Methods of sizing HVAC equipment
METHOD 1: Go by square feet alone
The worst way to size a system is by square feet, though many contractors do just that. The first problem is that it doesn't take into account the climate (hot or cold), how much insulation you have, whether the house is shaded, how leaky the house is, etc. The other problem is that it estimates only cooling; there's no rule of thumb for heating based on only square feet, because such an estimate would often be even farther off.
But for what it's worth, which is not much, the basic formula for cooling is 1 ton of cooling for every 600 s.f. of house. So, for example, a 2400 s.f. home would need 2400 s.f. x 1 ton/600 s.f. = 4 tons. Newer homes that are insulated well and aren't leaky would be closer to around 1 ton for every 1000 s.f. You could invert the figures if that's more comfortable. Another way of saying 1 ton for every 600 s.f., is 20 BTU per square foot. Another way of saying 1 ton per 1000 s.f. is 12 BTU per square foot.
METHOD 2: Go by square feet + climate
HVAC System Sizing
700-1100 sf 700-1050 sf 600-1000 sf 600-950 sf
1051-1350 sf 1001-1300 sf 951-1250 sf
1401-1650 sf 1351-1600 sf 1301-1600 sf 1251-1550 sf
1601-2000 sf 1601-1900 sf 1501-1850 sf
3.5 tons 2101-2300 sf 2001-2250 sf 1901-2200 sf 1851-2150 sf 1801-2100 sf
4 tons 2301-2700 sf 2251-2700 sf 2201-2600 sf 2151-2500 sf 2101-2400 sf
5 tons 2701-3300 sf 2751-3300 sf 2601-3200 sf 2501-3100 sf 2401-3000 sf
40-60 BTU/sf 35-50 BTU/sf 30-45 BTU/sf 20-40 BTU/sf 15-35 BTU/sf
[U.S. Heat Zone map]
Map from DoE. Data from AC Direct with permission, with my modifications for heat.By taking climate into account, you can estimate heating as well. While some people will estimate cooling based on square feet alone even though it's not accurate, nobody will estimate heating by square foot alone, because the results can be even farther off. At a minimum, heating estimates must take into account the climate as well as the area to be heated. (A study of office buildings showed that a building in Chicago needed three times less cooling than one in Miami, but it needed 48 times more heating. (source))
You can see the estimates in the table at right. To make this method a little less useless, I give it with ranges rather than a static number. You'll need more powerful equipment if you don't have good insulation and windows, less if you do.
METHOD 3: Size it to your existing system
If you're shopping for an HVAC to replace an existing system (rather than buying it for new construction), you can usually use the capacity of your existing system to gauge the size of the replacement system. The main thing is to make sure your existing system is sized correctly. A properly-sized system will run continuously (or nearly so) on the hottest and coldest days of the year. If your unit is shutting off even in the most extreme weather, it's probably oversized. Also, if you've installed energy efficiency upgrades (e.g., more insulation, double-paned windows) since you bought your system, it's probably oversized.
If your equipment doesn't appear to be oversized, just buy the same capacity equipment to replace it. If your current equipment is oversized, you could go with one that's a little smaller, but it's better to have the Manual J calculation performed (see way above) find out exactly how much smaller you can go.
For all your Home comfort needs
CHANGE'S AS ON 01/01/2017 EPA. REGULATES R-22 PRICES.
R-22 $ 167.50 PER. LB
R-410A $ 94.00 PER. LB
R-407-C $ 68.00 PER. LB
NU-22 $ 91.00 PER. LB
PLUS ALL SERVICE FEE'S OF $60.00 WILL BE ADDED
ALSO MORE EFFECTIVE AND REPLACEMENT FOR YOUR R-22 SYSTEM WORKS GREAT AND USED BY LOTS OF TECHS AND COMPANYS TO REPLACE R-22
MOST MORE EFFICIENT THEN R-22,NU-22 PRESSURES ARE THE SAME AS R-22 BUT MAKES YOUR SYSTEM MORE EFFICIENT AND OZONE SAFE.
Michael's Heating LLC.
WE DO NOT SAVE STORED IFORMATION