Guide · Running costs
Do heat pumps work in cold weather in the UK?
The most common objection — and what the design temperature data actually says.
Yes. Air source heat pumps sold in the UK operate down to about -20°C outdoor temperature, and UK systems are designed to meet full heat demand at a local design temperature of -1°C to -4°C depending on region. Efficiency falls in a cold snap — a unit achieving a COP of 4.0 at 7°C typically delivers 2.2–2.8 at -5°C — so bills rise for those days, but the house still reaches temperature.
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What design temperature means
Every UK region has a design outdoor temperature used for heat loss calculations — colder in the Highlands than in Cornwall. Your installer sizes the heat pump so it meets the house’s full heat loss at that temperature without backup heat.
That is why sizing must come from a room-by-room survey. A unit sized for average weather will struggle in January.
Defrost cycles are normal
Between roughly -5°C and +7°C with damp air, frost forms on the outdoor coil. The unit briefly reverses to melt it, then resumes heating. You may see steam and hear a change in tone. A few short defrosts an hour in damp cold weather is normal operation, not a fault.
Cold-weather checklist
- Keep the outdoor unit clear of snow, leaves and drifting debris.
- Do not turn the system off overnight in a cold snap — steady low-temperature running is cheaper than a big morning recovery.
- Check the condensate run is protected against freezing.
- Ensure weather compensation is enabled so flow temperature rises automatically as it gets colder.
Frequently asked questions
- At what temperature do heat pumps stop working?
- Most models sold in the UK keep operating to around -20°C, far below typical UK winter lows.
- Do heat pumps struggle in winter?
- Efficiency falls in deep cold, so running costs rise for those days, but a correctly sized system still heats the home without backup.
- Should I turn my heat pump off when it is freezing?
- No. Continuous low-temperature operation is more efficient than switching off and recovering the house from cold.
- Energy Saving Trust — Heat pump efficiency, running cost and SCOP benchmarks (UK).
- Nesta — How to Heat 28 Million UK Homes — Independent research on heat pump deployment and running costs.
- MCS Certified — Installer certification scheme required for grant eligibility.
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