Estimate the heat pump size, in tons, that a whole house needs for your climate. Use it to sanity-check installer quotes before you sign.
Estimated size
| Design heating factor | – |
| Design cooling factor | – |
| Typical design low temperature | – |
How heat pump size is worked out
The size that matters is the heat your house loses on the coldest normal night of the year (the “design temperature”) and the heat it gains on the hottest afternoon. The calculator estimates both from floor area, ceiling height, insulation and climate zone, using typical design factors for an average, reasonably insulated house (the insulation setting scales them up or down):
| Climate zone | Design low | Heating (BTU/hr per sq ft) | Cooling (BTU/hr per sq ft) |
|---|---|---|---|
| 1–2 (FL, south TX, AZ) | 35°F | 14 | 24 |
| 3 (GA, NC, central CA) | 25°F | 18 | 22 |
| 4 (VA, MO, KS) | 15°F | 22 | 20 |
| 5 (OH, IL, NY) | 5°F | 27 | 18 |
| 6–7 (MN, ME, ND) | −10°F | 33 | 16 |
The result is rounded up to the next half ton, because heat pumps are sold in 1.5, 2, 2.5, 3, 3.5, 4 and 5 ton sizes. One ton is 12,000 BTU per hour.
Size for heating or for cooling?
In the South the cooling load is usually larger, so the heat pump is sized for summer. In the North the heating load can be twice the cooling load. There you have two choices: size for heating with a cold-climate, variable-speed model that can slow down in summer, or size for cooling and accept that electric backup heat will run on the coldest nights.
Why you still need a Manual J
Square-foot rules ignore window area, air leakage, duct losses and which way the house faces. A proper ACCA Manual J calculation looks at all of that. Many homes turn out to need a smaller unit than the one they already have, because the old system was oversized. If a quote is more than half a ton above this estimate, ask the installer to show you the load calculation.