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

A heat pump in winter operates on the same principle as a refrigerator running backwards. Instead of removing heat from inside your home and dumping it outdoors, a winter heat pump extracts heat energy from the cold air, ground, or water outside and brings it indoors to warm your living spaces. This works because even cold air contains heat energy—it just takes a compressor and refrigerant to pull that energy out and concentrate it where you need it.

The outdoor unit contains a coil that absorbs heat from whatever source it's connected to. Refrigerant flowing through that coil picks up this heat and carries it indoors through copper lines. Inside, a second coil releases that heat into your home's air or water system. A compressor powers the whole cycle by pressurizing the refrigerant, which makes the heat transfer possible. On very cold days, most heat pumps switch on a backup electric heating element to supplement the heat being pulled from outside.

Key Takeaways

  • Heat pumps extract warmth from outdoor air, ground, or water and move it indoors, working even when outside temperatures drop below freezing.
  • Air-source heat pumps are the most common type for homes and remain effective down to about 0°F, though efficiency drops as temperature falls.
  • Ground-source and water-source heat pumps stay more efficient in winter because underground and water temperatures remain more stable than air temperature.
  • Most heat pumps include a backup electric heating element that turns on automatically during extreme cold to maintain comfort.
  • Heat pump efficiency in winter is measured by HSPF (Heating Seasonal Performance Factor), with higher numbers meaning lower operating costs.

How the refrigerant cycle moves heat indoors

The refrigerant inside your heat pump is the workhorse of the system. In winter mode, cold refrigerant flows through the outdoor coil, where it absorbs heat from the surrounding air, ground, or water. The temperature of the refrigerant is kept lower than the outdoor environment, so heat naturally flows into it. This warm refrigerant then travels indoors through insulated copper lines.

Inside your home, the warm refrigerant passes through an indoor coil. A fan blows your home's air across this coil, and the heat transfers from the refrigerant to your air. The now-cooled refrigerant returns outdoors, where it passes through an expansion device that lowers its pressure and temperature even further. This cold refrigerant re-enters the outdoor coil, and the cycle repeats. A compressor in the outdoor unit powers this entire loop by pressurizing the refrigerant, which is what makes heat move from cold to warm—something that wouldn't happen on its own.

Air-source heat pumps and the cold-weather challenge

Air-source heat pumps, the most common type in homes, pull heat directly from outdoor air. They work well down to about 0°F, though their efficiency drops as the temperature falls. Below 0°F, the outdoor coil can begin to frost over, which blocks heat transfer. To prevent this, air-source heat pumps automatically run a defrost cycle every 20 to 30 minutes in very cold weather. During defrost, the system reverses direction briefly and uses indoor heat to melt frost off the outdoor coil—a necessary step that temporarily reduces heating output.

The colder it gets outside, the less heat energy is available in the air, so your heat pump has to work harder and longer to extract enough warmth. This is why most air-source heat pumps include a backup electric heating element. When outdoor temperature drops below a certain threshold (usually around 35°F, depending on the model), the system automatically switches on this element to help meet your heating demand. The electric element is less efficient than the heat pump itself, so your energy bills will rise on the coldest days, but the system ensures your home stays warm.

Ground-source and water-source heat pumps in winter

Ground-source heat pumps (also called geothermal) and water-source heat pumps stay much more efficient in winter because they pull heat from underground or from a water source, where temperatures remain stable year-round. A few feet below the surface, ground temperature stays around 50°F even when air temperature is freezing. This means the heat pump doesn't have to work as hard to extract warmth, and it rarely needs the backup electric element.

Ground-source systems require either a loop of pipe buried horizontally in your yard or drilled vertically into the ground. Water-source systems need access to a pond, lake, or well. Because of the installation cost and complexity, these systems are less common than air-source heat pumps in residential homes, but they deliver superior winter performance and lower operating costs in cold climates. If you have the space and budget, a ground-source system is the most efficient heat pump option for winter heating.

