Do Heat Pumps Work in Cold Weather? The Honest Answer

Why a Heat Pump Can Pull Heat Out of Freezing Air

It sounds like a trick, so it's worth thirty seconds on how it works. Air at -5C still holds a lot of heat energy. It's only "cold" compared with your living room. A heat pump runs a refrigerant that boils at a far lower temperature than water (R290, the propane used in many new units, boils at about -42C at atmospheric pressure), so even freezing air is warm enough to evaporate it. The compressor then squeezes that vapour, which heats it up well past the temperature your radiators need, and the heat is handed over to your heating water.

The colder the air, the less heat there is to grab and the harder the compressor has to squeeze. That's the whole story of cold-weather performance: it keeps working, it just uses more electricity for each unit of heat it delivers.

What Cold Actually Does to Efficiency

Efficiency is measured as COP (coefficient of performance): units of heat out for every unit of electricity in. A COP of 3 means 3 kWh of heat for 1 kWh of electricity. Over a whole year it's called SCOP or SPF (the seasonal figure), and that's the one that sets your bills.

Two things move it. The first is the weather: the colder it is outside, the lower the COP. The second is the flow temperature, the temperature of the water the heat pump sends to your radiators, and that one you control. The lower it is, the more efficient the heat pump, which is why good installers fuss about radiator sizes. More on that in our guide to whether you need new radiators.

Heat pumps are sized on cold-day output, not mild-day output, so a correctly chosen unit has already been specified for the frost: its output at your local design temperature has to cover the house's heat loss.

What real British homes did in winter

Lab ratings are one thing. The best evidence we have from actual houses is the government-funded Electrification of Heat trial, run by Energy Systems Catapult. It installed and monitored 742 heat pumps in homes in the north east of England, the south east and Scotland. Its interim findings (March 2023):

• On the coldest day each one ran, the median air source heat pump managed a COP of 2.44 (the middle half 2.20 to 2.70), across 484 systems.
• Over a full year, the median air source system reached a seasonal efficiency of 2.94 for the heat pump itself, and 2.80 once backup heaters and circulating pumps are counted (291 systems).
• The mean flow temperature was 38.2C, and the lower a system's flow temperature, the higher its seasonal efficiency.

We'd expect a newer install designed for a low flow temperature to do better. Those homes were fitted between October 2020 and September 2021 (the trial's installation phase, per its interim report), in a trial that installed into a broad spread of house types.

If you want a sanity check from somewhere properly cold: Norway has about 650 heat pumps for every 1,000 households and Finland just over 540, according to the European Heat Pump Association. Nobody in Oslo is worried that it gets a bit chilly in Leeds.

How Low Can They Go? Minimum Operating Temperatures

For context, MCS's design standard gives design temperatures for eight UK reference locations, from about -1.5C in the south west to -5.9C in the west of Scotland. The operating limit is a safety margin, not a target. What you care about is output and efficiency at your design temperature, which is exactly what the heat loss survey pins down.

Defrost Cycles: The Steam Is Normal

The first frosty morning with a new heat pump can catch you out. In damp air around freezing, moisture freezes onto the outdoor unit's fins. Left alone, that ice would block the airflow, so the heat pump runs a de-icing cycle for a few minutes to warm the outdoor coil and melt it. Vaillant, for example, says its aroTHERM plus does this when the air drops below 2.5C.

What you'll notice:

• A cloud of steam pouring off the unit. It looks alarming and it's completely harmless.
• The fan stops and there's a hiss or a whoosh as the refrigerant changes direction, then a trickle of meltwater from underneath.
• Radiators cool slightly for a few minutes, then pick straight back up.

That meltwater needs somewhere to go. A good installer mounts the unit on feet or a bracket with a clear drain path, because meltwater that refreezes into a sheet of ice under the unit (or across your path) is the one genuinely annoying defrost problem.

Defrost is also why a heat pump can sound different in winter. If you're worried about noise, our heat pump noise guide covers what's normal and what isn't.

What a Freezing Day Actually Costs

Here's the honest version, using the 1 October to 31 December 2026 price cap (electricity 26.32p/kWh, gas 7.97p/kWh). A gas boiler at 90% efficiency delivers heat at 8.86p per kWh. A heat pump's cost per kWh of heat is the electricity price divided by its COP:

So yes, on the very coldest days a kWh of heat from a heat pump on a standard tariff costs more than a kWh from a boiler. Take a home that needs 100 kWh of heat on a freezing day: that's about £10.79 through the heat pump at the trial's coldest-day COP, against £8.86 of gas. On a heat pump tariff the same day comes to roughly £8.20. The "about 20p" is our estimate of the blended rate a heat pump realistically gets on a heat pump tariff, which prices its cheap windows regionally rather than at one national figure.

