Why the 20 BTU per Square Foot Rule Oversizes Most Air Conditioners

Where the rule comes from

The 20 BTU per square foot rule, which works out to a ton for every 600 square feet, is a shortcut from a time of leaky houses and single-pane glass. It has no input for climate. A 2,000 sq ft house gets the same 3.5 tons in Seattle and in Phoenix.

What the climate data says

Sizing each city from its NOAA design day, the chart lands more than 15% too big - past what ACCA Manual S allows - in 2,150 cities. It is close in 885 and too small in 171. Square feet per ton runs from 394 in the hottest city to 1,526 in the mildest, against the chart's fixed 600.

Why bigger is worse

An oversized unit hits the thermostat setting in minutes and shuts off. The coil never gets cold long enough to condense much water, so the house is cool and damp. Short cycles also wear the compressor and waste the start-up energy each time. The biggest gaps, where even the smallest central unit is far too much: San Francisco, CA (167% over), Anchorage, AK (167% over), Berkeley, CA (167% over), Richmond, CA (167% over), Boulder, CO (167% over).

Where the chart is too small

In the hottest places the chart undershoots. Bullhead City, AZ needs 60,800 BTU/hr, Maricopa, AZ needs 59,000 BTU/hr, Lake Havasu City, AZ needs 59,000 BTU/hr, Palm Desert, CA needs 59,000 BTU/hr, Cathedral City, CA needs 59,000 BTU/hr - all more than the chart's 40,000. There, the unit runs flat out on design days and the house drifts warm by late afternoon.

Where to go next

See how far apart the answers run: Bullhead City, AZ, Houston, TX, San Francisco, CA.

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