Heat Pump Size Calculator

How this estimate works, and its limits

Cooling: a rule-of-thumb range of about 25–30 BTU per sq ft in zones 1–2, 20–28 in zones 3–4 and 18–25 in zones 5–7, scaled for insulation (older, leaky homes, often pre-1980: +15%; newer, tight homes: −15%). 12,000 BTU/h is one ton. IECC climate zones run from 1 (South Florida, Hawaii) and 2 (Gulf Coast, southern Arizona) through 3 (most of the Southeast), 4 (Mid-Atlantic, Pacific Northwest), 5 (Midwest, New England) and 6 (upper Midwest, northern New England) to 7 (northern Minnesota, high mountains); your local building department can confirm yours. There is no official BTU-per-square-foot table, and the DOE discourages rules of thumb for whole-home equipment, so treat this as a starting point for talking to installers.

Heating: the house loses heat in proportion to the gap between indoors (70°F) and outdoors. We use a rough loss of 0.25–0.45 BTU/h per sq ft per °F depending on insulation, and work it out at your winter design temperature, the cold that is only beaten about 1% of hours. Your local design temperature is in your building department's code tables or a contractor's Manual J.

Cold weather: heat pump output falls as it gets colder. Cold-climate units on the NEEP list must keep at least 70% of their 47°F capacity at 5°F. We use that floor to translate the heat you need at design into a nominal size; many models do better, so compare their extended performance tables at your design temperature.

What size heat pump do I need for a 2,000 sq ft house?

For cooling, rules of thumb give roughly 3 to 5 tons depending on climate and insulation. In cold climates the heating load usually decides: check that the heat pump's output at your winter design temperature covers the house's heat loss. A Manual J load calculation gives the real number.

Do heat pumps lose capacity in the cold?

Yes. Output falls as the outdoor temperature drops. Cold-climate models listed by NEEP must still deliver at least 70% of their 47°F capacity at 5°F, and many do better. Check the maker's extended performance table at your design temperature.

Do I need backup heat with a heat pump?

Not if a cold-climate unit covers your full heating load at your design temperature. Otherwise, keep a furnace, boiler or electric strips for the coldest hours, below the balance point where the heat pump can no longer keep up.

Sources: DOE guide to sizing and selecting air-source heat pumps; PNNL cold-climate heat pump sizing; NEEP cold-climate ASHP specification; IECC climate zones. ACCA Manual J/S and ASHRAE design-temperature tables are copyrighted and not reproduced.