kind was relevant to your home. Secondly, the 3.5 °C is a guesstimate for the heating caused by electrical consumption, and people in the house. It's basically a measure of how much colder it gets outside before one puts the heat on. For very well insulated houses, the 3.5 °C can get larger - the heating sources inside the house are very effective because teh heat losses are low. For poorly insulated houses it might be a bit less, but 3.5 °C seems to be about right. Best wishes Michael

[@markegg262]

@@michaeldepodesta001I don’t think absolute usage is, er useful but specific. I use 28MWh on 350m2, the first is absolutely large but quite reasonable on specific.

[@michaeldepodesta001]

@@markegg262 Good Evening. Context is everything. In the context of domestic energy consumption, 43 MWh is large - it's about three times the UK average. Obviously a larger property will have larger heating requirements, but it is nonetheless large and will require a non-standard heating installation. Dividing by the floor area of teh lived-in areas gives a rough measure of fabric efficiency. In your case 28 MWh/350 m^2 gives about 80 kWh/m^2/year which is not bad in UK terms. For comparison, a Passivhaus is less than 15 kWh/m^2/year and modern building standards specify around 50 kWh/m^2/year. Best wishes Michael

[@jack504]

I'm curious about adding capacity based on location/ elevation etc. Surely local gas consumption/ degree days would be increased anyway and so would be accounted for in the initial figure without adding capacity? Say an equivalent house in Scotland uses 20% more gas than the same in southern England, if you have the gas use figures you would still correctly size the heat pump without adding design capacity? Or is this to account for a drop in heat pump efficiency?

[@michaeldepodesta001]

Jack, Hi. Good Question. The key to getting the right heat pump size is to know the Heat Transfer Coefficient (HTC). One then works out the heat pump size by multiplying the HTC by the maximum likely demand. So for two dwellings with the same HTC, then just as you say [local gas consumption/ degree days] should be the same. We would expect a house in Scotland to use (say) 20% more gas than a dwelling in (say) Manchester because the number of degree days is ~ 20% greater. Once one has the HTC Estimate one may need to adjust the likely design temperature depending on location in the UK. I didn't explicitly discuss this in the video because although different parts of the UK have different numbers of degree days, the design temperatures don't vary that much (IIRC). So I have just used 20 °C as the likely maximum temperature demand - remember: it's just a rule of thumb. The video doesn't consider heat pump efficiency at all - it just considers heat flow. The heat pump efficiency will drop on colder days, on my system (internal temperature is 21°C) it is just below 3 when the external temperature is -3°C and around 4 when the external temperature is +10 °C. Best wishes Michael

[@kevinleesmith]

If its annual kwh/had then my 20,000 woukd mean in Norwich (2235) it would require a 8.9kw heat pump, but the same 20,000 if I were in Edinburgh would be 20,000/2687 = 7.4kw! So the more North and colder I go, the less powerful heatpump I need?? Is that because the colderoutside it is, the more efficient the heatpump is?

[@michaeldepodesta001]

Kevin, Hi. Imagine identical two houses, both with 20,000 kWh of annual gas usage. One is in mild Norwich, and the other is in cold Edinburgh. Clearly the house in Edinburgh is better insulated. How do I conclude that? Well it's colder in Edinburgh than Norwich, but the gas usage hasn't increased. That means the HTC for the house in Edinburgh is lower than the HTC for the house in Norwich. So for 20 °C of demand, the house in Edinburgh will need a smaller heat pump because its HTC is lower. Does that make sense? Best wishes Michael

[@TeaBreak.]

Can you confirm 1227 units of gas used annually. Times 11.3 (my rough conversion estimate) equates to 13865kWh gas used annually. So heat pump size required is just a 4.78kw? I have been looking at 10kw heat pumps so a 5kw heat pump sounds more affordable. This is totally ignoring the pipes being the wrong size, so temp would need to be set high at the exoense of COP. So potentially you could run thr heating and hot water soley from solar pv? Something more difficult to do with a 10kw heat pump.