HSPF rating and what it tells you about winter performance

HSPF stands for Heating Seasonal Performance Factor and measures how much heat a heat pump delivers for every unit of electricity it uses over an entire heating season. A higher HSPF means the system is more efficient and will cost less to operate. Current federal standards require new heat pumps to have an HSPF of at least 8.8, though high-efficiency models can reach 10 or higher.

HSPF is different from COP (Coefficient of Performance), which measures efficiency at a single outdoor temperature. HSPF accounts for the fact that your heat pump will run at many different temperatures throughout the winter, including very cold days when efficiency drops. When comparing heat pumps, HSPF gives you a more realistic picture of what your heating bills will actually be. A system rated HSPF 10 will cost noticeably less to run than one rated HSPF 8.8, especially if you live in a cold climate where the heating season is long.

When backup heating kicks in and why

Most heat pumps have a built-in electric resistance heating element that activates automatically when the outdoor temperature drops below a set point or when the heat pump alone cannot keep up with demand. This backup element is not a failure—it's a designed feature. On moderately cold days, your heat pump handles all the heating. On very cold days, the backup element supplements the heat pump's output. On the coldest days, both systems run together.

You cannot control when the backup element turns on; the system decides based on outdoor temperature and indoor demand. Some newer heat pumps use a "smart" backup strategy that minimizes electric heating to save energy. If you notice your electric heating bill spiking during winter, it usually means the outdoor temperature has dropped below the point where your heat pump can meet demand alone, which is normal. If the backup element is running constantly even on mild winter days, that may signal a problem with the heat pump itself and warrants a service call.

Maintenance that keeps winter heating reliable

Heat pumps need regular maintenance to work efficiently in winter. Before the heating season starts, have a technician check the refrigerant charge, inspect the outdoor coil for debris, and test the defrost cycle on air-source systems. A clogged outdoor coil or low refrigerant will force the system to work harder and reduce heating output when you need it most.

During winter, keep the outdoor unit clear of snow and ice buildup. After a heavy snow, brush accumulated snow away from the coil, but do not chip away ice—let the defrost cycle handle it. If your heat pump is in a windy location, consider a windbreak to reduce heat loss around the unit. Check your indoor air filter monthly and replace it every one to three months, depending on dust levels. A dirty filter blocks airflow and reduces heating efficiency. These straightforward steps will keep your system running at peak performance through the cold months.

Frequently Asked Questions

Can a heat pump heat a home when it's 0°F outside?

Yes, most air-source heat pumps work down to 0°F, though efficiency drops significantly and the backup electric element will run frequently. Below 0°F, the system becomes less reliable and may struggle to keep up with demand. Ground-source heat pumps remain efficient well below 0°F because they pull heat from stable underground temperatures.

Why does my heat pump run more often in winter than summer?

In winter, there is less heat available in the outdoor air, so the heat pump must run longer and more frequently to extract enough warmth to heat your home. On very cold days, the backup electric element also runs, which increases total runtime. This is normal operation, not a sign of a problem.

Does a heat pump use a lot of electricity in winter?

Heat pumps are more efficient than electric resistance heating, but they do use more electricity in winter than in summer because heating demand is higher. On the coldest days when the backup element runs, electricity use rises further. Despite this, a heat pump typically costs less to operate than a furnace or baseboard heating in most climates.

What happens to a heat pump during a defrost cycle?

During defrost, the system reverses direction and uses indoor heat to melt frost off the outdoor coil. This temporarily stops heating your home for a few minutes. You may notice warm air from the outdoor unit or see steam rising from it. Defrost cycles are automatic and necessary to keep the system working in cold, humid conditions.

Is it normal for my heat pump to make noise in winter?

Some noise is normal, especially during defrost cycles when the system reverses direction. You may hear hissing, gurgling, or a brief loud sound as refrigerant direction changes. If the noise is loud, continuous, or accompanied by reduced heating, contact a technician to check for refrigerant leaks or compressor problems.