Do air-to-air heat pumps work in cold weather? What happens to your air conditioning in a UK winter

Short answer: yes. Air-to-air heat pumps, the reverse-cycle wall splits we review on this site, keep heating in cold weather. Every current mainstream unit sold in the UK is rated to heat down to at least minus 15°C outside, and several go to minus 20°C or minus 25°C. The UK's average January temperature, on the Met Office's 1991 to 2020 figures, is around 4°C (a mean daily high of 6.7°C and low of 1.2°C). Britain is a mild place to run one.

What does change as it gets colder is how much heat the unit produces per kilowatt of electricity, and how often the outdoor unit pauses to defrost. Neither is a fault, but both catch people out in their first winter.

How cold is too cold for a heat pump?

Colder than the UK gets, for practical purposes. These are the heating limits manufacturers publish for the wall splits we cover most.

Daikin UK's own winter advice says UK temperatures "can dip to around minus 10°C", true of a hard cold snap in a rural spot. Even then, every unit in the table is inside its rated range. Mitsubishi Electric also makes a Hyper Heating version of the MSZ-LN (the VGHZ) that runs to minus 25°C and holds full rated output at minus 15°C; it is a Nordic-market product, so ask an installer whether you can get it.

What actually changes when it gets cold

A heat pump moves heat from the outside air into the room. Cold air still holds heat, but the colder it is, the harder the compressor works. Two things follow.

Efficiency falls. Daikin's technical databook for the Perfera FTXM35R paired with the RXM35R outdoor unit gives these Ecodesign test figures:

Source: Daikin Air Conditioning Technical Data EEDEN23, FTXM35R + RXM35R, space-heating conditions A to D and the temperature operating limit. The COPs are Daikin's declared part-load values at each test point; a unit running flat out will sit a little lower. Electricity at the Ofgem price cap, direct debit, 1 July to 30 September 2026 (26.11p/kWh). October's price cap moves electricity to 26.32p, a fraction of a penny's difference.

Maximum output falls too. Panasonic's European catalogue is one of the few that prints the figure: its 3.5kW-class Etherea gives 4.00kW of heat at 7°C and 3.40kW at minus 7°C, about 85 per cent. Across the range it is roughly 80 to 90 per cent on the small and medium sizes, less on the largest. Those are 2018 figures for an earlier Etherea, but the pattern holds for standard (non-Nordic) splits.

Why that still beats a panel heater

Look at the last row of the table. A panel heater, fan heater, oil-filled radiator, storage heater or infrared panel turns one kilowatt-hour of electricity into one kilowatt-hour of heat, never more: COP 1.0 in any weather. An air-to-air unit at minus 7°C is still producing 3.6 units of heat per unit of electricity. Even at minus 15°C, which most of England sees for a few hours a decade, it is at 2.5, so 2kWh of heat costs 21p rather than 52p.

Field data agrees. A 2023 review in the journal Joule (Gibb, Rosenow, Lowes and Hewitt, for the Regulatory Assistance Project) pooled seven field studies of around 550 heat pumps in the UK, Germany, Switzerland, the USA, Canada and China. Between 5°C and minus 10°C the mean COP was 2.74, and the authors concluded that "concerns over the need for back-up heating in mild cold climates may be unfounded".

The winter question is not whether an air-to-air unit works at minus 7°C. It does. The question is whether it is still cheaper than what you have, and at COP 3.6 against COP 1.0 the answer is not close.

For the running-cost maths across a whole winter, see heating with air conditioning.

The defrost cycle: steam, a pause, and a minute of cool air

This is the part nobody warns you about. On a damp British day at 0°C to 5°C the outdoor coil runs colder than the air around it, so moisture freezes on to the fins. Frost is worse in humid cold than in a dry hard frost, and left alone it would block the airflow.

So the unit defrosts itself. The controller briefly reverses the refrigerant cycle, sending hot gas through the outdoor coil while the outdoor fan stops. Indoors, the fan slows or stops so it does not blow cold air at you. Outside, a plume of water vapour rises off the unit as the ice melts, which looks alarmingly like smoke, and water drips from the base. Grant UK describes the cycle as lasting "only minutes"; a UK MCS installer, quoting manufacturer documentation, puts it at two to ten. Then the unit flips back and carries on.

