HVAC Rule of Thumb Tested with Manual J Calc: Using Wrightsoft to Prove 500 sq ft/ton is Dumb

[@davidhoover2446]

This is amazing. People need to see this. I appreciate you doing that! At the beginning the of the video the fact that it was about 2.5 tons for that is already blowing the rule of thumb out of the water! I didn’t know any of this when we built our house here in FL. On our house they fudged the manual J so make it look as if we needed 4 tons for 2,000 sq ft, but with a 61/39% sensible/latent ratio. I started looking through it after I had commissioned the system myself with fan speed and a number of other things and measuring with an ecobee 4. It does work well with the fan speed bottomed out, dehum mode when called for, thermostat differential temp at 1.5 degrees (it turns on at 75, off at 73), and only ever using stage 1 at 33,800btu. It still cycles off. I might hit 85% run time at design temps of 91 on the data. We had like 1 week of 96 degree temps and that would finally make it run for hours. I’m running 254 cfm/ton in dehum mode and 282 cfm/ton normally. I have about 80 or more cfm hot humid fresh air dumping in there also plus the sante fe cuts out when cooling since it is sent to the return. Even if I run my fresh air maxed out which is about 130 cfm, it doesn’t make much difference - the differential between the return and supply just gets bigger. Sometimes I do that if a bunch of people come over. I had a smart technician check over my refrigerant numbers and it is actually still working correctly within refrigerant specs set up like this. Isn’t that craziness? I’m glad the two stage Lennox Merit had a lot of options to choose from on the air handler.

[@HomePerformance]

Great work, David, wow. Thanks for sharing the specs for us nerds.

[@OwensandCompany]

Homeowners have been trained or at least have misconceptions that more “A/C” is better. Recently a homeowner had his HVAC contractor upgrade his 48k BTU package unit to a 60k BTU just because the “smaller” unit ran 16 hours per day during peak heating season. No knowledge of infiltration, duct design, envelope design, just “give me more cold air!” I’m curious how things are going to work out for them after they have that new unit for a while. They probably won’t see the results until next summer. That’s something that I’ve noticed with homeowners, because it takes some time for the effect of changes to show up, they forget what changes were made and what caused it to happen, thus they end up overlooking the fact that their contractor didn’t know what they were doing and just assume it’s something else.

[@HomePerformance]

Very important point, Owens- since our boss is always the homeowner, the project might tank with bad management, just like any company would. More education needed- keep up the good work out there.

[@dbarrington1]

I live in Hawthorne Florida. My wife and I built a Steel Hangar. 50ft X80ft with 16.5' eves and a 3/12 roof pitch. It has a 42ft x 13 foot hangar door on the front west facing side that has not been insulated yet. We have 8" Perlins and gerts. We filled that with Fiberglass on the walls for an R25 wall and with the same in the roof with an additional 3.5" over top the perlins for a thermal break and total R35 Roof. We have 3 casement windows for egress. two on the east and one on the South. I installed two Pioneer 22 SEER2 Mini splits on the south wall side both 2 tons each. I am cooling this 4000 square foot and 80,280 cubic foot building from 99 outside to under 80 and dry inside with that 4 tons It is also very air tight for a steel hangar with the fabric liner that is inside it. The system is the High R value system for steel building by thermal design if you want to check it out on youtube Online calculators had me thinking I needed 10 tons to cool this hangar.... In reality just one 2 ton unit actually made it fairly comfortable inside on a hot day after it ran all night to dry it out and cool down....... So ya 1 ton per 500sq foot is ridiculous. Also... My wife and I built this complete hangar ourselves from Erection to the insulation. We hired day help for the roof since I did not want her on the roof. If your in FL near Gainesville and want to check it out let me know. I am the only air conditioned hangar at Melrose Landing Airport right now.

