HVAC Sizing. Formulas, Rule of Thumb, or Heat Load Calculations

[@michaelcinaz]

Proper sizing of an air conditioner, gas furnace, coil box, refrigerant lines, return air ducts, air filters, and supply air ducts are critical. Have the new HVAC system tested after it’s installed. Have it checked for static pressures (return, supply, total), proper airflow per room, and proper temperature. 👍

[@robertlong7723]

All of the factors that you mention certainly contribute to heat and cooling loads. These factors combine many variables into the calculation of a single load factor. Wouldn't the best indicator for a replacement HVAC be evaluating how well your existing heating and cooling units have performed and how often each one cycles at various outdoor temperatures? Then use that information in conjunction with the size of the current units to decide if capacity of the replacement units need to be larger or smaller.

[@philipramsden4975]

I just had my furnace and A/C replaced. I originally had a 2 ton unit, but they said it was undersized based on my vaulted ceilings and orientation of my house/no shade (rear of the house faces south). They recommended a 3 ton variable speed unit and it's been working great. My old unit struggled to keep up even before it started failing.

[@shericantor1031]

I’m in Richmond - can you put a link to your Load Calc Contractor?

[@NewHVACGuide]

He’s old school. No link. Call us and we will get you the info. Thanks

[@noahfigueroa3534]

Any insight on how to read/understand a heat/cool analysis? I had to pay $200 for my analysis but there is a ton of information on there. I just need to know what size mini split to install. I am a licensed general engineering contractor that installs my own mini-spits but then I hire a local licensed hvac guy to inspect and charge the system.

[@bloomqmx]

Does it make sense to upsize the coil compared to the compressor. ie 2.5 ton compressor and a 3 ton coil. Thx

[@redsadventures5654]

I worked for a company that did the basic load calc for every install but it definitely wasn’t a “real” load calc. They advertised that it was. I no longer work for that company

[@mmmallory1649]

I have 2 floors, one of which is a basement. Each floor is about 900 sq. feet, so 1800 total. I've read that when calculating the BTU and Tonage requirement that you only use 1/2 of the basement square footage, thus reducing the total square footage to 1350 if true. Can someone tell me what size BTU gas furnace (80%) and A/C tonnage (14 seer) I'd need for my situation? I'm in zone 3, Virginia. Thank you.

[@NewHVACGuide]

The only way to properly size a system is to do a manual J calculation typically with a pro. Watch this video if you need one performed. kzbin.info/www/bejne/a3LJeKevi7ZjfZIsi=3-M-YH4hyfGgJsmZ

[@Garth2011]

We have never heard of any HVAC folks doing a heat load calculation prior to installing a new system or replacing an existing system. They all seem to be fine with replacing the hardware with the same capacity that is existing and for new construction they always choose the cheap "contractor" models with cheap low grade ducting, supply and return vents. In the southwest, Type of roofing material might also have some play into the heat calcs as lots of folks use "spanish tile" materials that remain hot long after the sun goes down continuing to radiate heat.

[@privatename3621]

I'm in S Fla and the situation is the same. I've talked to a few HVAC guys that have come by to maintain my units over the years, and whenever I mention whether the would do a Manual J on replacement installations, the first reaction is somewhat bewilderment, followed by my awkwardly trying to explain to them what it is, followed by them pretending to know what it is, and blowing it off by saying that on a replacement system they don't typically do or need to do those. The idea being that the original developer of the gated community already did the calculations prior to the homes construction. So if nothing has changed (same windows, insulation, ac sizes, etc) then they are perfectly fine just replacing like for like systems. And this seems to be both the norm, and also is allowed by law if all things are the same as when the original home was built. But also, it does say something like "and you have the load calculations" ...which I don't believe anyone in our community was ever provided by the builder. Just took their word that it was done. But I also know that the same home models in our community have the exact same AC specs, regardless of orientation to the summer sun, which in S FL is important. I happen to have an entire wall of multiple sliding glass doors along the back side of my home, which faces the low hanging long summer sun. Other homes with the same equipment, may be oriented in the complete opposite direction, or at 90 degrees to mine. So clearly, the calculation was done in a way that ignored direct sun exposure, or assumed it was the same for all homes in the development. So where I'm at, I guess as long as they can get away with replacing like for like systems at the permitting office, that is typically what they do, unless it is new construction, or involves impactful remodeling changes.

[@Garth2011]

@@privatename3621 You can be guaranteed that the original builder "specs" were lean and the HVAC system was minimal, one return, small supplies and supply registers located for convenience/cost vs. function. Custom homes tend to be way better in that area.

[@padrepennyr7284]

Infiltration plays a major role in a load calculations. It can be natural infiltration or mechanical such as leaky ducts outside of the building envelope.

[@NewHVACGuide]

Great point

[@KeneOliver]

Sound advice.

[@NewHVACGuide]

Thanks pal

[@bigytv8392]

Nobody argues validity of properly sizing the HVAC system. However, in your video you throw in a lot of terms just to fill the space and mostly confuse people without offering much help. Basically, lots of words and not much advice, except do it. It's not rocket science. Plenty of online resources to figure out how to do manual J calculation accounting the factors you mentioned. Anyone with basic technical education who wants to do it should be able to get it pretty close. All useful info in your video can be easily summed up in 1 min.

[@bobboscarato1313]

Three minutes is more reasonable!

[@alberthartl8885]

This is not rocket science. Furnaces are typically 48, 60, 80 or 100 000 btu. Ac are 24, 36, 48, or 60 000 btu. If you are doing a heat pump, size to the heating load. Humidity will tweak the number up or down a 1/2 ton. Don't oversize or you will short cycle. That pretty much sums it up. (30 year general contractor)

[@alexkrokhov9652]

So by your logic if house need 65 000 btu of heat you will put 5 ton heat pump in there?

[@NewHVACGuide]

I’d recommend looking elsewhere for logic Alex 🙂