How do heat pumps work in winter?

It's the doubt that sits behind almost every heat pump conversation. A system that takes heat out of the air sounds fine in April. What happens in February, when it's dark, damp and two degrees below?

The short version: it keeps working just fine. Heat pumps run through winter in Norway, Sweden and Finland, where the winters are considerably harder than ours. What changes in the cold is not whether your home gets warm, but how the system goes about it — and that difference is worth understanding before you get a quote.

The short answer

A heat pump works in winter the same way it works in summer: it moves heat from the outside air into your home rather than burning fuel to create it. Cold air still contains usable energy, so the system keeps extracting heat well below freezing. Most current models are designed to keep running down to around -25°C, and some cold-climate units to -35°C, according to Energy Saving Trust. For context, the UK's average winter temperature in 2024 was over 5°C.

Efficiency does fall as it gets colder. In the government-funded Electrification of Heat trial, which monitored 742 heat pumps in British homes, the median air source system efficiency was 2.80 across the year and 2.44 on the coldest days measured, when mean daily temperatures dropped as low as -6°C. In other words, on the worst days of a British winter, the heat pumps in that trial were still delivering roughly two and a half units of heat for every unit of electricity they used.

Do heat pumps work in cold weather?

Yes, and the evidence for it is not theoretical. The four European countries with the highest heat pump ownership — Norway, Sweden, Finland and Estonia — are also among the coldest. Around two-thirds of Norwegian homes have one, in a country whose average temperature is close to -7°C.

The physics behind it is not complicated. There is still heat energy in air at 0°C, or at -10°C, because "cold" is relative to us rather than to absolute zero. A heat pump uses a refrigerant to capture that energy, compress it to a higher temperature, and pass it into your heating system. Our guide to how heat pumps work covers the mechanism in full.

What the cold changes is the effort. The colder the outside air, the harder the compressor works to lift that heat to a useful temperature, and the more electricity it uses to do it. That is the efficiency drop the trial measured. It is a gradual slope, not a cliff edge, and it is exactly what a properly designed system is sized to absorb.

What actually changes when the temperature drops

Three things behave differently on a cold day, and all three are normal.

• The system runs for longer. A heat pump is designed to trickle heat into your home steadily rather than blast it in short bursts like a gas boiler. On a cold day it will run for most of the day at a higher output. Our piece on why a heat pump runs all the time explains why that steady running is a feature rather than a fault.

• The flow temperature rises. Most systems use weather compensation, which reads the outside temperature and raises the temperature of the water going to your radiators only as much as the weather demands. In the Electrification of Heat trial, these controls rarely needed to call for high flow temperatures, which is a large part of why efficiency held up.

• The unit defrosts itself periodically. In cold, damp weather, moisture in the air freezes onto the outdoor heat exchanger. Left alone, that frost would block the transfer of heat, so the system briefly reverses to melt it before returning to normal. You may notice a change in sound, a puff of vapour, or water draining beneath the unit. All of it is the system doing its job.

Will it keep the house warm on the coldest day?

A well-designed system should hold a steady temperature through the whole year, including the coldest days, without you falling back on plug-in heaters. Energy Saving Trust's position is the same: you shouldn't need additional heating to stay comfortable, and most systems carry a boost setting for exceptional conditions that uses a little more electricity for a short period.

The phrase carrying the weight there is "well-designed". A heat pump that struggles in January is almost always a sizing or design problem, not a technology problem. MCS-certified installers size a system against a local winter design temperature drawn from published UK climate data, so the calculation is done for the coldest conditions your postcode realistically sees, not for an average day.

This is also where the whole-system point matters. A heat pump installation is not a like-for-like boiler swap. It usually includes a hot water cylinder, and often larger radiators and some pipework changes, because lower-temperature water needs more surface area to give off the same heat. Bigger radiators don't necessarily mean bigger radiators on the wall — a double or triple panel gives more surface area in the same footprint. Those upgrades are what let the system run at a low flow temperature, which is what keeps it efficient when the weather turns. The comfort comes from the whole system, not from the box outside.

Winter running costs, without the guesswork

Running costs rise in winter with any heating system, because your home is losing more heat and needs more of it replaced. With a heat pump, the seasonal efficiency drop adds a little on top of that.

Energy Saving Trust's current assessment is that heat pump running costs sit around the same as a new gas boiler at present energy prices, and lower than an older boiler, with heat pumps generally around four times as efficient. The reason the efficiency advantage doesn't translate into a proportionate bill saving is the price gap between electricity and gas rather than anything to do with the technology. A heat pump time-of-use tariff, which charges different rates at different times of day, is where a lot of owners find the difference. Our comparison of a heat pump and a gas boiler goes through this in more detail.

Two things reduce a winter bill more than any thermostat habit: a low flow temperature, which depends on the design and the emitters, and a home that loses less heat in the first place. Insulation isn't a prerequisite for a heat pump, and a good installer will design around what your home has, but it does change what the system has to do in January.

Getting your heat pump ready for winter

Heat pumps need about as much maintenance as a gas boiler, and the same principle applies: service it before the cold arrives, not during it. An annual service in autumn catches the things that only show up under load.

Beyond that, keep the outdoor unit clear of leaves, snow and anything that has blown against it over autumn, since the unit needs free airflow to pull heat from the air. Resist the urge to turn the system off when you go away in winter; heat pumps are far more efficient holding a steady temperature than recovering a cold house from scratch. And if something sounds or behaves clearly differently from last winter, that is worth a call rather than a wait.

Every installation arranged through Ample is covered by our MCS certification and our installation standards, and on a Heat Pump Plan the annual service, remote monitoring and any repairs are included for the life of the plan, with no callout fee. Winter is when that matters.

See what winter would look like in your home

The figures above describe heat pumps in general. What matters is your home: how much heat it loses, what radiators it has, and what the coldest week of your year actually looks like. Get an instant estimate in minutes, with no survey and no obligation, and you'll have a real starting point rather than a national average.

Frequently asked questions

Do heat pumps stop working below freezing?
No. Cold air still holds usable energy, and most current models are designed to run down to around -25°C. Efficiency falls as it gets colder, but the system keeps producing heat.

Why does my heat pump run constantly in winter?
Because that's how it's meant to work. A heat pump delivers heat gently and continuously rather than in short, hot bursts, and cold weather simply means it runs for more of the day at a higher output.

What is a defrost cycle and should I worry about it?
It's the system briefly melting frost off the outdoor unit so it can keep absorbing heat from the air. It happens automatically in cold, damp conditions, lasts a few minutes, and is normal operation rather than a fault.

Do I need a backup heater for winter?
A properly sized system shouldn't need one. Most heat pumps include a boost function for exceptional cold, and Energy Saving Trust's position is that a well-designed system keeps a home comfortable year-round without additional heating.

Are heat pumps more expensive to run in winter?
Your heating bill rises in winter whatever you heat with, because the house is losing more heat. A heat pump's efficiency also dips slightly in the cold. Energy Saving Trust puts heat pump running costs at around the same as a new gas boiler at current prices, and cheaper than an older one.

Does my home need to be fully insulated first?
No. Insulation lowers the bill with any heating system, and it helps, but it isn't a condition of getting a heat pump. A good installer will design the system around the home you have.