Air source heat pumps pull warmth from outside air and move it indoors

An air source heat pump works by extracting heat from the air outside your home — even when it feels cold — and moving that heat indoors to warm your living space. In summer, it reverses the process and pulls heat out of your home to cool it. The system does not generate heat the way a furnace burns fuel. Instead, it uses electricity to pump existing heat from one place to another, which is why it can heat a home even on a 30-degree day.

The core of the system is a refrigerant — a liquid that changes state easily between liquid and gas. This refrigerant circulates through a closed loop of pipes, changing pressure and temperature as it moves. When pressure drops, the refrigerant absorbs heat from the outside air. When pressure rises, it releases that heat indoors. A compressor powered by electricity does the work of changing the pressure.

Key Takeaways

  • Air source heat pumps move heat from outside to inside using a refrigerant and compressor, rather than generating heat through combustion or electric resistance.
  • The outdoor unit contains a coil that absorbs heat from the air; the indoor unit releases that heat into your home through ducts or a fan-coil system.
  • A reversing valve lets the system switch between heating in winter and cooling in summer by changing the direction the refrigerant flows.
  • The system's efficiency drops as outdoor temperature falls, so most air source heat pumps include a backup electric heater for the coldest days.
  • Ductless (mini-split) systems mount an indoor unit on the wall; ducted systems use existing ductwork like a traditional furnace.

The outdoor unit absorbs heat from the air

The outdoor unit sits on the ground or mounted to a wall outside your home. Inside it is a coil filled with cold refrigerant. Air passes over this coil — pulled by a fan — and the refrigerant absorbs heat from that air. Even air at 20 degrees contains heat energy; the refrigerant straightforward pulls it out.

As the refrigerant absorbs heat, it evaporates and turns into a gas. This gas then travels through a copper pipe into your home, carrying that heat with it. The outdoor unit also contains a fan that runs year-round to keep air moving across the coil, so the system can continue pulling heat even on very cold days.

The compressor pressurizes the refrigerant to release heat indoors

Inside your home, the refrigerant gas enters the compressor — an electric pump that squeezes the gas, raising its pressure and temperature. This hot, pressurized gas then flows into the indoor coil. As air from your home passes over this coil, the refrigerant releases its heat into your living space. The refrigerant cools down and condenses back into a liquid.

The compressor is the part that uses the most electricity in the system. Its job is to do the mechanical work of raising the pressure so the heat can move from a cold place (outside) to a warm place (inside). This is the opposite of what happens naturally — heat naturally flows from warm to cold — so the compressor has to push against that tendency.

A reversing valve switches the system between heating and cooling

A reversing valve is a four-way switch inside the system that changes which coil is indoors and which is outdoors. In heating mode (winter), the outdoor coil absorbs heat and the indoor coil releases it. In cooling mode (summer), the indoor coil absorbs heat from your home and the outdoor coil releases it to the outside air.

When you switch your thermostat from heat to cool, the reversing valve flips, and the refrigerant flow reverses direction. The same pipes and coils now serve the opposite purpose. This is why a heat pump can both heat and cool without needing two separate systems.

Efficiency drops in very cold weather, so backup heat kicks in

As outdoor temperature falls below about 35 degrees, the air contains less heat energy, and the heat pump has to work harder to extract it. The compressor runs longer and uses more electricity. Below about 20 degrees, most air source heat pumps become less efficient than a backup heating source.

For this reason, most air source heat pump systems include a backup electric heater — usually a resistance coil similar to a space heater element. When outdoor temperature drops below a set point (often 20 to 30 degrees, depending on the system), the backup heater turns on automatically. This keeps your home warm without forcing the compressor to work at very low efficiency. In the coldest climates, the backup heater may run for weeks at a time during winter.

Ductless systems mount an indoor unit on the wall; ducted systems use existing ducts

A ductless (mini-split) system has a small indoor unit mounted on a wall or ceiling in the room you want to heat or cool. Refrigerant pipes run from the outdoor unit to this indoor unit through a small hole in the wall. The indoor unit has its own fan and blower, so it heats or cools the air in that room directly. Ductless systems are common in homes without existing ductwork, or when you want to heat only certain rooms.

A ducted system uses the same ductwork as a traditional furnace. The indoor unit sits in your attic, basement, or utility closet and connects to your existing ducts. Air flows through the ducts to each room in your home. Ducted systems work well if you already have ductwork in place and want to replace a furnace with a heat pump.

The refrigerant cycle repeats continuously to maintain temperature

Once the refrigerant releases heat indoors and condenses back to a liquid, it flows back outside through the return pipe. The pressure drops as it enters the outdoor coil, and the cycle begins again. The refrigerant circulates continuously — typically 4 to 6 times per minute — as long as your thermostat calls for heating or cooling.

The system modulates (adjusts) the compressor speed to match the heating or cooling load. On a mild day, the compressor runs at lower speed and cycles on and off. On a very cold or very hot day, it runs at higher speed more often. Modern heat pumps use variable-speed compressors that can adjust their output smoothly, rather than running at full power or shutting off completely.

Frequently Asked Questions

Can an air source heat pump work when it's freezing outside?

Yes, but with reduced efficiency. Air at 20 degrees still contains heat that the system can extract. However, the compressor has to work much harder, and the backup electric heater usually turns on to help. Most systems are designed to operate down to about minus 10 degrees, though efficiency drops significantly below 20 degrees.

Why does my heat pump make noise?

The outdoor fan and compressor create noise similar to an air conditioning unit. A reversing valve can make a hissing sound when it switches between heating and cooling. Some noise is normal. If the sound is loud, grinding, or new, the system may need service.

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

A heat pump uses electricity to run the compressor and fans, while a furnace burns gas or uses electric resistance. Heat pumps are more efficient in mild weather but use more electricity in very cold weather when the backup heater runs. Total cost depends on your local electricity and gas prices.

What happens if the refrigerant leaks?

The system will stop heating or cooling effectively. Refrigerant is sealed inside the pipes and should not leak under normal conditions. If a leak occurs, a technician must find and repair it, then refill the refrigerant. The system cannot operate without the correct amount of refrigerant.

Do I need to maintain an air source heat pump?

Yes. The outdoor coil should be cleaned of leaves and debris at least once a year. Indoor filters need changing every few months, like a furnace. A technician should inspect the system annually to check refrigerant levels and compressor operation. Regular maintenance keeps the system running efficiently and extends its life.