Heat pumps become less efficient as outdoor temperature drops, and most stop heating effectively around 0°F to –10°F
A heat pump does not straightforward turn off at a specific temperature the way a light switch flips. Instead, its heating power gradually weakens as the air outside gets colder. Most air-source heat pumps—the kind mounted on the outside of your home—deliver noticeably less heat when the outdoor temperature falls below 32°F (0°C). By the time you reach 0°F to –10°F, the system is pulling so little warmth from the outside air that it cannot keep your home warm without help from a backup heating source.
The reason is physics: a heat pump works by extracting heat from outside air and moving it indoors. The colder the outside air, the less heat is available to extract. Below freezing, the outdoor unit also develops frost, which reduces airflow and efficiency further. Your system may keep running, but it will not maintain your desired indoor temperature on its own.
Key Takeaways
- Heat pump heating output drops significantly below 32°F and becomes unreliable below 0°F to –10°F depending on the model.
- Most heat pump systems include a backup electric resistance heater that turns on automatically when outdoor temperatures fall too low.
- A heat pump that runs constantly but does not warm your home is a sign the backup heater is working, not that the system has failed.
- Proper maintenance—cleaning the outdoor unit and checking refrigerant levels—helps a heat pump perform better in cold weather.
- If you live in a climate where temperatures regularly drop below –10°F, a heat pump alone may not be sufficient without a secondary heating system.
How outdoor temperature affects heat pump performance
Heat pump efficiency is measured in COP (coefficient of performance), which compares the heat output to the electricity used. At 47°F, a typical air-source heat pump has a COP of 3 to 4, meaning it produces three to four units of heat for every unit of electricity consumed. At 32°F, that COP drops to around 2. By 0°F, it may fall to 1 or below, meaning the system uses nearly as much electricity to produce heat as a standard electric resistance heater would.
The outdoor unit also accumulates frost and ice as temperatures drop, especially when humidity is present. The system runs a defrost cycle to melt this buildup, which temporarily stops heating your home and sends warm air outside. In very cold weather, these defrost cycles happen frequently and waste energy.
Different heat pump models have different cold-weather cutoff points. Some newer cold-climate heat pumps are designed to operate down to –13°F or lower, but standard models typically lose practical heating ability around 0°F to –10°F. Check your system's specifications or your installation manual to know your unit's rated minimum operating temperature.
When the backup heater turns on automatically
Most residential heat pump systems include a backup electric resistance heater, also called auxiliary heat or emergency heat. This is a strip of heating elements similar to those in an electric space heater. When the outdoor temperature drops below a threshold—usually between 32°F and 50°F, depending on how the system is programmed—the backup heater activates automatically to supplement the heat pump.
You may see a light on your thermostat labeled "Aux Heat" or "Emergency Heat" when this happens. This is normal and expected. The backup heater is doing its job. However, backup heat is expensive to run because it converts electricity directly to heat with no multiplier effect like the heat pump has. If your backup heater is running constantly, your heating bills will be higher than they would be if the heat pump alone could handle the load.
Some thermostats let you set the temperature at which backup heat engages. If your system is running backup heat at 40°F when it could still rely on the heat pump alone, you may be wasting money. A technician can adjust this threshold, or you can check your thermostat manual to see if you can change it yourself.
Signs your heat pump is struggling in cold weather
A heat pump that is struggling does not always fail completely. Watch for these signs that your system is at its limit:
- The outdoor unit is running constantly but your home is not reaching the temperature you set on the thermostat.
- The backup heater is running frequently, even on days when the outdoor temperature is above 0°F.
- You hear the outdoor unit cycling on and off rapidly, which often signals a defrost cycle repeating too often.
- Warm air from the indoor vents feels noticeably less warm than it did in fall, even though the system is running.
- Your heating bills spike sharply during the coldest weeks, a sign the backup heater is doing most of the work.
None of these necessarily means the system is broken. They mean the heat pump has reached the edge of what it can do in cold conditions, and the backup heater is compensating. If you live in a climate where these conditions happen regularly, this is the normal operating pattern for your system.
