Heat pumps work in winter by pulling warmth from outside air, ground, or water and moving it indoors, even when outdoor temperatures drop below freezing

A heat pump reverses its normal cooling cycle to extract heat from cold air and concentrate it inside your home. The outdoor unit contains a refrigerant that absorbs whatever heat exists in the air, ground, or water source — even at 20°F or lower, heat energy is still present. A compressor then pressurizes this refrigerant, raising its temperature further, and an indoor coil releases that warmth into your home. The cycle repeats continuously, delivering steady heat without burning fuel.

The key difference from a furnace is that a heat pump moves existing heat rather than creating it by combustion. This is why heat pumps remain efficient in winter, though their output does decline as outdoor temperatures fall. Most air-source heat pumps (the most common type) include a backup electric heating element that kicks in during extreme cold to maintain comfort when the outdoor air becomes too cold for the heat pump alone to meet demand.

Key Takeaways

  • Heat pumps extract warmth from outdoor air, ground, or water and move it indoors using refrigerant and a compressor, working even in freezing temperatures.
  • An electric backup heating element activates during very cold weather to supplement the heat pump when outdoor temperatures drop too low for efficient operation.
  • Air-source heat pumps lose efficiency as outdoor temperature falls, while ground-source heat pumps maintain steadier performance because underground temperatures stay more stable.
  • The defrost cycle reverses the heat pump briefly to melt ice buildup on the outdoor coil, which is normal operation and does not indicate a problem.
  • Winter heating costs depend on your local climate, electricity rates, and how cold it gets — heat pumps cost less to run than electric resistance heating in most regions but more than natural gas furnaces.

The refrigerant cycle that extracts heat from cold air

Inside the outdoor unit sits a coil filled with refrigerant — a liquid that boils at very low temperatures. Even when outdoor air is 20°F, that air contains heat energy. The cold refrigerant absorbs this heat and evaporates into a gas. A compressor then pressurizes the gas, which raises its temperature to perhaps 100°F or higher. This hot gas flows through the indoor coil, where a fan blows indoor air across it, transferring warmth into your home. The refrigerant cools back into a liquid and returns outdoors to repeat the cycle.

The compressor is the engine of this process — it requires electricity to run, which is why heat pumps draw power continuously during winter. The warmer the refrigerant becomes after compression, the more heat it can deliver indoors. This is why heat pumps work better in mild winter weather than in extreme cold: the temperature difference between the refrigerant and outdoor air grows smaller as it gets colder, so the heat pump must work harder to extract warmth.

Why the backup heating element turns on during extreme cold

Most air-source heat pumps include a second heating source — usually electric resistance coils similar to those in a space heater. This backup activates automatically when outdoor temperatures drop below a set point, typically between 25°F and 35°F depending on the system. At these temperatures, the heat pump alone cannot deliver enough warmth to meet your home's heating demand, so the backup element supplements it.

Backup heating is normal and expected. It does not mean your heat pump is broken. However, it does increase your electricity bill because resistance heating is less efficient than the heat pump cycle. If your backup element runs constantly even during mild cold snaps, or if your heating bills spike unexpectedly, have a technician inspect the outdoor unit for refrigerant leaks or a failing compressor.

The defrost cycle and why your heat pump cycles off briefly

During winter, moisture in outdoor air can freeze on the heat pump's outdoor coil, reducing its ability to absorb heat. To clear this ice, the system reverses itself temporarily — it runs in cooling mode, warming the outdoor coil while cooling your home for a few minutes. Once the ice melts, the system switches back to heating. This defrost cycle typically runs for 5 to 15 minutes and may happen several times per day in humid, freezing weather.

You may notice your indoor temperature drop slightly during defrost, or hear the outdoor unit switch sounds. Some systems set up an electric backup element during defrost to keep your home warm while the outdoor coil is being cleared. This is all normal operation. Defrost cycles do not indicate a malfunction and are part of how air-source heat pumps survive winter weather.

