How a heat pump warms a home when it is freezing outside

The cycle, step by step

1. Evaporation outside. Liquid refrigerant at low pressure passes through the outdoor coil. Because its boiling point is far below the air temperature, it absorbs heat from the air and turns into a gas ¹.
2. Compression. The compressor raises the pressure of that gas. Compressing a gas raises its temperature, so the refrigerant leaves the compressor at 50°C or more ¹.
3. Condensation inside. The hot gas flows through the indoor coil, where it gives up heat to the air or water of the heating system and condenses back to a liquid ¹.
4. Expansion. An expansion valve drops the pressure again, the refrigerant cools sharply, and the loop begins again ¹.

The same machine runs in reverse in summer to cool the house, which is why most heat pumps double as air conditioners ³.

Why it beats a heater on efficiency

An electric resistance heater turns one unit of electricity into one unit of heat. A heat pump uses one unit of electricity to move two, three or four units of heat from outside. That ratio is the coefficient of performance, or COP ⁴. The COP falls as the outdoor temperature drops, because there is a bigger temperature gap for the compressor to bridge, but it stays above 1 down to very low temperatures ¹⁴. A 2023 analysis of field data from seven studies in Europe and North America found average efficiencies of 2 to 3 at temperatures around -10°C, with cold-climate models still well above resistance heating at -30°C ².

What changes in a cold-climate model

• Variable-speed compressors that run faster when the outdoor air is colder, keeping output up instead of cycling on and off ³.
• Enhanced vapour injection, which adds a second stage to the compression so the refrigerant reaches a useful temperature from a lower starting point ¹.
• Refrigerants such as R-32 or propane with very low boiling points, which extract heat more effectively from freezing air ¹.
• Defrost cycles. Frost forms on the outdoor coil in damp, cold weather; the unit briefly reverses to melt it, which is why you may see steam rising from it on a winter morning ¹³.

Norway, Sweden and Finland, which have some of the coldest winters in Europe, also have the highest heat pump ownership per household, which is a practical demonstration that the technology works in the cold ⁵.

When a heat pump struggles

Output does fall on the coldest days, so sizing matters. A unit chosen for the average winter day may need backup, either an electric resistance element built into the air handler or an existing boiler in a "hybrid" setup ³. Poorly insulated houses that need very hot radiators are the hardest case, because raising the water temperature lowers the COP. Bigger radiators, underfloor heating or better insulation restore the efficiency ¹⁵.