[@michaeldepodesta001]

Oh Yes, one more thing. You can check the sizing quite precisely on the coldest day of the year whenever it arrives. protonsforbreakfast.wordpress.com/2022/11/07/what-to-do-on-the-coldest-day-of-the-year/ All the best Michael

[@fredbloggs8816]

Hi Michael, thanks for this very informative video - I have a query arising from an unsettling comparison of your rule of thumb sizing compared with my longform calculation of overall heat loss by u values. Should the answer of your rule of thumb be divided by the anticipated SCOP or is this for the birds?

[@michaeldepodesta001]

@fredblogs8816 good evening. No, the rule of thumb - dividing annual gas usage in kWh by 2,900 - gives an estimate for the required maximum heating power of the heat pump to provide equivalent heating to a gas boiler. I can't quite make out what you mean by your comment "is it for the birds? " In general the 'rule of thumb' will give a lower estimated heat pump size than a heat loss calculation, but that's because it's reality based! Heat loss calculations frequently overestimate heat pump size by a factor 2 to 3. The rule of thumb can also give funny results if - for example - you currently only heat your home in short bursts so that your gas use isn't representative of the heating required to keep your home at a steady temperature - as a heat pump will. Do let me know your calculation results if you would like to discuss this further. Best wishes Michael

[@fredbloggs8816]

@@michaeldepodesta001 many thanks for your reply Michael, I would appreciate any comments you have. Our house is an ongoing project to insulate and reduce heat requirement of a solid brick structure, 194M2 internal floor area, on 2 floors. According to my U value calculations, the total heat loss/heat requirement is 19.5kW which (if I have followed Heat Geek's videos correctly) I assume that total can be reduced by the SCOP which I'm including at 3 which gives me a heat pump size of say 7kW (less if a greater SCOP is assumed). (Current radiator output capacity exceeds the 19.5kW heat requirement by a reasonable amount; total heat requirement includes 7kw for HWS; DT range assumed as -3 to +21degrees C.) For the rule of thumb comparison, our annual oil consumption is 3000 litres, electricity is 3000 kWhrs for 194M2 internal floor area which suggests 10--11kW - a result 50% the opposite way to what you suggest is normally the case. Or, is this simply showing the true inefficiency of our current boiler and heating/hw provision? I hope this is clearer than "for the birds" and makes sense. Really appreciate the way you engage with all comments - it's very much appreciated.

[@michaeldepodesta001]

@@fredbloggs8816 Good Evening. Sorry for the delay in replying: it's been a busy day. I have two significant comments. You state that according to your U-value the total heat loss/heat requirement is 19.5kW. This is an estimate of the heating power required on the coldest day. If that truly is your heat loss then to meet it you need a 19.5 kW heat pump. The 'name plate' labels of heat pump label the THERMAL output. The electrical power required is found by dividing the 19.5 kW THERMAL power output by the COP - nominally 3 - to get 6.5 kW ELECTRICAL power required. Is 19.5 kW really your maximum heating requirement? At the moment you use 3,000 litres of heating oil a year. By chance, one litre of heating oil is NOMINALLY equivalent to 10 kWh of gas, so that's equivalent to about 30,000 kWh a year of gas consumption. The rule of thumb would suggest that the required heat pump size is 30,000/2,900 = 10.4 kW. For your building area that seems more likely. In fact feedback to this talk has suggested that oil boilers are considerably less efficient that gas boilers - closer to 75%. So the amount of heating entering your home is likely to be less than 30,000 kWh. perhaps as little as 23,000 kWh/year. It is my experience that heat loss calculations frequently overestimate the required heat pump size. This is not the place to discuss why that is. But if you just heat your home very extremely for short periods then teh rule of thumb can be wrong, but nearly 20 kW seems just VERY big for a property of that size. You use 3,000 kWh of electricity a year which is modest (~8 kWh/day): are just one or two people living there? Kids generally use quite a bit of electricity. Does that make sense to you? Best wishes: M

[@fredbloggs8816]

