The old rule that “heat pumps don’t work when it’s cold” is out of date. Cold-climate air-source heat pumps (ccASHPs) now heat homes through Minnesota and Maine winters. What is true is that their efficiency falls as it gets colder — which is exactly why our calculator uses a lower seasonal COP for colder climate zones.
Why COP drops in the cold
A heat pump moves heat from outdoor air into your home. The colder the air, the less heat is available and the harder the compressor works, so the coefficient of performance (COP) falls. A unit that hits COP 3.5 at 47 F might be COP 2 at 5 F. The seasonal COP — the winter average — is what drives your bill.
| Climate zone | Assumed seasonal COP | Rough HSPF2 |
|---|---|---|
| Mild (Gulf South, Southwest) | 3.2 | ~10.9 |
| Mixed (Mid-Atlantic, Pacific NW) | 2.8 | ~9.6 |
| Cold (Northeast, upper Midwest) | 2.4 | ~8.2 |
| Very cold (N. New England, Mountain West) | 2.0 | ~6.8 |
See climate zones and COP for how we use these.
What to look for in a cold climate
- A cold-climate (ccASHP) rating — look for rated capacity at low temperatures, not just the mild-weather number.
- A strong HSPF2 (8.5+) — see what HSPF means.
- Correct sizing via a Manual J load calculation, so the unit isn’t oversized for mild days or undersized for cold ones.
- A plan for the coldest snaps — backup electric strips or a dual-fuel pairing.
Running cost in a cold state
Even at a seasonal COP of 2.0, a heat pump still beats propane, oil and electric resistance in most cold states — and often beats gas where electricity is moderately priced. Check your own state on its state page, or read is a heat pump cheaper than gas? The cold doesn’t break the math; it just shifts the numbers, and the calculator lets you model your zone honestly.