Photo: Tracy Adams on Unsplash
The most common worry about heat pumps in northern states is simple: will it keep the house warm when it's 0°F outside? The answer depends less on the brand and more on one specification most buyers never see — heating capacity at low outdoor temperatures. ENERGY STAR publishes it for every certified model, and we put it front and center on each model page.
Three temperatures, three numbers
Certified heat pumps report heating capacity at 47°F (the nameplate condition), 17°F (a cold day) and, for most modern variable-speed units, 5°F (a very cold day). A conventional single-stage heat pump might deliver 36,000 BTU/h at 47°F but only 22,000 at 17°F and perhaps 17,000 at 5°F — under half its rated output. A good cold-climate inverter unit can deliver 100% or more of its 47°F capacity at 5°F, because its compressor speeds up as temperatures drop.
The capacity-retention ratio
We calculate capacity retention at 5°F as heat output at 5°F divided by heat output at 47°F. Across the 308 models in our database the median is 76%. Anything above 80% is strong; above 100% means the unit was designed with deliberate headroom for cold weather.
Practical rule: in a cold climate, size the heat pump so its 5°F (or 17°F) capacity covers your home's heat loss at your local design temperature — not so its nameplate tonnage matches your old air conditioner.
What "cold-climate" certification means
ENERGY STAR's cold-climate designation (version 6.1 and later) requires, among other things, a COP of at least 1.75 at 5°F and a 5°F capacity of at least 70% of the 47°F capacity. Units that meet it are flagged with a blue Cold-climate badge throughout this site.
Backup heat and the balance point
When outdoor temperature drops below the point where the heat pump's output equals the house's heat loss — the balance point — something else has to make up the difference. In most ducted systems that is electric resistance heat strips, which run at a COP of 1.0, two to three times more expensive per unit of heat than the heat pump. A unit with strong 5°F capacity pushes the balance point lower and keeps those strips off for more of the winter. That single difference can matter more to your bill than a point of HSPF2.
How to use the numbers when shopping
- Ask your contractor for a Manual J heat-loss calculation and your local 99% design temperature.
- Find the model's capacity at 17°F and 5°F on its page here.
- Compare those numbers with the heat loss at design temperature. If the 5°F capacity covers it, backup heat will rarely run.
- Check the COP at 5°F — 2.0 or higher is excellent.
Defrost, snow and siting
In freezing humid weather the outdoor coil frosts and the unit periodically reverses to defrost. Modern controls do this efficiently, but the outdoor unit should be mounted on a stand or wall bracket above typical snow depth, with clear space for melt water to drain away from walkways.
Browse the cold-climate models in our database, or compare two units side by side with the comparison tool.