[@trickstothetrades1801]

Wow that was a fun 21 min.! I loved seeing all those drastic off the wall changes. It really shows you the cause and effect of how and what you do during construction and why. Thanks

[@HomePerformance]

Thanks for calling it FUN, you're so welcome buddy

[@halledwardb]

Can't wait for the course. My 13 and 15 yr daughters install these things, and we could all use another perspective on sizing.

[@clownhands]

1400 square foot industrial loft in nyc, top floor of a 120 year old brick building. Our 3 ton unit ran 24x7 from mid May to mid September and never got the interior temperature below 74 degrees, usually between 77-80. The electric bill is approximately equal to my parent’s mortgage payment. Old non-insulated buildings suck. Looking forward to moving to the suburbs.

[@HomePerformance]

Haha I hear you- the penthouse always gets the biggest shaft, watch this: kzbin.info/www/bejne/g2a1cmxmgMuBq68si=VrAyMHjnvy8HyHM1

[@pauldelvaglio8827]

Great illustration. We just completed a 3000 sq ft remodel in Southern California we have a 12kbtu ducted mini split for the main floor 2000 sqft and 9kbtu for the 1000 sqft upstairs R15 in walls r50 attic radiant barrier sheathing on the roof ducts and equipment in the conditioned space

[@pauldelvaglio8827]

Because of the design I hired a consultant to do the Manual J because I wasn’t confident in my ability to design this remodel with my wrightsoft program

[@DB57RB]

Crazy timing. My parents just had a Manual J done by an HVAC company for their house. I haven't looked at the details but the company got it to my Dad this morning. They quoted replacing his 5 ton ac with two 3 tons for 6 tons of cooling 🙄. Comes in right about 500sq per ton. The house cooled ok with the 5 ton, the only problem was the upstairs getting slightly warmer (4 degrees) than the ground floor. Its so frustrating getting correct sizing.

[@davidgagnon2849]

Another great video, Corbett! Hopefully this next year we'll be pulling the trigger on hiring you to size and design our replacement AC/Make up air/dehumidifier for our house here in Central Florida. Not exactly Miami but close. ;-)

[@HomePerformance]

Happy to David

[@bradleysargent9803]

I'm not a builder and I'm only familiar with mass builder type homes in Texas. On a single story home, they typically locate the AHU in the unconditioned attic or in the garage. All the supply air is located in the attic (duct board and flex). I doubt there is much of any building envelop quality control. To ensure the home is cool (setpoint set at 70 F) during a heat wave of 100+ degree day, that unit is sized well north of 400 SF/ton. That way they don't get any warranty calls that the house isn't cooling due to other issues with the home build. I agree, on a new home build, you can definitely get closer to 250 SF/ton or lower, but the building has to be constructed correctly. Great Video!

[@theomass]

This is exactly my concern as I am about to change out my 4T 1998 original AC. (1850sq ft 1st floor and detailed-room based ManJ done on coolcalc) Regular conditions shows 49K BTUH while improving windows to dual pane and maybe improving som attic insulation (need a complete redo and sealing) drops to 42K BTUH)

[@VagelisPertsinis]

The lower the sf/ton you get the worse it is. For example, new houses that are tightly built, and with energy efficiency in mind can do 2000 sf per ton, with a single unit, and multi-level. BUT, 70F design temp when it’s 100+ outside is not how these systems are designed. It’s probably going to be 75F. That said I don’t think people have experienced living at 75F at 40% Relative Humidity so they are trying to compensate for high humidity with blasting the AC.

[@superspeeder]

Canada here. Generally cold, but we do need significant cooling in the summer months. In my townhouse subdivision standard practice is 2-3 tons in 1500 sqft townhomes. Crazy!!!

