Heat pumps work in winter by pulling warmth from the cold air outside and moving it indoors

A heat pump doesn't create heat the way a furnace does by burning fuel. Instead, it moves heat that already exists in the outdoor air into your home, even when the temperature outside is well below freezing. The outdoor unit contains a refrigerant—a liquid that changes state easily between liquid and gas—that absorbs whatever warmth is present in the cold air. That warmth is then compressed and transferred indoors through the indoor unit, where it releases the heat into your home.

This process works because heat exists in air at almost any temperature above absolute zero (−459°F). Even at 20°F outside, there is still thermal energy in the air that a heat pump can extract and use. The colder it gets, the less efficient the process becomes, but most modern heat pumps can still pull usable heat from air down to around 0°F without help.

Key Takeaways

  • Heat pumps move existing warmth from outdoor air into your home rather than generating heat by burning fuel, and they work effectively down to around 0°F.
  • A refrigerant circulates between the outdoor and indoor units, absorbing cold-air heat outside and releasing it inside through a reversing valve.
  • When outdoor temperatures drop below the heat pump's effective range, a backup heating element (usually electric resistance heat) turns on automatically to supplement warmth.
  • Heat pumps are most efficient in mild winters and lose efficiency as outdoor temperature falls, but they still use less energy than electric resistance heating in most climates.
  • Proper maintenance—keeping the outdoor unit clear of snow and ice, and checking refrigerant levels—helps your heat pump maintain winter performance.

The refrigerant cycle that extracts heat from cold air

The heart of winter heat pump operation is the refrigerant cycle. Refrigerant enters the outdoor unit as a low-pressure liquid. As it passes through the outdoor coil, it absorbs heat from the surrounding air and evaporates into a gas. This happens even when the outdoor air feels bitterly cold to you, because the refrigerant boils at a much lower temperature than the air around it.

The refrigerant gas then flows to the compressor, which pressurizes it. Compression raises the temperature of the gas significantly—this is the key step that makes winter heating possible. The hot, pressurized gas moves indoors through copper tubing and enters the indoor coil. There, it releases its heat into the air that your furnace blower circulates through your home. As the refrigerant cools, it condenses back into a liquid and the cycle repeats.

A reversing valve is what allows the same heat pump to heat in winter and cool in summer. In heating mode, the valve directs the refrigerant flow so that the outdoor coil absorbs heat and the indoor coil releases it. In cooling mode, the flow reverses.

Why heat pumps need backup heat when it gets very cold

Below a certain outdoor temperature—usually around 0°F to 20°F depending on the heat pump model—the system cannot extract enough heat from the air to keep your home warm on its own. At that point, a backup heating element activates automatically. In most homes, this is an electric resistance heater built into the indoor unit or furnace. It works like a space heater, converting electricity directly into warmth.

When backup heat is running, your system is less efficient because electric resistance heating uses more energy per unit of warmth than the heat pump cycle does. However, the heat pump still runs alongside the backup heat in many cases, contributing whatever warmth it can extract. Your system's controls decide when to switch to backup heat based on outdoor temperature and how much heating your home needs.

Some newer heat pump systems use a cold-climate heat pump design that can extract heat efficiently down to −13°F or lower, reducing the need for backup heat. These systems use larger outdoor coils and more powerful compressors, and they cost more upfront but can save money on winter energy bills in very cold climates.

How defrost cycles keep ice from blocking the outdoor unit

In winter, moisture in the outdoor air can freeze on the outdoor coil, blocking airflow and reducing the heat pump's ability to absorb warmth. To prevent this, heat pumps run a defrost cycle automatically—usually every 30 to 90 minutes when conditions are right for frost buildup.

During defrost, the reversing valve switches the system into cooling mode temporarily. Hot refrigerant from the compressor flows to the outdoor coil, melting the ice. At the same time, your backup heat turns on indoors so you don't feel a sudden drop in warmth. The defrost cycle typically lasts 5 to 15 minutes. You may notice warm air stopping briefly or even cool air coming from vents during this time—that is normal and necessary.

