There is heat in cold air
"Heat" in physics just means energy in the movement of molecules, and air at minus five degrees still has plenty of it. What matters for a heat pump is whether the refrigerant in its outdoor coil is colder than the air passing over it. It is: the expansion valve drops the refrigerant to a temperature well below the outside air, so heat flows from the air into the refrigerant even in a hard frost. The cycle carries on; it just has further to lift that heat.
What changes as it gets colder
- Efficiency falls. The gap between the outside air and the water the home needs grows, so the compressor does more work per unit of heat.
- Output falls. A unit's maximum heat output is lower in cold air than in mild air, at the moment the home needs most.
- The home needs more heat. Heat loss rises as the outside gets colder, so demand and capacity move in opposite directions.
That combination is why heat pumps are sized for the cold end of the year rather than the average. A design picks a cold outdoor temperature for the location and checks that the heat pump can meet the home's heat loss at that temperature.
Defrosting
When the air is cold and damp, moisture freezes on the fins of the outdoor coil, just as frost forms on a car windscreen. Ice blocks the air flow and cuts the heat pump's ability to collect heat. So the unit defrosts itself: it briefly runs the cycle in reverse, sending warm refrigerant to the outdoor coil to melt the ice, then switches back. During a defrost the home is not being heated for a few minutes, and the meltwater may give off a cloud of vapour that looks like smoke. It is normal, and the system is designed for it.
Defrosting uses energy without heating the home, which is one reason measured seasonal efficiency is lower than laboratory figures. Ground source heat pumps take heat from the ground, which does not freeze around the loop in the same way, so they do not need to defrost.
What the field evidence shows
The Electrification of Heat demonstration measured 742 heat pumps in British homes through real winters. Its interim median seasonal figure for air source systems, 2.80, already includes cold spells and defrosting, because it covers the whole season. The project concluded that heat pumps can be installed successfully in all the property types it tested. The government's general figure of around 3 units of heat per unit of electricity is likewise a seasonal average, not a mild-day figure.
Living with a heat pump in winter
- Leave the heating running steadily rather than switching it off and on; recovering a cold house needs a high flow temperature.
- Keep snow, leaves and ice clear of the outdoor unit and its drain.
- Expect radiators to feel warm rather than hot, even in a frost; the room temperature is what counts.
- If the backup heater runs often, ask the installer to check the sizing and settings.
The controls guide explains how weather compensation raises the flow temperature only as much as each cold day needs.
Ground source in winter
A ground source system draws heat from a loop buried where temperatures change slowly through the year, so its source stays steadier in a cold snap than outside air does. It still feels the cold indirectly, because the home needs more heat and the loop is asked for more, but it does not lose output to frosted coils.