Heat pumps lose the most efficiency in very cold weather, typically below 32°F (0°C), because they have to work much harder to pull heat from frigid air

A heat pump moves heat from outside air into your home, even when it is cold. But as outdoor temperature drops, less heat is available to move, so the system runs longer and uses more electricity to reach your target indoor temperature. Most heat pumps are designed to work well down to about 32°F. Below that point, many models switch to electric resistance heating (essentially a space heater), which costs significantly more to run than the heat pump itself.

The efficiency loss is not sudden—it happens gradually as the temperature falls. At 47°F, a typical air-source heat pump operates near its rated efficiency. At 32°F, it may deliver only 50 to 70 percent of the heating capacity it promises on the label. At 0°F or below, some units become so inefficient that the backup electric heating kicks in automatically, and your heating bills spike.

Key Takeaways

  • Heat pumps lose efficiency below 32°F because there is less heat available in cold air to extract and move indoors.
  • Most air-source heat pumps switch to electric resistance heating below freezing, which costs two to three times more per unit of heat than the heat pump mode.
  • Frost buildup on the outdoor coil forces the system to defrost regularly, consuming energy and temporarily stopping heating while it clears ice.
  • Ground-source heat pumps stay efficient in cold weather because soil temperature stays above freezing, but they cost much more to install.
  • Regular maintenance—cleaning coils, checking refrigerant charge, and replacing filters—helps a heat pump maintain whatever efficiency it is designed for at each temperature.

How outdoor temperature affects heating output

The colder the air outside, the less heat energy is available for the heat pump to extract. Think of it like trying to squeeze water from a nearly dry sponge—the sponge is still there, but you have to work much harder to get anything out. A heat pump's compressor (the part that does the work) runs longer and draws more power to move the same amount of heat indoors.

Manufacturers publish a heating capacity rating, usually measured at 47°F. This is the amount of heat the unit can deliver at that temperature. As outdoor temperature falls, the actual output drops. At 32°F, output may be 70 percent of the rated amount. At 17°F, it might be 50 percent. At 0°F, some units deliver only 30 to 40 percent of their rated capacity. Your home loses heat to the outdoors at a constant rate (depending on insulation), so when the heat pump cannot deliver enough heat, the backup electric resistance heating turns on to make up the difference.

Frost and ice buildup on the outdoor coil

When outdoor air is cold and humid, moisture condenses on the heat pump's outdoor coil and freezes into frost or ice. This layer acts as insulation, blocking heat transfer between the outdoor air and the refrigerant inside the coil. The system detects this and enters defrost mode: it reverses the refrigerant flow to heat the outdoor coil and melt the ice, then drains the water away.

Defrost cycles use energy and interrupt heating. During defrost, the outdoor coil is being heated instead of absorbing heat from outside, so no heat is being delivered to your home. The cycle typically lasts 5 to 15 minutes and may run several times per hour in wet, freezing conditions. In very cold, humid climates, defrost cycles can account for 10 to 20 percent of the system's total energy use during winter. Some newer heat pumps use smart defrost controls that run only when necessary, reducing this waste.

Electric resistance heating and backup systems

Most air-source heat pumps include electric resistance heating (also called auxiliary or emergency heat) as a backup. When outdoor temperature drops below the system's balance point—typically 32°F to 40°F depending on the model—the heat pump alone cannot meet the home's heating demand, and the resistance heating turns on automatically.

Electric resistance heating is about three times more expensive to run than a heat pump, because it converts electricity directly to heat with no efficiency gain. A heat pump delivers 2 to 4 units of heat for every 1 unit of electricity it uses (this ratio is called the coefficient of performance, or COP). Electric resistance delivers only 1 unit of heat per 1 unit of electricity. In a cold winter, if your heat pump spends weeks running with backup heating, your monthly bill will be noticeably higher than if the heat pump alone could meet demand.

