Do heat pumps work in cold weather?
Short answer: Yes. Heat pumps keep working below freezing, but their capacity and efficiency drop as it gets colder. To earn ENERGY STAR's cold-climate designation, a model must reach a COP of at least 1.75 at 5°F and keep at least 70% of its 47°F heating capacity. At 18¢/kWh, heat at COP 3 costs 6¢ per kWh, versus 18¢ for electric resistance strips.
How does a heat pump heat a house when it’s cold outside?
A heat pump moves heat rather than making it. Even 20°F air holds heat energy; the refrigerant absorbs it outdoors and releases it indoors.
Efficiency is measured as COP (coefficient of performance):
COP = heat delivered ÷ electricity used
A COP of 3 means 3 kWh of heat for every 1 kWh of electricity. Electric resistance heat, like a space heater or heat strip, has a COP of 1.
Does a heat pump lose efficiency as it gets colder?
Yes. The colder the outdoor air, the harder it is to pull heat from it. Two things fall:
- Capacity: how much heat per hour the unit can deliver.
- COP: how much heat you get per kWh.
Meanwhile, your house loses heat faster on colder days, so demand rises as supply falls. That is why sizing and model choice matter.
What is a cold-climate heat pump?
These are models designed to keep useful output at low temperatures. Two common benchmarks exist. Note that the ENERGY STAR figures are minimum certification criteria, not typical performance of every heat pump.
| Benchmark | What it requires |
|---|---|
| ENERGY STAR cold-climate criterion | COP of at least 1.75 at 5°F |
| ENERGY STAR cold-climate criterion | 5°F heating capacity at least 70% of 47°F capacity |
| NEEP ccASHP specification | Lists heat pumps suited to IECC climate zone 4 and colder |
The NEEP list helps check a specific model’s capacity at your design temperature, which NEEP says helps confirm required capacity “without unnecessary oversizing.”
Even at the 1.75 minimum, a qualifying unit at 5°F delivers 75% more heat per kWh than resistance heat.
Why is steam coming out of my heat pump?
Frost builds on the outdoor coil in winter, so the unit periodically runs a defrost cycle. Trane says the steam cloud you may see is ice melting and evaporating, and is “perfectly normal.” According to Trane:
- Its demand-defrost cycles usually last 3 to 5 minutes. Other brands’ timed defrost may differ.
- It is more common when temperatures are in the 20s or 30s.
- Your thermostat may show “AUX” during defrost while backup heat covers the gap.
If the coil stays encased in ice for hours, call a technician.
How much does auxiliary or emergency heat cost to run?
Most ducted heat pumps use electric resistance heat strips as backup. They run at COP 1.
Cost per hour = strip size (kW) × price ($/kWh)
For a 10 kW strip at 18¢/kWh: 10 × $0.18 = $1.80 per hour.
| Electricity price | 10 kW heat strip, per hour |
|---|---|
| 12¢/kWh | $1.20 |
| 18¢/kWh | $1.80 |
| 30¢/kWh | $3.00 |
Compare a 1,500-watt unit in our space heater cost guide.
Heat pump vs resistance heat: cost per unit of heat
Cost per kWh of heat = electricity price ÷ COP
EIA expects the 2026 U.S. average residential price at about 18.2¢/kWh. Using 18¢:
| Heat source | COP | Cost per kWh of heat | Cost per million Btu |
|---|---|---|---|
| Resistance (strips, space heater) | 1 | 18.0¢ | $52.75 |
| ENERGY STAR cold-climate minimum, 5°F | 1.75 | 10.3¢ | $30.14 |
| Heat pump, colder day | 2 | 9.0¢ | $26.38 |
| Heat pump, milder day | 3 | 6.0¢ | $17.58 |
One million Btu equals about 293 kWh. At 12¢ or 30¢, COP 3 heat costs 4¢ or 10¢ per kWh.
Tips for running a heat pump in winter
- Keep the outdoor unit clear. Snow or debris blocking the coil can restrict airflow. Many owners brush snow away gently; follow your manufacturer’s manual, turn the unit off first if it advises, and never chip ice off the fins.
- Watch for “AUX” outside defrost. Strip heat costs three times as much per unit of heat as a heat pump at COP 3.
- Be cautious with big setbacks. On some systems, a large morning recovery may bring on backup heat. Results vary by thermostat and equipment, so check its heat pump settings or ask your installer.
- Electrical safety: a frequently tripping heat-strip breaker needs an electrician, not a bigger breaker.
More ways to cut costs are in our guide to lowering your electric bill in winter.
Quick answers
Do heat pumps work below freezing? Yes, with lower capacity and efficiency as it gets colder.
At what temperature does a heat pump stop being efficient? There is no single cutoff. ENERGY STAR’s cold-climate criterion requires a COP of at least 1.75 at 5°F, which beats resistance heat.
Is steam from my heat pump normal in winter? Yes, during defrost. Trane says it is melting ice evaporating.
How much does emergency heat cost per hour? A 10 kW strip costs $1.80 per hour at 18¢/kWh.
Is a heat pump cheaper than a space heater? Per unit of heat, yes, whenever COP is above 1.