How to size an air conditioner

Three habits stack up. The first is the rule of thumb: 500 square feet per ton, applied to a house whose envelope nobody looked at. The second is the safety margin — a contractor rounds up "to be safe", and the supply house only stocks half-ton steps, so the rounding always goes one way. The third is replacement by nameplate: the old unit was four tons, so the new one is four tons, even though the house has had new windows and a foot of attic insulation since.

Each of those is a small decision. Together they routinely put a four-ton condenser on a house with a 2.8-ton load.

A cooling load is the sum of everything adding heat to the house on a design afternoon. Conduction through walls, ceilings and glass. Solar gain through glass, which depends entirely on orientation — a west window at four o'clock is worth several times a north window of the same size. Infiltration, the air leaking in that has to be cooled and dried. And internal gain: people, cooking, lighting, the equipment that is always on.

Floor area appears nowhere in that list except as a proxy. It correlates with wall area and window area in an average house, which is why the rule of thumb works at all — and why it fails badly on a house with a wall of glass, a converted attic, or an envelope from a different decade than the one the rule came from.

This is the number that surprises people. The cooling design temperature difference is the 1% summer design dry-bulb temperature minus your indoor setpoint. In most of the country that is about 20 degrees: 95 outside, 75 inside. In Chicago it is 19. Even in Phoenix, at 109 design, it is 34.

Compare that to the heating side, where the same house in zone 5 works against a 72-degree difference. Cooling loads are simply smaller than heating loads across most of the United States, and sizing intuition built on "it gets really hot here" consistently overshoots.

Screening values. Your jurisdiction publishes its own design conditions and those are the ones a plan reviewer will use.

Run the AC size calculator in "check what's installed" mode with the tonnage you were quoted. If it comes back more than 25% above the calculated load, ask the contractor for the load calculation. If they do not have one, that is the answer.

For a real number, spend twenty minutes in the Manual J worksheet instead. Room by room, with your actual windows and insulation, it will land far closer than any per-square-foot figure.

Equipment comes in half-ton steps. A 3.3-ton load has to be a 3.0 or a 3.5. Size down. A slightly undersized system runs longer, dehumidifies better, and on the handful of days it cannot quite keep up, the house drifts a degree or two — which is far less objectionable than a year of short cycles.

The exception is a house with a genuine latent problem: high humidity, lots of infiltration, a basement. There, variable-capacity equipment is the right answer rather than an extra half ton of single-stage capacity.

• Can I see the load calculation, room by room?
• What design temperatures did you use, and where did they come from?
• What is the AHRI reference number for the indoor and outdoor pair you are quoting?
• Did you measure static pressure before deciding the duct system is adequate?
• What airflow will the blower actually deliver at that static pressure?

A contractor who answers all five is worth paying more. One who cannot answer the first is guessing with your money.

Whatever the size, the efficiency floor is set federally: split-system air conditioners must meet 13.4 SEER2 nationally, and 14.3 SEER2 under 45,000 BTU/h in the Southeast — Alabama, Arkansas, Delaware, Florida, Georgia, Hawaii, Kentucky, Louisiana, Maryland, Mississippi, North Carolina, Oklahoma, Puerto Rico, South Carolina, Tennessee, Texas, Virginia, the District of Columbia and the U.S. Territories — and the Southwest.

Sources: 10 CFR 430.32(c)(6)(i)(A) (read 2026-09-04)