What size heat pump do I need?
Both loads, the one that decides the size, and the shortfall a cold climate leaves.
A heat pump has to do two jobs with one piece of equipment, and the two loads are rarely the same size. Which one wins depends on where you live, and that single fact explains most of the confusion about heat pump sizing.
Before you buy anything. This is a square-foot estimate, not an ACCA Manual J load calculation. A Manual J uses your building’s real construction, orientation and air leakage, and on a well-sealed house it usually comes out smaller than this. Use this to sanity-check a quote, not to replace one.
Worked example: 2,000 sq ft in climate zone 5
Zone 5 is Chicago, Denver, Boston and Detroit. Four occupants, kitchen inside the conditioned area, average insulation, 8 ft ceilings.
In zone 5 the heating load is the larger of the two, so a heat pump sized purely for cooling leaves 29,500 BTU uncovered on the coldest design day. That gap is what supplemental heat exists for: electric resistance strips, a gas furnace in a dual-fuel setup, or a cold-climate heat pump that holds more of its capacity at low outdoor temperatures.
Sizing a heat pump to heating instead
The tempting fix is to size the equipment to the heating load and be done with it. In a cold climate that oversizes the cooling side badly, and you get the short-cycling and humidity problems described on the oversizing page every summer.
The three conventional answers, in the order most contractors consider them:
- Size to cooling, add supplemental heat. The default. Cooling behaves correctly all summer and the strips or furnace cover the coldest days.
- Use a variable-capacity (inverter) unit. These modulate down, so a unit large enough for the heating load can still run gently in cooling. This is the main reason inverter-driven equipment took over in cold climates.
- Dual fuel. Heat pump above the balance point, gas furnace below it.
Heating and cooling load by climate zone
For a 2,000 sq ft house with average insulation and 8 ft ceilings, so the only thing changing is the climate:
| Zone | Cooling | Heating | Driven by | Example cities |
|---|---|---|---|---|
| 1 | 63,750 | 37,500 | Cooling | Miami, Key West, Honolulu |
| 2 | 56,250 | 47,500 | Cooling | Houston, Orlando, Phoenix, New Orleans |
| 3 | 50,250 | 57,500 | Heating | Atlanta, Dallas, Los Angeles, Charlotte |
| 4 | 46,250 | 67,500 | Heating | Washington DC, Seattle, St Louis, Nashville |
| 5 | 42,250 | 77,500 | Heating | Chicago, Denver, Boston, Detroit |
| 6 | 39,000 | 87,500 | Heating | Minneapolis, Burlington, Helena |
| 7 | 35,000 | 97,500 | Heating | Duluth, Grand Forks, International Falls |
| 8 | 32,250 | 107,500 | Heating | Fairbanks, Barrow |
The crossover sits in zone 4: south of it a house is cooling-driven, north of it heating-driven. That is the single most useful line in this table.