What Size Central Air Unit Do I Need? Why Square Footage Isn't Enough

Tons, Btu and a rough range

AC capacity is sold in tons. One ton is 12,000 Btu per hour of cooling, so DOE's example lists a 2-ton heat pump as 24,000 Btu/hr (DOE).

If you want a ballpark before a contractor visits, DOE's two figures give a wide range: 400 to 500 square feet per ton for the common rule of thumb, and 800 to 1,000 square feet per ton for a very efficient house (DOE Consumer Guide). The table below is just that arithmetic. Use it to sanity-check a quote, not to buy a unit.

The gap between the two columns is the point. A quote far outside the range for your house deserves a question, and only a load calculation settles it.

Same house size, very different AC size

DOE's sizing fact sheet compares a hypothetical 2,000-square-foot home in Raleigh, North Carolina, built two ways (DOE):

Same floor area, more than double the cooling load. That's why a square-foot chart can be off by a wide margin.

What a Manual J calculation looks at

ACCA describes Manual J as the national ANSI-recognized standard for producing HVAC equipment sizing loads for homes, and says a load calculation done to it is required by national building codes and most state and local jurisdictions (ACCA). According to DOE, Manual J determines heating and cooling loads from (DOE):

• Wall, ceiling, window and floor areas and their insulation values
• Air leakage of the building and the ducts
• Building orientation, roof surface color and occupancy

Once the loads are known, Manual S is used to select equipment that matches them.

Why oversizing is a problem

It's tempting to go bigger "just in case." The ENERGY STAR fact sheet on right-sized air conditioners explains why that backfires (EPA ENERGY STAR):

DOE also cites a Florida study in which units oversized by 50 percent or more used about 9 percent more cooling electricity a year and had 13 percent higher summer peak demand (DOE).

DOE adds that oversized systems use more fan power and often show more duct leakage because of higher duct pressures. Our guide on how long an AC should take to cool one degree explains how to spot short-cycling at home.

Ducts can eat the capacity

Ducts in a hot attic matter as much as the unit. DOE's fact sheet says simulation studies show cooling systems can lose 30 percent of their peak sensible capacity from heat gained by ducts in unconditioned space, and duct leakage can have even larger effects (DOE). That's a reason to fix ducts before upsizing, not to buy a bigger unit to make up for them.

DOE also gives an airflow target for heat pumps of 400 to 500 cubic feet per minute per ton of cooling capacity, and notes that inadequate airflow can cost 5 percent or more of a system's performance.

Heat pumps and cold climates

A heat pump is sized for both heating and cooling. DOE notes that Manual S advises that, in colder climates, heat pumps can be sized up to 25 percent larger than the required cooling size to reduce use of backup resistance heat (DOE). In that case, ask about units that adjust blower speed for humidity, which DOE says can help keep cooling comfortable.

Don't size for fast cool-downs

Some people want a bigger unit so the house cools fast after work. DOE suggests a programmable thermostat instead, so the house is cooled before you arrive. That avoids the temptation to oversize for rapid cool-downs (DOE).

What to ask your contractor

DOE's fact sheet advises insisting on documented Manual J sizing calculations and using Manual S to select the equipment. Questions that follow from that guidance:

1. Will you do a Manual J for my house? Ask for the written report, not a verbal estimate.
2. What design temperatures did you use? DOE says not to alter the suggested design temperatures for your location, because using annual extremes leads to oversizing. For cooling, Manual J uses the 97.5 percent design temperature, and it generally recommends 75°F indoors.
3. Did you account for my upgrades? New windows, insulation or air sealing lower the load. DOE says the recommended size should drop as a home's efficiency improves.
4. How did you pick the model? The answer should reference Manual S and the Manual J loads.
5. Are the ducts sized for it? A new unit is only as good as the ducts that carry its air.

Replacing an old unit with the same size

Don't assume the old size was right. DOE's fact sheet cites a large survey in which nearly 40 percent of contractors said they purposely oversized equipment. If your home has had energy upgrades since the old unit went in, the load may be lower now. A fresh Manual J is the way to know. When choosing among correctly sized units, the next decision is efficiency; our guide on what a good SEER rating is covers that.