An air heat pump moves heat from outside air into your home instead of burning fuel to create warmth

An air heat pump works by capturing heat that already exists in the outdoor air—even cold air contains some heat energy—and moving it inside your home. It does this using a refrigerant (a special liquid that changes states easily) that circulates through indoor and outdoor coils. The outdoor unit pulls in air, the refrigerant absorbs the heat from that air, and then the system compresses the refrigerant to make it hotter. That hot refrigerant moves indoors, releases its heat into your home through the indoor coil, and then the cycle repeats. In summer, the process reverses: the system pulls heat out of your home and releases it outside, working like an air conditioner.

The key difference from a furnace or space heater is that a heat pump does not generate heat—it transfers it. This is why heat pumps work even when outdoor temperatures are well below freezing. As long as there is a temperature difference between inside and outside, the system can move heat from the colder space to the warmer one.

Key Takeaways

  • Air heat pumps move existing heat from outside air into your home rather than creating heat by burning fuel or using electric resistance.
  • A refrigerant circulates between outdoor and indoor coils, absorbing heat outside and releasing it inside through compression and expansion cycles.
  • Heat pumps can heat your home even in freezing weather because cold air still contains usable heat energy.
  • The same system reverses in summer to cool your home by pulling heat out and releasing it outdoors.
  • Efficiency depends on the temperature difference between inside and outside—the smaller the gap, the less work the compressor has to do.

The refrigerant cycle: how heat moves through the system

The heart of an air heat pump is a closed loop of refrigerant that continuously cycles through four main stages. The refrigerant starts as a cold, low-pressure liquid in the outdoor coil (called the evaporator). As outdoor air passes over this coil, heat from the air transfers into the refrigerant, causing it to evaporate into a gas. This happens even when the outdoor air feels cold to you—the refrigerant is straightforward colder than the air, so heat naturally flows toward it.

Once the refrigerant becomes a gas, a compressor (powered by electricity) squeezes it. Compression raises both the pressure and temperature of the gas significantly. This hot, high-pressure gas then flows to the indoor coil (called the condenser). As indoor air is blown across this coil, the hot refrigerant releases its heat into your home, and the refrigerant cools back into a liquid. Finally, the liquid passes through an expansion valve that lowers its pressure, and the cycle begins again. This continuous loop is what allows the system to move heat from outside to inside without creating any heat itself.

Why the compressor is the most important part

The compressor is what makes heat pumps possible. Without it, heat would only flow from hot to cold naturally, and you could not pull heat from cold outdoor air into a warm home. The compressor uses electricity to do the work of raising the refrigerant's temperature and pressure high enough that heat will flow into your home even when it is warmer indoors than outdoors.

The compressor is also the reason heat pump efficiency varies with outdoor temperature. When it is very cold outside, the compressor has to work much harder to extract heat from the air and raise the refrigerant temperature enough to heat your home. This is why most air heat pumps include a backup heating system (usually electric resistance heat) for the coldest days. The system automatically switches to backup heat when outdoor temperatures drop below a certain point—often around 35°F to 40°F, depending on the model—because at that point the compressor would use more electricity than a straightforward electric heater.

Indoor and outdoor units working together

An air heat pump has two separate units that must work in sync. The outdoor unit contains the compressor, the outdoor coil (evaporator), and a fan that pulls air across the coil. The indoor unit contains the indoor coil (condenser), a blower fan, and usually a filter. Refrigerant lines connect the two units, carrying the refrigerant back and forth.

The outdoor unit is where the hard work happens—it extracts heat from the outside air and compresses the refrigerant. The indoor unit is where you feel the results: warm air blows from vents into your home. In cooling mode, the roles reverse slightly. The indoor unit becomes the evaporator (pulling heat out of your home), and the outdoor unit becomes the condenser (releasing that heat outside). A reversing valve inside the system switches the direction of refrigerant flow to make this happen.