@@michaeldepodesta001 Evening Michael. Many thanks for your considered response, it's much appreciated and yes it all makes sense. ' Re your query on u value calculation, thank you for correcting my misunderstanding. I mis-understood until now that total heat requirement was to be divided by the COP. Phew! There's another factor which may be relevant for the comparison of rule of thumb and u value calc results which prompted my original query. I included 8.5kW for HWS within the u value 19.5 kW total - I was uncertain as to whether that is required for heat pump sizing, if that's wrong that would bring the two methods closer? Again, many thanks for all your help, no doubt you can tell I'm not naturally scientifically minded but I think we all need to learn about this subject as it's not something many boiler engineers are currently interested in diversifying into. And yes, you're right there are just two of us now and the annual elec consumption is accurate.

[@michaeldepodesta001]

@@fredbloggs8816 Good Evening again. HWS? Is that Hot Water Storage? If so that could explain the very high heat demand you calculated. The way a heat pump does space heating and hot water heating is as follows. It spends most of its time heating your house using radiators or under-floor heating. To do this it heats hot water up to (maybe) 30 °C if it's not very cold outside and (maybe) 45 °C if it's very cold outside. And then it periodically stops heating your house and instead - for may be an hour or so - stops heating your house completely and devotes itself to heating water in your hot water cylinder to (maybe) 50 °C. If you have a 10 kW heat pump heating your home then when it switches to heating hot water it will devote 10 kW to heating hot water. It takes around 7 kWh of energy to heat up a 200 litre storage tank so with a 10 kW pump this would take less than 1 hour. Is that what you meant by HWS? All the best Michael

[@andrewludwig9251]

Are British air source heat pumps for radiators also able to cool the water using the refrigeration cycle during hot weather or is it just for heating?

[@michaeldepodesta001]

Andrew, Good Evening, Firstly, few of the heat pumps are British. Vaillant is German, Grant is British, but Mitsubishi, Sanyo and Samsung... And yes, many of them can cool as well as heat, but it's not that effective. Typically they can't cool below 10 °C because otherwise moisture will condense on teh radiators and pipes. It is possible to cool with fan-coil units similar to the air conditioning fan-coil units, but these are expensive and not as good as AC at cooling. For this reason my wife and I chose to get an addition small AC unit for summer cooling and as a winter backup for heating in case of failure of the heat pump. All the best Michael

[@adzy166]

We’re still in the Iron Age here and use an automatic coal fed boiler. How could I apply rule of thumb for this use case? We’re currently going down the solar PV plus ASHP route

[@michaeldepodesta001]

@adzy166, Good Evening. Sadly I don't think I have enough information to be able to help you. Do you know how much coal you use in a typical year? If I knew that figure and your location I could probably make a guesstimate. "We’re currently going down the solar PV plus ASHP route". Can I suggest that you consider purchasing a battery as well? In summer the solar PV can charge the battery which can run your home overnight. In winter you can buy cheap rate electricity and then run the heat pump from that for some fraction of a day. The savings can be very large. Best wishes Michael

[@rymoe6299]

❤ 👊🏻👍🏻🍻

[@darrenadams2640]

Hmm, not sure about this. My annual gas consumption is 11,460 which gives just under 4kW HP. But I currently have a 30 years old non-condensing, poorly running (for at least a couple of years!) gas boiler in a detached 4 bed with 4 adults and only reasonable insulation (loft could be better). Should I not be expecting more like 6kW?

[@michaeldepodesta001]

Good Morning. A gas consumption of 11,460 kWh is below average for the UK, but your detached 4-bed house is probably larger than average in the UK. And yes, 6 kW peak heating requirement would sound like a good first guess. This is just a rule of thumb, and it can give misleading indications for a couple of reasons. If the house is unheated for long periods of the day, e.g. when everyone is out at work, and the temperature falls then the gas consumption corresponds to a much lower average temperature - and every degree Celsius change in average temperature represents about 11% in gas consumption. Alternatively I note you have 4 adults. If they were in the house all day, then this corresponds to 400 W of heating i.e. 9.6 kW/day of heating, or 3,500 kWh of additional heating. Similarly, all electrical consumption ends up as heat, so if your electrical consumption were particularly high, this too could be heating the house. Without details I can't really help. You could try measuring gas consumption and electrical consumption on a very cold day and then work out the required heat pump size from that. Best wishes Michael

