Do Heat Pumps Work in Cold Weather in the UK?

Yes, heat pumps work in cold weather in the UK, and modern air source models keep heating homes well below anything a normal British winter throws at them. Leading units are rated to run at outdoor temperatures down to -25°C or lower, while the design temperatures installers use for the UK’s main reference locations sit between -1.5°C and -5.6°C.

What changes in the cold is efficiency, not whether the heat pump works. A heat pump uses more electricity to deliver each unit of heat on a freezing day than on a mild one, but it still delivers well over twice the heat of a plain electric heater.

How Cold Can a Modern Air Source Heat Pump Operate?

Leading air source heat pumps from major manufacturers are rated to operate at outdoor temperatures down to -25°C, and some models to -28°C. The UK’s lowest recorded temperature is -27.2°C, and UK design temperatures sit roughly 20°C or more above these limits.

An operating limit is the temperature at which the unit will still run and produce heat. It is not the temperature at which it delivers its full rated output. Output falls as the air gets colder, which is why the heat pump has to be designed around the coldest conditions your home is likely to face.

How Cold Does It Actually Get in the UK?

UK winters are mild, with average winter temperatures around 5°C and design temperatures for heat pump sizing of -1.5°C to -5.6°C across the 14 reference locations MCS uses. Using Met Office data, the government’s long-term means for 1991 to 2020 are 5.2°C in December, 4.9°C in January and 5.2°C in February.

Installers do not size heat pumps for the average day. They use an external design temperature, which is a cold figure that is only beaten for a small fraction of the year. The MCS Heat Load Calculator takes these from CIBSE Guide A, using the 99.6% figure by default. That means the outdoor temperature is expected to be at or above the design figure for 99.6% of the time.

Homes on higher ground get a colder design figure. The MCS calculator lowers the design temperature by 0.6°C for every whole 100m that a property sits above its reference weather station. A farmhouse in the Pennines, the Welsh hills or the Scottish Highlands will therefore be designed for colder conditions than a house in the nearest city.

Truly extreme cold is rare. The UK’s lowest recorded temperature is -27.2°C, measured at Braemar in 1982 and at Altnaharra in 1995, according to the Met Office.

How Efficient Is a Heat Pump When It Is Freezing Outside?

A typical UK air source heat pump still produces around 2.3 to 2.4 units of heat for every unit of electricity at -3°C to -5°C, compared with about 3.4 at 10°C. That comes from the Electrification of Heat project, a government-funded trial that monitored 742 heat pump systems in homes across Great Britain. The project’s interim findings said heat pumps performed well even on the coldest observed days, with only a relatively small reduction in performance.

International research tells the same story at much lower temperatures. A 2023 review in the journal Joule by researchers including Jan Rosenow and Richard Lowes gathered field and lab studies from cold climates. It found an average COP of 2.74 between -10°C and -5°C, and that COP stayed well above 2 at 0°C in every study reviewed.

The UK project figures are measured at the “H4” boundary. That includes the heat pump, circulation pumps and any backup electric heater, so they reflect what homeowners pay for in real homes. Ground source heat pumps hold their efficiency better in the cold because the ground stays at a steadier temperature than the air.

What Does a Lower COP Mean for My Energy Use?

A lower COP means you use more electricity per unit of heat on cold days, but you still use far less than direct electric heating. For example, if a typical three-bed semi needed 100 kWh of heat on a freezing day, the electricity required would look roughly like this:

These figures are illustrative. Cold days cost more to heat with any system, but a heat pump keeps that cost well below direct electric heating.

What Do Norway, Sweden and Finland Tell Us About Heat Pumps in the Cold?

The Nordic countries have some of the coldest winters in Europe and some of the highest heat pump ownership rates. The 2023 Joule review reported that in 2021 Norway had around 60 heat pumps per 100 households, with Sweden and Finland at around 45 each.

These countries regularly see temperatures far below anything in the UK. The evidence is clear that the technology copes with cold. If heat pumps work at scale in Oslo and Helsinki, they will work in Leeds, Belfast or Aberdeen when they are designed and installed properly.

How Should a Heat Pump Be Sized for Cold Weather?

A heat pump should be sized to meet your home’s full heat loss at the local design temperature, without relying on a backup heater. The MCS design standard says the heat pump must provide at least 100% of the calculated heat load at the design flow temperature, without input from any supplementary electric heater.

In practice that means your installer should:

• Carry out a room-by-room heat loss calculation for your property.
• Use the correct design temperature for your location and altitude.
• Check the heat pump’s output at that outdoor temperature and flow temperature, rather than its nameplate rating.
• Allow for defrosting, which reduces output in cold, damp weather.

