A heat pump moves warmth from outside air into your home, rather than burning fuel to create heat

A heat pump works by capturing heat that already exists in the air outside — even cold air contains some heat energy — and moving it indoors. It does this using a refrigerant (a liquid that changes state easily) that cycles through four main parts: a compressor, a condenser coil, an expansion valve, and an evaporator coil. In winter, the pump pulls heat from outside air, compresses it to raise the temperature, and releases that heat inside your home. In summer, it reverses the cycle to pull heat out of your home and dump it outside, working like an air conditioner.

The reason this matters is efficiency. A furnace burns gas or electricity to create heat from scratch, which wastes energy. A heat pump straightforward moves heat that is already there, so it can deliver more warmth per unit of electricity used — typically 2 to 3 times more efficient than electric resistance heating. The tradeoff is that heat pumps work best in moderate climates; in very cold regions, they lose efficiency and may need a backup heating source on the coldest days.

Key Takeaways

  • Heat pumps move existing heat from outside air into your home rather than generating heat by burning fuel, making them more efficient than furnaces.
  • The system uses a refrigerant that circulates through a compressor, condenser, expansion valve, and evaporator to transfer heat in either direction.
  • In winter the pump pulls heat from outside air and moves it indoors; in summer it reverses to cool by moving heat outside.
  • Heat pumps deliver 2 to 3 times more heating or cooling per unit of electricity than electric resistance heating or cooling.
  • Performance drops in very cold climates, and many cold-region installations include a backup heating system for temperatures below freezing.

The refrigerant cycle: how heat actually moves

The heart of a heat pump is a closed loop filled with refrigerant — a chemical that boils at very low temperatures. As the refrigerant circulates, it changes between liquid and gas states, and each state change either absorbs heat or releases it. This is the same principle that makes a refrigerator work, except a heat pump is designed to move heat into your home instead of out of it.

In heating mode, the cycle starts at the evaporator coil, which sits outside. Cold refrigerant flows through this coil and absorbs heat from the outdoor air — even air that feels cold to you contains thermal energy that the refrigerant can capture. The refrigerant boils into a gas as it absorbs this heat. That gas then flows to the compressor, an electric motor that squeezes the gas, raising its temperature and pressure. The hot, pressurized gas moves to the condenser coil inside your home, where it releases its heat into the air or into water (depending on the system type). As the refrigerant cools, it condenses back into a liquid. Finally, the liquid passes through an expansion valve, which lowers its pressure and temperature, and the cycle begins again.

The compressor is the only part that uses significant electricity. Everything else happens because of the refrigerant's natural properties — it wants to boil at low pressure and condense at high pressure. By controlling the pressure at different points in the loop, the system controls where heat is absorbed and where it is released.

Winter heating: pulling warmth from cold air

This is the part that confuses most people: how does a heat pump extract heat from air that is already cold? The answer is that "cold" is relative. Air at 32°F still contains heat energy compared to the refrigerant inside the evaporator coil, which can be as cold as 0°F or lower. Heat always flows from warmer to cooler, so the refrigerant absorbs heat from the outdoor air even when that air feels freezing to you.

The efficiency of this process depends on the temperature difference. On a 40°F day, the heat pump works well because there is a large gap between the outdoor air and the cold refrigerant. On a 0°F day, the gap is smaller, so the evaporator coil has to work harder and the compressor has to run longer to move the same amount of heat. This is why heat pump efficiency drops in very cold climates. Many homes in cold regions pair a heat pump with a backup heating system — usually electric resistance heat or a gas furnace — that turns on automatically when outdoor temperatures fall below a certain point (often 25°F to 35°F, depending on the system).

The outdoor unit of a heat pump will have frost or ice on it during winter operation, because the evaporator coil is so cold. Most heat pumps have a defrost cycle that periodically reverses the refrigerant flow to melt this buildup, then resumes heating.

Summer cooling: the cycle in reverse

In cooling mode, the heat pump straightforward reverses direction. The outdoor unit becomes the condenser (where heat is released), and the indoor unit becomes the evaporator (where heat is absorbed). Warm indoor air passes over the indoor coil, loses its heat to the cold refrigerant, and the heat is pumped outside. This is identical to how a central air conditioner works, except a heat pump can switch back to heating mode when you need it.

Because the cycle is reversible, a heat pump provides both heating and cooling with a single system. You do not need a separate furnace and air conditioner, which saves money on installation and maintenance. The switchover happens automatically — most systems have a thermostat setting that tells the heat pump which mode to run in, or a smart thermostat that switches based on the outdoor temperature.

