Do heat pumps work in cold weather?

How can a heat pump get heat out of freezing air?

Cold air still contains heat. What we call freezing is only cold relative to us: air at -5°C holds a great deal of heat energy compared with the far colder refrigerant inside the outdoor coil. That refrigerant boils at a very low temperature, so as long as it is colder than the air, heat flows into it. The compressor then raises it to a temperature your radiators can use.

What changes in cold weather is how hard the compressor has to work. The bigger the gap between the outside air and the water temperature your radiators need, the more electricity it takes to bridge it. So efficiency drops gradually as the temperature falls. It does not fall off a cliff at 0°C.

How well did heat pumps actually perform on the coldest days?

The best British evidence is the Electrification of Heat demonstration project, funded by the government. It installed 742 heat pumps across Scotland, the north east and the south east of England, in a broad mix of house types and ages, and metered them between November 2020 and August 2022.

The report puts the mean outdoor temperature on that coldest day at -0.4°C. On the worst day of the winter, the typical system still produced 2.44 kWh of heat for every 1 kWh of electricity. A direct electric heater, by comparison, produces 1 kWh of heat per kWh on every day of the year.

The report is also candid that performance over a single day varies a lot from home to home, and that efficiency falls as the gap between indoor and outdoor temperature widens. That is exactly what the physics predicts, and it is why sizing matters.

How cold does it really get in the UK?

Met Office records put the UK's average winter (December to February) temperature at 4.1°C over 1991 to 2020, with an average daily minimum of 1.3°C. Winter 2025 to 2026 averaged 4.9°C, with a mean daily minimum of 2.5°C. Frosty nights and cold spells happen, but long stretches well below freezing are unusual for most of the country.

Heat pumps are not designed for the average day, though. They are designed for a cold day that your area only rarely drops below. MCS, the certification body for heat pump installers, publishes those design temperatures from CIBSE data.

Put simply, a heat pump in West London is sized to keep your rooms at full comfort when it is -1.7°C or -3.0°C outside, depending on which column the installer uses. It keeps running on the rare hours that are colder still; the design simply accepts that those hours are few.

How is a heat pump sized so it copes in a cold snap?

Under the MCS heat pump design standard (MIS 3005-D), an installer must follow a set procedure. It is worth knowing, because it tells you what a proper quote should contain.

1. A room-by-room heat loss calculation to BS EN 12831-1:2017. Heat loss is how many watts each room leaks through walls, windows, floor, roof and draughts at the design temperature.
2. Indoor design temperatures of at least 21°C in living and dining rooms, 22°C in bathrooms and 18°C in bedrooms, halls and kitchens.
3. The outdoor design temperature for your location, from the table above.
4. A heat pump chosen to supply at least 100% of that calculated heat load at the planned flow temperature, without counting any electric backup heater.
5. A system able to hold those indoor temperatures across multiple defrost cycles.

Flow temperature is the temperature of the water sent to your radiators. The lower it can be while still meeting the heat loss, the more efficient the heat pump is in every kind of weather. That is why radiator sizing is part of the design, not an afterthought.

What happens to the outdoor unit in frost and ice?

In cold, damp weather, typically around freezing with high humidity, moisture from the air can freeze onto the outdoor coil. The heat pump detects this and runs a short defrost cycle, briefly reversing to warm the coil and melt the ice. You may see a cloud of vapour and hear the fan stop and restart. It is normal, and a well designed system keeps your rooms at temperature through it.

• Leave space around the unit so air can flow freely, and keep it clear of drifting snow and leaves.
• Make sure meltwater can drain away, not pool and refreeze on a path where someone could slip.
• Do not cover the unit in winter. It needs the air.

What does a cold spell do to your bills?

Your bill rises in a cold week for two reasons: the house needs more heat, and each unit of heat takes more electricity. Here is the arithmetic at the Ofgem price cap for 1 October to 31 December 2026, where electricity is 26.32p per kWh and gas 7.97p per kWh (Direct Debit averages; electricity is VAT-free from October 2026 to March 2027).

On the coldest days a heat pump costs a little more per unit of heat than gas at these prices, and on mild days less. What you pay over the year follows the seasonal figure, and a heat pump tariff with cheaper hours changes the picture again. Our running costs guide works through whole-year numbers.

Why do some heat pumps struggle in winter?

When someone tells you their heat pump could not cope with the cold, it is almost always one of these, and none of them is the weather's fault.

• Undersized: the unit was picked without a room-by-room heat loss calculation, so it cannot meet demand at the design temperature.
• Radiators too small for the flow temperature, so the system is turned up hotter and its efficiency drops just when it is needed most.
• Controls set up like a gas boiler: short bursts of heat twice a day instead of steady, weather-compensated running.
• A leaky house. If the loft and walls are uninsulated, heat escapes faster than a sensibly sized unit can replace it. Our insulation guides explain where to start.

The Electrification of Heat analysis found that systems with lower average flow temperatures achieved higher efficiency, and that the spread between installations remained wide. Design and commissioning quality is the variable. As an MCS-certified installer, Smart Energie has to follow the MIS 3005-D procedure above on every heat pump, and we will show you the heat loss figures room by room.

What should you ask an installer before winter?

• Which outdoor design temperature did you use for my postcode, and which column?
• Can I see the room-by-room heat loss figures?
• What flow temperature is the system designed for, and which radiators change?
• What is the heat pump's output at that design temperature and flow temperature, from the manufacturer's data?
• How much of the year's heat, if any, do you expect the backup heater to supply?
• Is weather compensation set up and explained to me at handover?

If you are weighing up whether your home is ready, the retrofit planner takes you through it, and you can check the Boiler Upgrade Scheme at the same time.

Common questions