Ground-source heat pumps and cold-weather performance

Ground-source (geothermal) heat pumps pull heat from soil or groundwater instead of outdoor air. Soil temperature stays relatively constant year-round—typically 45°F to 55°F depending on depth and location. Because the outdoor heat source is always warmer than winter air, a ground-source heat pump maintains high efficiency even when outdoor air is well below freezing.

A ground-source system may deliver 80 to 90 percent of its rated capacity even at 0°F outdoor air temperature, and it rarely needs backup electric heating. The trade-off is cost: installing a ground-source system requires drilling or excavating to bury pipes, which can cost $15,000 to $30,000 or more depending on soil conditions and property size. For most homeowners in cold climates, the long-term energy savings do not offset the installation cost, but for those with suitable land and a long time horizon, ground-source is the most cold-weather-efficient option.

Maintenance steps that preserve efficiency in winter

A heat pump that is not maintained loses efficiency faster than one that is. Check and replace your air filter every one to three months during heating season—a clogged filter restricts airflow and forces the system to work harder. Keep the outdoor unit clear of snow, ice, leaves, and debris so air can flow freely through the coil. After heavy snow, brush accumulated snow away from the unit (do not use a pressure washer, which can damage the coil).

Have a technician inspect the system before winter: they will check refrigerant charge, clean the coils if needed, inspect electrical connections, and test the defrost control. A system running low on refrigerant loses efficiency significantly and may not heat adequately. Dirty coils reduce heat transfer. Loose electrical connections increase resistance and waste energy. These checks cost $100 to $200 but can prevent efficiency loss of 10 to 20 percent or more.

Sizing and installation issues that affect cold-weather performance

A heat pump that is undersized for your home will run constantly in winter and rely heavily on backup heating, making it appear inefficient even if the unit itself is working correctly. Proper sizing requires a load calculation based on your home's insulation, air leakage, window area, and local winter temperatures. A unit sized for mild climates will struggle in a cold one.

Installation quality also matters. Poor ductwork design, leaky ducts, or improper refrigerant charge all reduce efficiency. If your heat pump was installed by someone without heat pump experience, or if you have never had it serviced, a professional inspection may reveal problems that are straightforward and inexpensive to fix. Sealing duct leaks, for example, can recover 10 to 15 percent of lost heating output.

Frequently Asked Questions

At what temperature does a heat pump stop working?

Most air-source heat pumps continue to operate down to about –13°F (–25°C), but efficiency drops sharply below 32°F and backup electric heating usually turns on. Below 0°F, the heat pump may deliver only 30 to 40 percent of its rated capacity. Some newer cold-climate heat pumps are designed to work efficiently to –22°F or lower, but these cost more.

Will my heat pump heat my home in winter if I live somewhere that gets below freezing?

Yes, but you will likely use backup electric heating on the coldest days, which will increase your heating bill. If you live where winter temperatures regularly drop below 20°F, a heat pump alone may not be cost-effective unless you choose a cold-climate model or combine it with another heating source like a furnace or boiler.

How much more does it cost to run a heat pump in winter versus summer?

Winter heating costs depend on your climate, home insulation, and how often backup heating runs. In mild climates, winter heating may cost only slightly more than summer cooling. In cold climates where backup heating runs frequently, winter heating can cost two to three times more per month than summer cooling, because electric resistance heating is so expensive.

Can I improve my heat pump's winter efficiency myself?

You can keep the outdoor unit clear of snow and debris, replace air filters regularly, and seal obvious air leaks in ducts or around windows. For anything beyond that—refrigerant charge, coil cleaning, defrost control testing—you need a technician. These professional checks are worth the cost because they often uncover problems that waste 10 to 20 percent of your heating energy.

Is a heat pump worth it if I live in a cold climate?

It depends on your current heating system and how cold your winters are. Heat pumps work well in climates where winter temperatures stay above 20°F most of the time. In colder regions, a heat pump paired with a furnace or boiler (called a hybrid system) can be more cost-effective than a heat pump alone, because the furnace handles the coldest days when the heat pump is least efficient.