You will notice a short pause in heating, sometimes a click from the reversing valve, and a brief puff of cooler air when it restarts. On a raw wet day it may happen every hour or so. That is normal.

Frozen heat pump: when it is a fault

A thin, even coating of white frost on the outdoor coil between defrosts is normal. What is not:

• Thick ice that never clears. If the coil stays encased after the unit has had time to defrost, suspect low refrigerant, a failed sensor, a stalled fan or a defrost control fault. One UK installer's rule of thumb: frozen solid for more than three or four hours means call a professional.

• An ice sheet under the unit. Defrost water that cannot drain refreezes and builds up until it reaches the coil. That is a drainage problem, not a refrigerant one.

Do not chip at the ice or pour hot water over it. Leave the unit on heat, give it time, and if it does not recover call your installer. Anything on the refrigerant circuit needs an F-Gas certified engineer.

Placing the outdoor unit for winter

Most icing problems are placement problems, so get it right on day one.

• Off the ground. Wall brackets or a raised base keep the coil above puddles, leaves and drifting snow.

• Drainage for defrost water. MCS installation standards require condensate to discharge safely to a drain or soakaway, and it is checked at commissioning. Water running across a path or step becomes an ice sheet in January. A gravel trench away from footpaths, a 40mm waste pipe to a drain, or a soakaway all work.

• Sheltered but not boxed in. A wall under an eave is fine; a tight enclosure or a fence hard against the fan is not, because the unit recirculates its own chilled air. Keep about 60cm clear and never cover it.

Size it for January, not July

A unit sized only for summer cooling can come up short in winter. A badly insulated room with a big window can lose more heat on a cold night than it gains in a heatwave, and the unit's output falls as the temperature does. Installers who quote on a floor-area rule of thumb for cooling sometimes miss this.

Ask for a heating heat-loss figure as well as a cooling one, and check the unit's heating output at 2°C or minus 7°C rather than its 7°C nominal. Our BTU calculator gives a starting point for both, and what size air conditioning you need explains what pushes the number up. MCS-certified installers now have an air-to-air performance estimate calculator (MCS 031 b, December 2025) for the same job.

Running it through a cold snap

• Leave it running. Steady, low-level operation is how a heat pump stays efficient in sub-zero weather. Switching off overnight and blasting it in the morning costs more, because it then runs for hours at maximum output warming a cold room. Longer run times at a low hum are normal in cold weather. Turn it down a degree or two rather than off; the most efficient way to run an air-to-air heat pump has the detail.

• Keep the outdoor unit clear. After snow, brush the top and coil face with a soft brush. No scraping, no hot water.

• Keep the indoor filters clean. A clogged filter cuts output when you need it most. See heat pump servicing and winter maintenance.

Not heating at all? Check these first

1. Mode. The remote is on heat (usually a sun symbol), not cool, dry or fan. The most common "fault" of all.

2. Defrost. Wait ten minutes; if the outdoor fan was stopped and the unit was steaming, it was defrosting.

3. Airflow. Filters clean, outdoor unit clear of snow, leaves and ice.

4. Still nothing, or persistent ice. Note any error code on the indoor unit and call your installer. Refrigerant leaks, sensors and fan motors need an F-Gas engineer.

The Nordic proof

Norway has around 630 heat pumps for every 1,000 households, the highest in Europe, and Finland just over 520, according to the European Heat Pump Association's 2025 market report. NOVAP, the Norwegian heat pump association, records almost 1.76 million air-to-air units sold there since 1987, around 95 per cent of them used for heating. They are the same kind of wall split you would put in a British living room, often the Nordic variant rated to minus 25°C, in winters where minus 20°C is unremarkable.

The Energy Saving Trust notes there is not yet much UK field data on air-to-air units. But the physics does not change at the border, and the UK winter is a good deal gentler than the ones these units were built for.