Vaillant’s installer guidance for the aroTHERM plus suggests allowing a 12 to 15% drop in output during design conditions for most of the UK to account for defrosting. The MCS standard also says an air source system should be able to maintain the design indoor temperatures across multiple defrost cycles.

Undersizing is the main reason a heat pump struggles in a cold snap. Oversizing is not the answer either, because an oversized unit cycles on and off in mild weather and runs less efficiently for most of the year.

Does a Heat Pump Need a Backup Heater in Cold Weather?

A properly designed heat pump does not need a backup heater to keep your home warm, but many systems include one as a safety net. The MCS design standard says any supplementary electric heater must be designed not to operate above the external design temperatures, so it should only switch on in conditions colder than the system was designed for.

Common types of backup include:

• A backup heater in the indoor unit or cylinder: a direct electric element that can top up space heating if the heat pump cannot keep up.
• An immersion heater in the hot water cylinder: used for hot water if the heat pump is out of action, and on many systems for a periodic high-temperature cycle.
• A hybrid system: a heat pump paired with a boiler. MCS requires the heat pump in a hybrid to cover at least 55% of the calculated heat load.

Direct electric backup works at a COP of 1, so it costs roughly two to three times more per unit of heat than the heat pump itself. The Energy Saving Trust notes that a boost function may kick in during extreme cold and that this is normal and should not significantly increase yearly costs. If your backup heater runs often in ordinary winter weather, the system may be undersized or set up incorrectly.

How Do Flow Temperature and Weather Compensation Help in a Cold Snap?

Weather compensation automatically raises the flow temperature to your radiators as it gets colder outside and lowers it when it is milder, so the heat pump delivers just enough heat for the conditions. The Energy Saving Trust describes it as automatically adjusting the heat pump flow temperature as the outside temperature varies, using an outdoor sensor.

This matters in a cold snap because it lets the heat pump run steadily at the lowest flow temperature that keeps your home warm. Lower flow temperatures mean higher efficiency. For example, on a mild day the flow might be around 30°C, and at the design temperature it might rise towards 45 to 55°C, depending on your radiators. The MCS design standard requires the heat pump to be capable of a flow temperature of at least 55°C at design conditions.

A well-set weather compensation curve means you should not need to touch the controls when a cold spell arrives. The heat pump will work harder on its own. If your home feels cool only on the coldest days, a small adjustment to the top end of the curve is usually better than turning up the thermostat or switching on a boost.

What Should I Expect From My Heat Pump During a Cold Snap?

During a cold snap you should expect your heat pump to run for longer periods, use more electricity and defrost more often, while still keeping your home at a comfortable temperature.

Does Defrosting Stop a Heat Pump Working in the Cold?

No, defrosting does not stop a heat pump working in the cold; it pauses heating for a few minutes at a time so the outdoor unit can melt frost off its coil. Vaillant notes that de-icing happens most often around 2.5°C and less often below 0°C, because colder air holds less moisture. So the damp, just-above-freezing days common in the UK usually bring more defrosting than hard frosts do.

You may see a cloud of steam from the outdoor unit and hear the fan stop and start. Vaillant’s operating instructions say you do not have to do anything when this happens. Our separate guide to the heat pump defrost cycle covers this in more detail.

How Can I Get the Best From My Heat Pump in Freezing Weather?

The best thing you can do in freezing weather is leave the heat pump running steadily on its weather compensation settings and keep the outdoor unit clear. Heat pumps are most efficient when they run low and slow rather than being switched on and off.

• Keep the heating on. The Energy Saving Trust advises keeping a heat pump on for extended periods and using a small setback overnight or when you are out, rather than switching it off.
• Leave the power on. Vaillant says the unit should stay switched on for frost protection unless separate frost protection is fitted.
• Clear snow and leaves. Gently brush snow away from the air inlet and outlet grilles and keep plants and clutter back from the unit.
• Do not cover the unit. Covers and tarps block airflow.
• Avoid big thermostat changes. Turning the thermostat right up or down forces the heat pump to work at high flow temperatures and lowers efficiency.
• Check the drainage. Make sure defrost water can drain away and is not freezing under the unit or across a path.
• Watch for error codes. The Energy Saving Trust advises not to ignore warnings on the control screen.

A heat pump that has been designed to your home’s heat loss at the right design temperature will keep you warm through a UK winter. If you want to know how a heat pump would cope in your property, our engineers can carry out a full heat loss survey and quote.