Maintenance that helps heat pumps work better in cold
You cannot change the outdoor temperature, but you can keep your heat pump running at peak efficiency for the conditions it faces. Clear snow and ice from the outdoor unit regularly during winter. A blocked unit cannot pull air through, which makes performance worse. Keep the area around the unit clear of leaves, branches, and debris.
Check that the indoor air filter is clean. A clogged filter forces the system to work harder and reduces the amount of warm air that reaches your home. Most filters should be replaced every one to three months during heating season.
Have a technician inspect the system before winter arrives. They can check refrigerant levels, clean the outdoor coil, and verify that the defrost cycle is working properly. Low refrigerant is a common reason heat pumps lose efficiency in cold weather, and it requires a professional to diagnose and repair.
Seal air leaks around windows, doors, and ductwork. The better your home is insulated, the less heat the system has to produce to maintain your comfort. This is especially important if you rely on a heat pump in a cold climate.
When a heat pump alone is not enough
If you live in a region where temperatures regularly drop below –10°F, a heat pump may not be your primary heating source. Many homes in cold climates use a heat pump paired with a gas furnace, wood stove, or other backup system. The heat pump handles the milder winter days and shoulder seasons (fall and spring) when it is efficient, and the backup system takes over during the coldest stretches.
This is not a failure of the heat pump—it is the right tool for the job in that climate. A heat pump is most cost-effective in regions where winter temperatures stay above 0°F most of the time. If you are considering installing a heat pump in a very cold climate, discuss this with an HVAC contractor who can recommend a model rated for cold operation and advise whether a backup heating system makes sense for your area.
What to do if your heat pump stops heating
If your heat pump is not producing any heat at all—not even weak heat—the problem is different from normal cold-weather performance loss. Check these things before calling a technician:
- Verify the thermostat is set to "Heat" mode, not "Cool" or "Off."
- Look at the outdoor unit to see if it is running. You should hear the fan and compressor operating.
- Check whether the backup heater is working. If the thermostat shows "Aux Heat" is on and warm air is coming from the vents, the heat pump itself may have failed, but the backup is keeping your home warm for now.
- Look for ice or snow completely blocking the outdoor unit and clear it away.
If the outdoor unit is not running at all, or if the backup heater is on but producing no heat either, call an HVAC technician. This indicates a mechanical failure that requires professional repair. Do not wait in a cold home—a failed heating system in winter is a serious problem.
Frequently Asked Questions
Can I use my heat pump in freezing weather?
Yes, but with reduced efficiency. Most heat pumps continue to operate at temperatures well below freezing, but they produce less heat and use more electricity to do so. Your backup heater will likely run alongside the heat pump to maintain comfort. Check your system's manual for its minimum operating temperature.
Why is my heat pump running but my house is cold?
In cold weather, this usually means the heat pump is working at its limit and the backup heater is on. The system is producing heat, but not enough to keep up with how fast your home is losing it. Check that your thermostat shows "Aux Heat" is active. If it is, the system is working as designed. If it is not, the heat pump may have a refrigerant leak or compressor problem and needs service.
Does a heat pump stop working at 32°F?
No. A heat pump continues to work below 32°F, but its heating output drops significantly. At 32°F, it is still reasonably efficient. By 0°F to –10°F, it is much less efficient and usually cannot maintain comfort without backup heat. The exact temperature at which performance becomes inadequate depends on your specific model.
Should I turn off my heat pump in winter?
No. Leave it on and let it run. The backup heater will engage automatically when needed. Turning the heat pump off and relying only on backup heat wastes money because resistance heat is expensive. The heat pump is still more efficient than backup heat, even in cold weather.
How much does it cost to run a heat pump in very cold weather?
Costs vary widely depending on your local electricity rates, how well your home is insulated, and how cold it gets. When the backup heater is running frequently, your heating bills will be higher than during milder months. If you are concerned about winter costs, ask an HVAC technician to check whether your backup heater threshold is set appropriately for your climate.