Heat pumps work in freezing weather, but their heating power drops as outdoor temperature falls
A heat pump extracts heat from cold air and moves it indoors, even when the outdoor temperature is well below freezing. Most air-source heat pumps continue operating down to around 0°F (−18°C), though some newer models work reliably to −13°F (−25°C) or lower. The catch: as outdoor temperature drops, the heat pump has to work harder to pull heat from the cold air, so it produces less warmth indoors and uses more electricity to do it.
Think of it this way—there is still heat energy in 10°F air; it is just less concentrated than heat in 50°F air. The heat pump's compressor has to run longer and cycle more often to extract enough of it. At some point, usually between −5°F and 0°F depending on the model, the system switches to backup electric resistance heating (often called "emergency heat") to meet your home's demand. That backup is less efficient and costs more to run, but it keeps your house warm when the outdoor air is too cold for the heat pump alone.
Key Takeaways
- Most air-source heat pumps continue heating down to 0°F, though output drops significantly below 20°F.
- As outdoor temperature falls, the heat pump cycles more often and uses more electricity to extract the same amount of heat.
- Below roughly −5°F to 0°F, the system automatically switches to electric resistance backup heating, which is less efficient but necessary.
- Cold-climate heat pumps rated to −13°F or lower are available but cost more upfront and are most useful in regions with frequent deep freezes.
- A heat pump's heating capacity at 0°F is typically 50 to 70 percent of its capacity at 47°F, the standard rating temperature.
Why heat pump output drops in cold weather
Heat pumps work by circulating refrigerant through an outdoor coil, where it absorbs heat from the surrounding air, then moving that refrigerant indoors to release the heat. The colder the outdoor air, the slower that heat transfer happens. At 47°F (the standard test temperature), a 3-ton heat pump might deliver 3 tons of heating. At 0°F, that same unit might deliver only 1.5 to 2 tons—roughly half.
The compressor, which pressurizes the refrigerant to make the heat transfer work, has to run longer and harder in cold weather. This increases electricity use. Your thermostat senses that the indoor temperature is dropping and keeps the compressor running more often, but the system still cannot keep up with your home's heat loss through walls and windows. That is when the backup electric resistance heating kicks in automatically.
When backup heating turns on and what it costs
Your heat pump has a second heating element—usually electric resistance coils—built into the indoor unit or the air handler. When the outdoor temperature drops below a certain threshold (often called the "balance point," typically between −5°F and 0°F), the thermostat switches on this backup heating to help the heat pump meet demand. You may see "auxiliary heat" or "emergency heat" light up on your thermostat display.
Electric resistance heating is less efficient than the heat pump itself because it converts electricity directly to heat with no amplification. A heat pump can move 2 to 3 units of heat for every unit of electricity it uses; resistance heating produces only 1 unit of heat per unit of electricity. In a region where winter temperatures regularly drop below 0°F, the backup heating can add 20 to 40 percent to your winter electric bill. In milder climates, it may run only a few times per year.
Cold-climate heat pumps and their limits
Manufacturers now offer cold-climate heat pumps designed to work efficiently down to −13°F (−25°C) or even lower. These units use larger outdoor coils, variable-speed compressors, and improved refrigerants to extract heat more effectively in extreme cold. They cost 15 to 25 percent more than standard models but reduce reliance on backup heating in regions with frequent deep freezes.
Even cold-climate models have limits. Below −13°F, heating output continues to decline, and backup heating becomes necessary. No air-source heat pump works well below roughly −20°F (−29°C). In regions where temperatures regularly drop below −10°F, a ground-source (geothermal) heat pump may be a better choice, since ground temperature stays around 50°F year-round and does not fluctuate with outdoor air.
How to know your heat pump's cold-weather rating
Your heat pump's specifications sheet lists its heating capacity (in BTU or tons) at different outdoor temperatures. Look for a table showing output at 47°F, 32°F, 17°F, and sometimes 0°F or lower. The difference between the 47°F rating and the 0°F rating tells you how much heating power you lose in deep cold.
You can also ask your installer for the unit's "minimum operating temperature"—the lowest outdoor temperature at which it will run. Most standard units are rated to 0°F; cold-climate models go lower. If you live in a region where winter temperatures regularly drop below 10°F, knowing this number helps you understand how often your backup heating will run and what your winter electric bill might look like.
What happens if you run a heat pump in extreme cold without backup
If outdoor temperature drops below the heat pump's minimum operating temperature, the system will not start. Some units have a safety lockout that prevents operation below a certain threshold to protect the compressor. Others will attempt to run but will cycle on and off frequently, using a lot of electricity while producing very little heat.
Running a heat pump below its rated temperature can also damage the compressor. Refrigerant that is too cold becomes too thick to circulate properly, and the compressor can overheat trying to force it through. This is why backup heating exists—it is not just for comfort, it is a safety feature that protects the equipment. If your heat pump is not turning on during a cold snap, check your thermostat to see if it has switched to backup heating mode, or contact your installer to verify the unit's minimum operating temperature.
Comparing heat pump performance across climate zones
| Climate Zone | Winter Low Temperatures | Heat Pump Type | Backup Heating Frequency |
|---|---|---|---|
| Mild (South, coastal) | Rarely below 20°F | Standard air-source | Rarely or never |
| Moderate (Mid-Atlantic, Midwest) | 10°F to 20°F common | Standard or cold-climate | Several times per winter |
| Cold (Northern, mountain) | Below 0°F common | Cold-climate recommended | Regularly, 20+ days per winter |
| Extreme (Far North) | Below −10°F common | Ground-source or cold-climate with large backup | Frequently, or ground-source preferred |
Frequently Asked Questions
At what temperature does a heat pump stop heating?
Most standard air-source heat pumps stop operating around 0°F (−18°C), though they produce very little heat below 20°F. Cold-climate models work down to −13°F (−25°C). Below the unit's minimum operating temperature, the system will not start or will cycle on and off without producing useful heat. Your backup electric heating takes over at that point.
Does a heat pump use more electricity in winter than summer?
Yes, significantly more. In winter, the heat pump runs longer and works harder to extract heat from cold air. When outdoor temperature drops below the balance point (usually −5°F to 0°F), backup electric resistance heating turns on, which is very electricity-intensive. A heat pump's winter electric use can be two to three times its summer cooling use, depending on your climate.
Can I use a standard heat pump in a place where it gets below 0°F?
Yes, but your backup heating will run frequently and your winter electric bill will be higher. A standard heat pump works down to 0°F, so it will continue operating, but it produces little heat in extreme cold and relies heavily on backup. If temperatures below 0°F occur more than a few times per winter, a cold-climate heat pump or ground-source system is more cost-effective over time.
What is the difference between a cold-climate heat pump and a standard one?
Cold-climate heat pumps have larger outdoor coils, variable-speed compressors, and improved refrigerants that allow them to extract heat efficiently down to −13°F or lower. Standard models are optimized for milder conditions and lose most of their heating capacity below 20°F. Cold-climate models cost more upfront but reduce backup heating use in regions with frequent deep freezes.
Should I switch to backup heating manually in winter?
No. Your thermostat switches to backup heating automatically when the outdoor temperature drops below the balance point. Manually switching to "emergency heat" mode bypasses the heat pump entirely and uses only electric resistance, which wastes electricity. Leave your thermostat on "heat" or "auto" and let the system manage the switch.