At what temperature does a heat pump stop working?
It doesn't stop. It keeps running and delivers less heat, which is a different problem and a better one. The number that matters is where its output crosses your house's load.
Heat pump capacity at low outdoor temperatures
Short answer: a standard heat pump delivers roughly 60 percent of its rated capacity at 17°F. A cold-climate inverter unit holds 75 to 85 percent there and keeps running to −13°F. Neither one "stops" — below the balance point the backup heat carries the difference, and that is where the bill goes up.
| Outdoor temp | Standard unit | Cold-climate unit |
|---|---|---|
| 47°F (rated) | 100% | 100% |
| 35°F | 80 to 85% | 90 to 95% |
| 17°F (rated) | 55 to 65% | 75 to 85% |
| 5°F | 40 to 50% | 65 to 80% |
| 0°F | backup heat dominant | 55 to 70% |
Percent of the 47°F rated capacity. Ranges span single-speed, two-stage and inverter equipment; your model's spec sheet has the exact curve.
Why the question is framed wrong
A heat pump does not have an off switch at some temperature. It has a capacity curve that falls as the outdoor air gets colder, because there is less heat in that air to move. At 47°F it moves a lot of heat per unit of electricity. At 5°F it moves less, works harder, and may not keep up with the house.
So the useful question is not "when does it stop" but "at what outdoor temperature does its output fall below what my house loses". That temperature is the balance point, and everything above it is a heat pump problem while everything below it is a backup heat problem.
The two rating points, and why the second one is the honest one
AHRI rates every heat pump at 47°F and again at 17°F. The 47°F number is the one on the box and in the brochure, and in most of the country it is not the condition the system spends January in. The 17°F number is closer to the truth for a cold-climate installation, and it is the one to ask for.
A standard single-speed unit typically lands at 55 to 65 percent of rated capacity at 17°F. A cold-climate inverter unit holds 75 to 85 percent, because it can spin the compressor faster and the coil is designed for a larger temperature difference. That gap is the entire reason the cold-climate category exists, and it is why the two cost different amounts.
The balance point, in one paragraph
Draw your house's heat loss as a line that rises as it gets colder. Draw the heat pump's capacity as a line that falls. Where they cross is the balance point. Above it, the heat pump alone holds the setpoint. Below it, something else has to make up the difference, and that something is usually electric resistance strips in the air handler.
Resistance heat is close to 100 percent efficient, which sounds good until you compare it to a heat pump running at 250 percent. The strips are not inefficient, they are just the most expensive way to make a BTU in the house. Every hour they run is an hour at two and a half times the cost per unit of heat.
What actually happens on a cold morning
- Output falls, runtime rises. The unit runs continuously instead of cycling. That is normal and not a fault.
- Defrost runs every 30 to 90 minutes. The outdoor coil ices, the unit reverses to melt it, and for a few minutes the indoor coil is cold. Some systems blow lukewarm or cool air during defrost. That is also normal.
- Backup heat stages in below the balance point. You may see the auxiliary or emergency heat indicator on the thermostat. Continuous aux heat in mild weather is a fault, not a feature.
- Efficiency drops but stays above one. A modern cold-climate unit still returns 1.5 to 2 units of heat per unit of electricity at 5°F. A standard unit at 0°F is close to parity, which is why the strips win there.
How to know what yours will do
Get the extended performance table for the specific model, not the brochure. It lists capacity and input at 47, 42, 35, 30, 25, 17, 10, 5 and 0°F, and sometimes lower. Compare the capacity at your design temperature to the heat loss from a Manual J. If capacity exceeds load with a margin, the system heats the house without strips; if it doesn't, you are buying resistance heat for the coldest weeks and should know that before the quote is signed.
For cold-climate shopping, the NEEP list is the practical filter: it only includes units that hold a meaningful share of capacity at 5°F. Starting there saves you from comparing 30 models that all claim to be "cold climate" in the marketing copy.
The number people forget to ask for: capacity at 5°F, not at 47°F. A unit rated 36,000 BTU at 47°F might deliver 20,000 at 5°F, and a house that loses 25,000 BTU at that temperature will be running strips all night. That is a sizing decision, and it is invisible in the headline rating.
Frequently asked questions
At what temperature is a heat pump no longer effective?
Effectiveness declines gradually rather than ending at a temperature. A standard heat pump is delivering roughly half its rated capacity near 5 F, which is below the load of most houses, so backup heat runs. A cold-climate unit still covers most of the load at that temperature. Neither stops operating.
Do heat pumps work below 0 F?
Yes. Cold-climate models are rated to operate at minus 13 F and some to minus 22 F. Output at those temperatures is a fraction of the 47 F rating, so whether the house stays warm depends on the load calculation and on whether backup heat is sized for the shortfall.
Why does my heat pump blow cold air in winter?
Usually defrost. The outdoor coil ices in cold, humid weather, and the unit reverses briefly to melt it, which makes the indoor coil cold for a few minutes. If it blows cold for long stretches and the house loses temperature, that is a refrigerant or reversing valve fault and needs a technician.
Is a heat pump cheaper than a furnace in a cold climate?
It depends on the balance point and on local prices for electricity and gas. Above the balance point the heat pump wins on cost per BTU by a wide margin. Below it, resistance backup can cost more than a gas furnace would, which is why cold-climate installations often pair a heat pump with a furnace rather than strips.
How long does a heat pump last?
Ten to fifteen years for the outdoor unit, with the compressor usually the limiting part. Cold-climate units that run continuously through long winters tend to land at the lower end. A well-sized system that is not fighting a load it cannot meet lasts longer.
Does a heat pump lose capacity in cold weather?
Yes, and the loss is the design constraint. Capacity falls because the temperature difference between the outdoor coil and the outdoor air shrinks, so less heat moves per cycle. Inverter-driven compressors and larger coils reduce the loss but do not remove it.
Disclaimer: Capacity figures are typical ranges for currently sold equipment, not specifications for any particular model. Every heat pump has its own extended performance table at 47, 17, 5 and 0 F, and that table, not this page, decides whether a unit will heat your house. Sizing and equipment selection belong to a Manual J load calculation and a Manual S match.
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