The thing that matters is the whole winter, not the worst day. Over a whole year the heat pump runs well above its coldest-day figure: the trial's year-round median was 2.94 against 2.44 on the coldest day. Across the year, a system at our standard SCOP of 3.5 averages out at around 7.52p a kWh of heat against the boiler's 8.86p. Break-even is a seasonal efficiency of 2.97. Be honest with yourself about what that means. The trial's median system, at 2.94 for the heat pump alone (8.95p a kWh) or 2.80 for the whole system (9.40p), comes out roughly level with gas or a little behind at today's prices, rather than ahead. The trial also found that the lower a system's flow temperature, the higher its seasonal efficiency, and getting the flow temperature down is what a good design is for. Our running costs guide goes through the annual numbers in detail, or try the calculator below with your own bill.

Winter Settings That Actually Help

Use weather compensation and leave it alone

Weather compensation reads the outdoor temperature and sets the flow temperature to match: cooler water on mild days, warmer on cold ones. It's the setting most worth getting right. Ask your installer to set the curve so the house is just warm enough on the coldest days, then leave it running.

Don't turn it off overnight

Boiler habits die hard. A boiler blasts the house back up to temperature quickly, so switching it off at night makes sense. A heat pump works best running steadily at a low flow temperature. Turn it off on a freezing night and it has to recover a cold house in the morning at a high flow temperature, which is its least efficient way of working. A small overnight setback of a degree or two is fine. Off is not.

Keep the outdoor unit breathing

Clear fallen leaves in autumn and brush off drifting snow so the fins and the drain stay clear. Don't build a box around it to "protect" it: it needs a free flow of air, and the manufacturer sets minimum clearances for a reason. A light cover over the top against heavy snow is fine if it doesn't restrict airflow.

Know how your system is protected from freezing

A monobloc heat pump has water pipes running outside, so it needs freeze protection. There are two common approaches. Some systems run a glycol (antifreeze) mix through the circuit, which protects it even in a power cut. Some service plans exclude these (British Gas's repair cover doesn't cover glycol systems). Others use plain water with freeze-protection valves that dump water from the outdoor pipework if it gets close to freezing during a power cut. The unit's own controls run the pump in cold weather to stop water sitting still. Neither is wrong. Just ask which one you have, because it changes what you'd do after a long power cut.

Check the immersion and backup heater settings

If your system has a backup electric heater, it can kick in during the coldest weather. A little use on the worst days is expected. If your electricity use jumps and the house still isn't warm, the backup may be doing work the heat pump should be doing, which is a sign the system is undersized or set up badly.

When Cold Really Is a Problem

When a heat pump leaves a house cold, these are the things to check, before blaming the weather:

• No proper heat loss survey. MCS's design standard requires a heat load calculation to BS EN 12831-1:2017, and MCS provides its own calculator for exactly that. It's what tells the installer how much heat each room needs on the coldest day. A quote based on the size of your old boiler is a guess.
• An undersized unit. If the heat pump can't meet the home's heat loss at design temperature, it'll hold on through mild weather and fall behind in a cold snap, leaning on the backup heater to catch up.
• A high flow temperature. Small radiators forcing the system to run at 55C or above push efficiency down and make every cold day more expensive.
• A leaky house. You don't need a passivhaus, but obvious gaps (no loft insulation, open chimneys, draughty doors) raise the heat loss and the size of unit you need. Our guide to insulating before a heat pump covers what's worth doing first and what isn't.

All four are design problems, and all four are sorted before installation by an installer who does the job properly. That's the real answer to "will it keep me warm in January": get at least two or three MCS installers to survey the house and compare how they've sized it. If one quotes a much smaller unit than the others without explaining why, that's your warning. The Boiler Upgrade Scheme only works through MCS installers anyway, so you'll need one either way. We've written up how to compare heat pump quotes, and you can get quotes from MCS installers near you in a couple of minutes.

If you only do one thing: ask every installer to show you the heat loss figure for your home and the unit's output at your design temperature. If the output covers the heat loss, the cold isn't your problem.