Types of heat pumps and where the heat goes

An air-source heat pump is the most common type. It exchanges heat with the outdoor air, so the outdoor unit is visible and the indoor unit is usually a wall-mounted cassette or a traditional air handler in a closet or attic. These work well in moderate climates and are cheaper to install than other types.

A ground-source heat pump (also called a geothermal heat pump) exchanges heat with the ground instead of the air. The ground stays at a relatively constant temperature year-round — usually 45°F to 55°F depending on your location — so ground-source systems are more efficient than air-source systems, especially in cold climates. The tradeoff is that installation requires digging trenches or drilling a well, which costs significantly more upfront. Ground-source systems are most common in new construction or major renovations where the cost can be spread over the life of the system.

A water-source heat pump works similarly but exchanges heat with a body of water — a pond, lake, or well. This is rare in residential settings but sometimes used in areas with suitable water sources.

Efficiency ratings and what they mean

Heat pump efficiency is measured in two ways. HSPF (Heating Seasonal Performance Factor) measures heating efficiency, and SEER2 (Seasonal Energy Efficiency Ratio) measures cooling efficiency. Both are ratios of heat output to electricity input, so a higher number means more efficient. A modern air-source heat pump typically has an HSPF of 8 to 10 and a SEER2 of 16 to 22, depending on the model and climate zone.

These numbers matter because they directly affect your heating and cooling bills. A heat pump with an HSPF of 10 delivers 10 units of heat for every 1 unit of electricity used. An electric furnace delivers only 1 unit of heat per 1 unit of electricity, so the heat pump is 10 times more efficient. In practice, the difference in your bill depends on your climate, how cold it gets, and whether you have a backup heating system that runs frequently.

Efficiency also degrades over time. A heat pump that is 10 years old and has not been serviced may perform 10 to 15 percent worse than when it was new. Regular maintenance — cleaning the outdoor coil, checking refrigerant levels, and replacing filters — helps preserve efficiency.

Common problems and how they affect performance

A refrigerant leak is the most serious problem. If the refrigerant level drops, the system cannot move as much heat, and the compressor has to work harder to compensate. A small leak might reduce efficiency by 5 to 10 percent; a large leak can make the system barely functional. Refrigerant leaks require a technician to find and seal the leak, then recharge the system.

Frost or ice buildup on the outdoor coil in winter is normal, but excessive buildup means the defrost cycle is not working properly. This reduces heat transfer and forces the compressor to run longer. A clogged outdoor coil (from leaves, dirt, or pollen) has the same effect. Cleaning the outdoor unit and checking the defrost cycle are part of regular maintenance.

A failed compressor is expensive to repair — often $1,500 to $3,000 — because the compressor is the most complex part. Compressors usually fail because of low refrigerant (which causes the compressor to overheat), lack of maintenance, or straightforward age. Most compressors last 10 to 15 years with proper care.

Frequently Asked Questions

Can a heat pump work in temperatures below zero?

Air-source heat pumps can technically operate below zero, but efficiency drops sharply and they cannot keep up with heating demand. Most systems are paired with a backup heating source that activates automatically when outdoor temperature falls below 25°F to 35°F. Ground-source heat pumps work much better in extreme cold because the ground temperature stays constant.

Why does my heat pump run constantly in winter?

Constant operation usually means the outdoor temperature is below the point where the heat pump is efficient, so the backup heating system is running, or the heat pump is struggling to meet demand. This is normal on very cold days. If it runs constantly even on mild days, the refrigerant level may be low or the outdoor coil may be dirty.

How much electricity does a heat pump use compared to a furnace?

A heat pump uses less electricity than an electric furnace for the same amount of heat, but more than a gas furnace (which uses gas, not electricity). The exact difference depends on your climate and how often the backup heating runs. In moderate climates, a heat pump typically costs 30 to 50 percent less to operate than electric resistance heating.

What is the difference between a heat pump and an air conditioner?

An air conditioner only cools; a heat pump cools and heats. Both use the same refrigerant cycle, but a heat pump has a reversing valve that allows the cycle to run in either direction. An air conditioner is cheaper upfront but requires a separate heating system.

Do heat pumps work in humid climates?

Yes. Heat pumps work in any climate, but they are most cost-effective in moderate climates where heating and cooling seasons are both significant. In very humid climates, the cooling function works well, but you may want to pair the heat pump with a dehumidifier if indoor humidity stays above 60 percent.