How efficiency is measured and what affects it

Air heat pump efficiency is measured by a number called COP, which stands for Coefficient of Performance. COP is the ratio of heat delivered to electricity used. A COP of 3 means the system delivers three units of heat for every one unit of electricity it consumes. The remaining two units come from the heat that was already in the outdoor air—you are not creating energy, you are moving it.

COP changes based on outdoor temperature. On a mild 50°F day, a heat pump might have a COP of 3.5 or higher. On a 20°F day, the same system might drop to a COP of 2 or lower, because the compressor must work harder to extract heat from much colder air. This is why heat pump performance is often rated at a standard temperature (usually 47°F) so you can compare different models fairly. The temperature difference between indoors and outdoors is the biggest factor: the smaller the gap, the easier the job, and the higher the efficiency.

Heating mode versus cooling mode

In heating mode (winter), the outdoor coil acts as an evaporator, pulling heat from outside air and sending it indoors. The indoor coil acts as a condenser, releasing that heat into your home. A fan in the outdoor unit pulls air across the cold outdoor coil, and a fan in the indoor unit blows warm air from the indoor coil into your living spaces.

In cooling mode (summer), the system reverses. The indoor coil becomes the evaporator, pulling heat and moisture out of your home. The outdoor coil becomes the condenser, releasing that heat outside. The reversing valve (a four-way solenoid valve) switches the direction of refrigerant flow. This is why a single air heat pump can both heat and cool—it is the same machine running in opposite directions. Some systems also have a defrost mode that briefly reverses to heating mode to melt ice that builds up on the outdoor coil during winter operation.

Common misconceptions about how air heat pumps work

Many people think heat pumps do not work in cold weather because they assume the outdoor air is too cold to contain usable heat. In reality, heat exists in air at any temperature above absolute zero (−459°F). Even air at 0°F contains heat that a heat pump can extract. The system works fine in cold climates—it just becomes less efficient and may need backup heat on the coldest days.

Another misconception is that heat pumps create heat. They do not. They move heat from one place to another using electricity to power the compressor. This is why a heat pump can never deliver more heat than the total energy (heat plus electricity) you put into it. A well-designed heat pump in mild weather can deliver more heat to your home than the electricity it uses, because it is moving heat from outside rather than creating it. But in very cold weather, when the compressor works hard and backup heat kicks in, the system may use more total energy than a straightforward electric heater—though it is still often more efficient than burning fossil fuels.

Frequently Asked Questions

Can an air heat pump work when it is freezing outside?

Yes. Cold air still contains heat energy that the heat pump can extract. However, efficiency drops significantly below freezing, and most systems include backup electric heat that activates automatically when outdoor temperatures fall below about 35°F to 40°F. The backup heat ensures your home stays warm without overworking the compressor.

Why does my heat pump make a hissing sound?

Hissing usually comes from the expansion valve, where high-pressure refrigerant is deliberately throttled down to lower pressure. This is normal operation. If the sound is very loud or accompanied by reduced heating, contact a technician—it may indicate a refrigerant leak.

What happens to the heat pump if the outdoor coil gets covered in ice?

Most heat pumps have a defrost cycle that automatically reverses the system briefly to heat the outdoor coil and melt ice buildup. During defrost, indoor heating stops temporarily (usually for 5 to 15 minutes) while the outdoor coil is cleared. Some systems use electric resistance heat to warm your home during defrost so you do not feel the interruption.

Why is my heat pump less efficient on very cold days?

The compressor must work much harder to extract heat from very cold air and raise the refrigerant temperature high enough to heat your home. The larger the temperature difference between indoors and outdoors, the more electricity the compressor uses per unit of heat delivered. This is why COP (efficiency) drops as outdoor temperature falls.

Can a heat pump heat a home as fast as a furnace?

Heat pumps typically heat more slowly than furnaces because they move heat gradually rather than creating it rapidly through combustion. On very cold days when backup heat activates, heating speed improves. If fast warm-up is important, ask about a system with electric resistance backup heat that can set up alongside the heat pump.