[@dellybeanhead]

Do you have any opinion on Agile electricity tariffs (like octopus) . I read about somebody who has made a device that switches the call for heat off if the agile price is too high and the savings in energy costs were massive. . I assume the heat pump stays running but does not produce heat. Another option I have personally costed out is to install a battery bank that gets fully charged during the EV tariff period and therefore gives you all day electricity at the low tariff. Payback on batteries is just 3 years (although mine is cheaper than most as I am a certified installer). I am not sure which method is best and currently have no experience of heat pumps. I imagine switching the pump on and off to utilise cheap agile tariffs is not a good idea.

[@michaeldepodesta001]

Dear @dellybeanhead, Good Afternoon. Running a heat pump with a battery is very smart. With Octopus;s different tariffs it is very hard to know which one is optimal, but I use Octopus Intelligent Go. The terms have got better than Octopus Go. Intelligent go: 7.5 p/kWh from 11:30 until 5:30 i.e. 6 hours Go: 7.5 p/kWh from 00:30 until 4:30 i.e. 4 hours. I use this with a Tesla Powerwall 2 (13.5 kWh with 5 kW of power). My house has a *peak* heating demand around 70 kWh which requires round 20 kWh of electricity to operate the heat pump. More normally the house requires ~ 20 kWh of heat which requires just 5 kWh of electricity. So in winter we charge the battery (and run the dishwasher etc) in teh cheap hours and then run off the battery until it runs out. On teh very coldest days his happens at around 1:00 p.m. and we have to run on full price electricity. But normally in winter the battery runs through to very late in teh evening, so the average price of electricity is generally less than 10 p/kWh. In summer we run off solar power and use teh battery to take us overnight so that we draw nothing from the grid for about 3 months. The cost of this is about £35/month averaged across the year - and £15/month is standing charges! So that covers all the electricity and heating. This article details the costs and covers battery degradation. protonsforbreakfast.wordpress.com/2023/12/30/solar-pv-review-of-2023/ But overall, a battery & solar PV natch very well with a heat pump. Do drop me a line if you have questions. If you want to find out what goes wrong with heat pump installations, try this article. protonsforbreakfast.wordpress.com/2024/02/10/5-reasons-heat-pump-installations-have-problems/ Best wishes Michael

[@michaeldepodesta001]

Dear Teabreak, Good Evening. Yes. 1227 cubic metres of gas corresponds to roughly 13,865 kWh of gas used annually. So yes, roughly 5 kW sounds about right. My home in Teddington, West London used 15,000 kWh/year before external wall insulation and measurements now indicate a 5 kW heat pump would have been fine (subject to radiator sizing). For a variety of reasons I did install a 5 kW pump but with external wall insulation the heating demand was cut by roughly half and the heat pump can barely get started! Last year we warmed the house using less electricity than if we had used a hairdryer (set on low!). (protonsforbreakfast.wordpress.com/2022/04/12/could-you-heat-your-house-with-a-hairdryer/) Some of the stories on my blog may be relevant protonsforbreakfast.wordpress.com/my-house/ My plan was to install the heat pump and see if the radiators would be good enough. They were. Changing radiators afterwards is easy - changing pipework is not, but I took a risk. Typically - using weather compensation - the flow temperature reaches 43 °C ish when the outside temperature is 0 °C. In the summer, the whole house runs from solar PV for about 4 months, but in winter solar is low and heating demand is high, so solar doesn't cover everything. If you are considering solar, I would encourage you get battery storage too. protonsforbreakfast.wordpress.com/2022/09/01/its-been-a-sunny-summer/ The real risk with a 10 kW pump is not really the cost. The real risk is that it won't be able to modulate so as to operate efficiently at normal autumn/winter/spring demand. It will just keep switching on and off. If you don't find what you are after on teh blog, do drop me another line. All the best Michael