[@HomePerformance]

Everybody’s doing it ;)

[@larryseibold4287]

I might have missed it in the video. Is the software showing peak hourly load to hit and maintain a set point, or the max daily cooling load (AC runs continuously for most of the day), to keep up with a set point from day to day? What is the set point (hitting 72F is way harder than hitting 76F on the hottest day)? The point this reveals, is it is important to know the set point as well as the heat load (temp and hours) on the hottest day. It also begs the question: how much if any thermal drift during the hottest day of the decade is acceptable? Insulation not only reduces the heat load, it also slows the heat rise, allowing thermal mass (ac battery) to carry cool walls through the sunny hours from the less hot hours. Related: It would be nice to see homes typically use way less air flow than the ducts can handle (delta P) while staying balanced, so that variable output heating and cooling systems work as intended in all conditions over time. It is best to run the air handler approximately 24-7, with a varying amount of heating and cooling, ventilation, humidification de-humidification as needed (VFD).

[@christophercooke9376]

Thank you for mentioning insulation. From my limited observations, Manual J is good if buildings built to code, but but how many places have spots where contractor failed to install some pieces or an entire wall, or a home remodel that never replaced insulation in ceilings... would be curious to know on a basic house things that can easily affect loads, just upgrading windows from single pane to dual pane. Wonder if there is a way to come up with a way to calculate in human behaviors... such as leaving windows open for ventilation but still running AC... (why you might ask, well a lot of homes built in southern California where a window was proper ventilation for a bathroom and dedicated vents not needed, so someone wanting "fresh" air). Overall good, wish more AC professionals knew more about Manual J & D. Replaced my heat pump about 5 years ago. I didn't know about them and this was never brought up, but trying to educate people from information you and others have provided.

[@jeremyjedynak]

Thanks for debunking myths like these by using actual calculations!

[@HomePerformance]

Glad you enjoy it Jeremy

[@douglaslindsaychapman5188]

The best thing for AC would be to put a return air duct from the highest part of the house. To pull the hottest air down into the AC. I live closer to Maine, so in the heating season I would have to switch the return air vents to the floor.

[@coldfinger459sub0]

Excellent example, the lack of knowledge that unfortunately prevails. The reason they need that 5 ton unit also is there poor duct design only providing two or 2 1/2 tons of cooling in many cases. Especially when they’re one of the guys who have a 14“ x 16“ filter with a 1 inch hog hair stuck in the grill with their 12 inch or 16 inch return . And a second 1” filter in the air cabinet nobody has ever changed.

[@HomePerformance]

Hog hair I love it

[@frankgall6]

This is how I do most of the calls I do cause these businesses ask me to make it work AFTER the size it wrong and install the equipment. Insane

[@jepito29]

2ton in my 1100sqft uninsulated 1950 bungalow couldnt keep up. Did new windows and r23 in attic and now it does. Makes me think that rule of thumb was created at the same time residential AC was and never got updated

[@HomePerformance]

Amen

[@machickman4041]

Would you be able to do an example of a post frame or pole barn home?

[@HomePerformance]

I’ll put it on my long list buddy

[@kanzzon]

You just described my house in miami and by the way I have a 5 ton for a 2200 sq ft house and my ac produces more water than I have aver consumed

[@HomePerformance]

Haha nailed it buddy

[@yehudagoldberg6400]

Is the software accounting for less than perfect building construction and installation?

[@HomePerformance]

Yes, the math has fat built in, it was put together by engineers who are very risk-averse people.

[@frommarkham424]

I mean right off the bat doesnt it depend on the volume of air, external surface area, etc. too

[@Armand9100]

This is not on the topic of the video but I would like to know what you think of the HVAC design of the houses Matt Reisinger features here: kzbin.info/www/bejne/i16mgoGolrCHoKM. The entire attic is a combination plenum/ductwork. It is his latest vedio as of Sept. 16.

[@KPHVAC]

I watched that video. It is a very strange design. For a well sealed and insulated home it might be alright. I still would have put both ductless units on the far walls so they blow air towards the center of the home.

[@seek3n]

I don't promote oversized to make up other poor condition. But most people I worked for only understand system capacity also think im trying to upsale more stuff or can't afford other improvements

[@cokeiceeeee]

Adds solarium and still under 500.

[@HomePerformance]

Nice :)

[@frommarkham424]

Ngl physics is hard tho

[@HomePerformance]

Agreed, but doing hard things is life