How a heat pump gets heat from cold air
Air at minus 3°C still holds heat; it is only cold compared with your living room. A heat pump circulates a refrigerant that boils at a very low temperature, so even freezing air warms it enough to turn it into gas. A compressor then squeezes that gas, which raises its temperature, and the heat passes into the water for your radiators and cylinder. The colder the air, the harder the compressor works for the same heat, which is why efficiency falls in winter. It does not stop.
In cold, damp weather the outdoor unit also defrosts itself from time to time, which is why you may see it steaming. That is part of normal operation.
What the UK trial measured
The Electrification of Heat Demonstration Project, funded by the government and run by Energy Systems Catapult, monitored heat pumps in 742 British homes through real winters. Its December 2024 performance report gives the efficiency of the median air source system at different outdoor temperatures, measured over 30-minute periods:
| Outdoor temperature | Heat out per unit of electricity | Heat cost per kWh at the cap (26.32p) |
|---|---|---|
| 10°C (peak) | 3.37 | 7.8p |
| Minus 3°C | 2.37 | 11.1p |
| Minus 5°C | 2.3 | 11.4p |
| Direct electric heater, any weather | 1.0 | 26.3p |
Over a whole year, the median air source system measured 2.78 on the same whole-system measure, and 85% of households said they were likely to recommend a heat pump after one heating season. The trial’s headline finding, from its earlier report, was that there is no property type or architectural era that is unsuitable for a heat pump (December 2021).
What a cold week does to the bill
Two things happen at once in a cold spell: the house needs more heat, and each unit of heat costs more to make. At the 1 October to 31 December 2026 cap, heat at minus 3°C costs about 11.1p per kWh from the median system, against 7.8p on a mild 10°C day. So a cold week costs noticeably more than a mild one. Over a whole season the two average out: the median annual figure was 2.78, between the cold-day and peak readings.
Why design matters most in winter
A heat pump sized and designed for your home’s heat loss on a cold day will keep it warm through a cold snap; one that is undersized, or running a high flow temperature to make up for small radiators, will struggle and cost more. In the trial, efficiency fell steeply as flow temperature rose, from 3.5 at a 35°C flow to 2.38 at 55°C. That is the reason a room-by-room heat-loss calculation and the right radiators matter more than the brand. The sizing and radiators guide explains what to ask for.
Air source or ground source in the cold
A ground source system draws heat from the ground, which stays at a steadier temperature than the air through winter. The trial’s ground source sample was small, but its median efficiency at minus 5°C was 3.1, against 2.3 for air source. Ground source costs more to install and needs land or a borehole; for most homes, a well-designed air source system is the practical choice. Both get the same grant: see the air source heat pump grant for what the £7,500 covers.
Getting through a cold snap
- Leave it running. Turning the heating off and back on makes the system work at a higher flow temperature to recover, which is less efficient.
- Let weather compensation do its job. It raises the flow temperature only as much as the outdoor temperature requires.
- Keep the outdoor unit clear of snow, leaves and anything that blocks airflow.
- Check any back-up heater. If an electric immersion or boost is cutting in often, ask the installer why.