If you see ice building up heavily on the outdoor unit even after defrost cycles, or if the unit is buried under snow, clear away the snow and ice by hand. Do not use a heat gun or pour hot water on it, as rapid temperature changes can damage the coil. A gentle spray from a garden hose (in milder weather) or straightforward brushing away snow works well.

Winter efficiency and how cold affects performance

Heat pump efficiency is measured by COP (coefficient of performance), which compares the heat output to the electrical energy used. On a 50°F day, a typical heat pump might have a COP of 3 or higher, meaning it produces 3 units of heat for every 1 unit of electricity consumed. On a 0°F day, the same heat pump might have a COP of 1.5 to 2, still better than electric resistance heating (which has a COP of 1) but noticeably less efficient.

This efficiency drop is why heat pumps work best in climates with mild winters. In regions where temperatures regularly drop below −10°F, a traditional furnace or a cold-climate heat pump may be more cost-effective. However, in most of the continental United States, heat pumps save money on winter heating compared to electric resistance or oil heat, even accounting for the efficiency loss in cold weather.

Your actual winter energy use also depends on how well your home is insulated, how you set your thermostat, and how much your backup heat runs. A well-insulated home with a heat pump set to 68°F will use far less energy than a poorly insulated home set to 72°F.

Maintenance steps that keep your heat pump working through winter

Heat pumps need less maintenance than furnaces because there is no combustion, but winter care is important. Before cold weather arrives, have a technician check the refrigerant charge, inspect the compressor, and test the reversing valve. A low refrigerant charge will cripple winter performance and should be corrected before temperatures drop.

During winter, keep the outdoor unit clear of snow and ice buildup. After heavy snow, brush away accumulation from the sides and top of the unit. Do not block the unit with a cover or fence—it needs airflow to absorb heat from the surrounding air. If ice forms on the coil between defrost cycles, that is usually a sign the system needs service.

Check your indoor air filter monthly during the heating season and replace it if it is visibly dirty. A clogged filter reduces airflow and makes your heat pump work harder. If your system has a humidifier, check its water level and clean or replace the pad as needed—dry indoor air is common in winter and can make your home feel colder than it actually is.

Frequently Asked Questions

Why does my heat pump sometimes blow cool air in winter?

Cool air during defrost cycles is normal—the system is melting ice on the outdoor coil while backup heat warms your home indoors. If cool air is blowing when the outdoor temperature is above 20°F and defrost is not running, the system may have a refrigerant leak or a faulty reversing valve and needs service.

Can I use a heat pump if I live somewhere that gets below zero?

Yes, but your backup heat will run frequently and your energy bills will be higher than in milder climates. A cold-climate heat pump model is more efficient in very cold weather. In regions with sustained temperatures below −10°F, a furnace or hybrid system (heat pump plus furnace) may be more economical.

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

Costs vary by climate, local electricity and gas prices, and your home's insulation. In most climates, a heat pump costs less to operate than electric resistance heat or oil heat. Compared to natural gas furnaces, a heat pump usually costs more in very cold climates but less in mild ones. A technician can estimate your specific costs based on your location and system.

What should I do if my heat pump stops heating on a cold night?

Check that the thermostat is set to heat mode and that the temperature setting is above the current room temperature. Verify the outdoor unit is not buried in snow or completely iced over. If those are fine, the system may have lost refrigerant or the reversing valve may be stuck—call a technician. Do not wait; use a space heater or fireplace to stay warm while you arrange service.

Does my heat pump use more electricity in winter than summer?

Yes, typically 30 to 50 percent more, depending on your climate and how cold it gets. Winter heating demands are usually greater than summer cooling demands in most regions. Backup heat, which runs only in winter, also increases electricity use. This is normal and expected.