Comparing air-source and ground-source heat pumps in winter

An air-source heat pump pulls heat from outdoor air and is the most common type. It works in winter but loses efficiency as temperatures fall, and it requires backup heating during extreme cold. Installation is straightforward and costs less than ground-source systems.

A ground-source heat pump (also called geothermal) pulls heat from the earth or groundwater, which stays between 45°F and 55°F year-round depending on your region. Because the heat source is warmer and more stable, ground-source systems maintain higher efficiency throughout winter and rarely need backup heating. The trade-off is higher upfront cost — installation requires drilling or trenching — and ground-source systems are not practical for all properties. Ground-source heat pumps make sense if you have space for the ground loop and plan to stay in your home for at least 10 years.

Winter heating costs and efficiency ratings

Heat pump efficiency in winter is measured by HSPF (Heating Seasonal Performance Factor). A higher HSPF means lower heating costs. Modern air-source heat pumps typically have an HSPF between 7 and 10, meaning they deliver 7 to 10 units of heat for every unit of electricity consumed. Ground-source systems often reach HSPF values of 10 to 15.

Your actual winter heating bill depends on your local electricity rates, how cold it gets, and how much your home needs to be heated. In regions where electricity is cheap and winters are mild, a heat pump will cost less to run than an electric furnace and may rival a natural gas furnace. In regions where electricity is expensive or winters are very harsh, heating costs will be higher. A technician can estimate your winter costs based on your climate, home size, and local utility rates.

Heat pumps also may have access to for federal tax credits and some state rebates when installed, which can offset upfront cost. Check the Database of State Incentives for Renewables and Efficiency (DSIRE) or your local utility for current programs in your area.

What to do if your heat pump isn't keeping up in winter

If your home feels cold even though the heat pump is running, check these items first. Verify that your thermostat is set to heat mode and the temperature setting is above the current room temperature. Look at the outdoor unit to see if ice or snow is blocking airflow — clear any debris by hand, but do not use a pressure washer, which can damage the coil. Check that all vents and returns indoors are unblocked and that your air filter is clean.

If the backup heating element is running constantly or your heating bill is much higher than expected, the heat pump may have a refrigerant leak, a failing compressor, or a faulty sensor. These require a technician to diagnose and repair. Do not attempt to add refrigerant yourself — this is a licensed task in most states. Call a heat pump service provider for an inspection if performance drops suddenly or if you notice ice buildup that does not clear after the defrost cycle.

Frequently Asked Questions

Can a heat pump heat a home in freezing weather?

Yes. Heat pumps extract warmth from outdoor air even at 0°F or below. However, their heating output declines as temperature falls, and most systems set up backup electric heating when outdoor temperature drops below 25°F to 35°F. In extreme cold, the backup element may run most of the time, which increases electricity use.

Why does my heat pump make noise during winter?

Heat pumps make several normal winter sounds: a hum from the compressor, clicking when the defrost cycle starts, and a whoosh from the fan. Loud grinding, squealing, or rattling usually means a mechanical problem and warrants a service call. Occasional hissing or gurgling as refrigerant flows is normal.

Is it normal for my heat pump to cycle on and off frequently in winter?

Yes, especially in mild weather. The system cycles on when indoor temperature drops below your thermostat setting and cycles off once the target temperature is reached. Frequent short cycles are normal and more efficient than running continuously. However, if the system cycles every few minutes without reaching your set temperature, the heat pump may be undersized or have a refrigerant leak.

How much does it cost to run a heat pump in winter compared to a furnace?

Heat pump winter costs depend on your electricity rates and local climate. In regions with mild winters and low electricity rates, heat pumps typically cost less than electric resistance heating and may rival natural gas furnaces. In regions with very cold winters or high electricity rates, winter heating costs will be higher. A local HVAC contractor can estimate costs for your specific situation.

What should I do if ice builds up on the outdoor unit?

Some ice on the outdoor coil during winter is normal and the defrost cycle clears it automatically. If ice accumulates faster than the defrost cycle can clear it, or if ice blocks the entire unit, turn off the heat pump and call a technician. Thick ice buildup usually signals a refrigerant leak or a faulty defrost sensor that needs professional repair.