How do heat pumps work when it's freezing outside?

Cold air still has energy

Air is made of tiny particles called molecules. Even on a freezing day, those molecules are moving. Temperature tells us something about their average energy: colder means less energetic motion, not no motion at all.

Heat is energy passing from something warmer to something colder. Put a warm mug on a cold table and energy flows from the mug to the table. Neither needs to be hot in an everyday sense; one just needs to be warmer than the other.

The outdoor part of a heat pump contains a fluid called refrigerant. If that refrigerant is colder than the air around it, heat can flow from the air into the fluid. Even air below freezing can be the warmer side of that exchange.

Your fridge already does this

A fridge moves heat out of its cold interior and releases it into your kitchen. That's why the back or sides of a working fridge can feel warm. The cold inside hasn't appeared from nowhere; heat has been carried somewhere else.

A heat pump uses the same basic idea to warm a home. Its refrigerant travels around a closed loop, collecting heat outside and releasing it inside. In many systems, that released heat warms water for radiators or underfloor heating. Others warm indoor air directly.

Think of the refrigerant as a delivery van. It keeps doing the same route, picking up energy at one stop and dropping it off at another. The fluid isn't used up on each journey.

The loop has four important parts

First comes the outdoor heat exchanger, a set of tubes and fins with lots of surface area. Cold refrigerant absorbs heat from the air and boils into a gas. Refrigerants are chosen so this can happen at temperatures far below the boiling point of water.

Next, a compressor squeezes the gas. This takes electrical work and raises the gas's pressure and temperature. That extra step matters: a fluid only slightly warmer than freezing couldn't give heat to an already warm living room.

The now-hot refrigerant reaches the indoor heat exchanger. It gives up heat to the home's air or heating water and turns back into liquid. Finally, an expansion valve drops its pressure, making it cold enough to collect outdoor heat again. Then the loop repeats.

How can it give out more heat than the electricity going in?

An electric resistance heater turns electrical energy into heat. A heat pump uses electricity to move heat that already exists outside, as well as adding heat from the work it does.

Suppose a system uses one unit of electricity and collects two units of heat outdoors. It can deliver roughly three units of heat indoors. Nothing has been created from nothing. The missing energy in the electricity-only comparison came from the air.

Engineers call the ratio of heat delivered to electricity used the coefficient of performance, or COP. A COP of three describes that example. It isn't a promise for every heat pump, every house, or every winter day.

Freezing weather does make the job harder

As outdoor temperatures fall, the system has to move heat across a bigger temperature gap. Its efficiency usually falls, and some models also deliver less heating power. Cold-climate designs are built to handle tougher conditions, but they still have operating limits.

Frost can also collect on the outdoor coil. The system periodically runs a defrost cycle to clear it, temporarily using energy that isn't heating the house. Some installations use backup heating when the weather or demand calls for it.

This is why the useful buying question isn't simply, 'Does it work below freezing?' Ask whether a particular model, correctly sized and installed, can meet your home's heat demand at your local winter temperatures. Insulation, radiator sizing, controls, and electricity prices all affect the result.

The idea to take with you

A heat pump doesn't need hot outdoor air. It needs refrigerant colder than that air, plus electricity to lift the collected heat to a useful indoor temperature.