Yes, heat pumps work in cold weather, and yes, they work in old houses. They are the standard way to heat homes across Scandinavia, where winters make ours look mild, and plenty of Victorian terraces and 1930s semis in Britain now run on one. The real question is never whether the machine can pull heat from freezing air, because it can. It is whether your particular house, with its radiators, its insulation and its draughts, lets the pump run at a sensible efficiency so the bills stay reasonable. Get that part right and it is a comfortable, cheap-to-run heating system. Get it wrong and it is an expensive way to be lukewarm.
The short answer. Air source heat pumps keep producing heat down to around minus 15 to minus 25 degrees, far below anything a normal British winter throws at them. Efficiency does fall as it gets colder, so the pump works harder on the frostiest mornings, but it never stops. In an older house the limiting factor is almost never the cold outside; it is whether the radiators are big enough to warm the rooms at a low water temperature, and whether the fabric loses heat faster than the pump can replace it. Fix the worst draughts and top up the loft first, size the emitters properly, and an old home heats fine.
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How a heat pump makes heat when it is freezing outside
The thing that trips people up is the word "cold". It feels wrong that a box outside can pull warmth from air that is below zero, but there is no contradiction. Cold air still holds a large amount of energy; "absolute zero", where there genuinely is none left, is minus 273 degrees. A heat pump uses a refrigerant that boils at a very low temperature, so even freezing air is warm enough to make it evaporate. A compressor then squeezes that vapour, which raises its temperature sharply, and that concentrated heat is handed to the water in your heating system. The same trick runs in reverse in your fridge, which happily pumps heat out of a cold interior into your kitchen.
So the machine does not "run out" of heat on a frosty day. What changes is how hard it has to work. The bigger the gap between the freezing air outside and the warm water it needs to make, the more electricity the compressor draws for each unit of heat delivered. That ratio is the coefficient of performance, or COP, and it is the number that really matters through winter.
What actually happens to efficiency in the cold
A well set up air source heat pump might run at a COP of around 3.5 to 4 in mild autumn weather, meaning roughly three and a half to four units of heat for every unit of electricity. On a properly cold morning, say zero to minus 5, that might slip to somewhere around 2.5 to 3. It is still turning one unit of power into two and a half or more units of heat, which a direct electric heater can never do, but the gap has narrowed. Averaged across a whole heating season, a decent installation lands at a seasonal figure, the SCOP, of roughly 3 to 3.5.
Here is the part that gets lost in the panic about cold snaps: the coldest days are also the fewest. Most of a British winter sits between about 2 and 10 degrees, where the pump is comfortably efficient. The handful of hard-frost mornings each year drag the average down a little, but they are not where most of your annual heat, or most of your annual cost, is spent. Judging a heat pump by its worst morning is like judging a car's fuel economy by one steep hill.
| Outside air | Typical COP | What it means |
|---|---|---|
| 10°C (mild) | ~4 | Four units of heat per unit of power |
| 2°C (average winter) | ~3 to 3.5 | The bulk of the season sits here |
| -3°C (cold snap) | ~2.5 to 3 | Working harder, still well ahead of direct electric |
| -10°C (rare in the UK) | ~2 to 2.5 | Efficient enough; happens a few days a year at most |
Illustrative figures for a typical air source unit; real numbers depend on the model, the flow temperature and how well the system is set up.
The defrost cycle, and why you might see steam
On a cold, damp day you may notice the outdoor unit briefly stop, sometimes with a puff of vapour rising from it. That is the defrost cycle, and it is normal. Moisture in the air freezes onto the cold outdoor coil, and if frost built up unchecked it would choke airflow and kill efficiency. So every so often the pump reverses for a few minutes, sends warm refrigerant out to the coil, melts the frost, then carries on heating. It costs a little energy and a few minutes of heating, which is already accounted for in the seasonal efficiency figures. A unit that seems to defrost constantly, though, usually points to a siting or drainage problem worth raising with the installer, because pooled meltwater refreezing under the unit makes it worse.
Why old houses are about the radiators, not the cold
This is the heart of it. A heat pump is happiest producing water at a low temperature, often around 35 to 45 degrees, rather than the 60 to 70 degrees a gas boiler typically pushes out. Lower water temperature is exactly what keeps the efficiency high. But a cooler radiator gives out less heat for its size, so to warm the same room at 45 degrees instead of 70 you need a bigger radiator, or more of them, or underfloor heating.
In an older house fitted with radiators sized for a hot boiler, that is the pinch point. It is not that the pump cannot cope with an old building; it is that the existing emitters were never sized to work gently. A good installer does a room-by-room heat loss calculation and upsizes the radiators that need it. Skip that step, run the pump too hot to compensate, and the efficiency drops toward that of a plain electric heater, which is where the horror stories about running costs come from. The link between water temperature and efficiency is the same principle that makes turning down a gas boiler's flow temperature worthwhile, just pushed further.
Insulation and draughts come first
A heat pump replaces the heat a house loses. The faster the house leaks heat, the more the pump has to make, and the harder it works on cold days. That is why the sensible order is to slow the losses before, or at least alongside, fitting the pump. You do not need a house sealed like a flask, and you certainly do not need to gut a period property, but the cheap wins matter more here than with a boiler that can simply blast out more heat to paper over the gaps.
Top up the loft insulation to a full depth, because a thin or trodden-down layer is one of the biggest losses in an older home. Deal with the obvious draughts around doors, floorboards and old sash windows. If the walls are solid or the cavities empty, look at what insulation is practical without wrecking the character of the place. None of this is unique to heat pumps, but a leaky house punishes a low-temperature system more visibly than a hot boiler, so it pays to do it.
A worked example of a winter's day
Take a modestly insulated three-bed semi that needs about 40 kWh of heat on a cold winter day when it is 2 degrees outside. With a gas boiler at, say, 90 per cent efficiency, that is roughly 44 kWh of gas burned, which at an example rate of 7p per kWh comes to about £3.08 for the day. Now the heat pump: at a COP of 3.2 in that weather, 40 kWh of heat needs about 12.5 kWh of electricity, which at an example rate of 26p per kWh works out at around £3.25.
On that single cold day, they are close, and the pump might even edge slightly higher because electricity is dearer per unit. But swing to a milder day where the pump runs at a COP of 4 and the picture opens up, and across the whole season the pump's better average efficiency usually keeps it in the same ballpark as gas or a little cheaper, especially on a heat pump friendly electricity tariff with cheaper overnight hours. The fuller side-by-side sits in heat pump running cost versus a gas boiler, and you can price your own electricity use for any appliance with the running-cost calculator. The honest read is that a heat pump rarely halves your heating bill against mains gas; the case for it rests more on getting off gas and on the running cost against oil, LPG or old electric heating, where it wins clearly.
Air source or ground source in the cold
Ground source heat pumps draw heat from pipes buried in the garden or a borehole, where the temperature barely changes across the year, sitting around 8 to 12 degrees whatever the weather above. That steadier source means a ground source unit holds its efficiency better through a cold snap than an air source one, and it never has to defrost. The catch is the cost and disruption of the groundworks, which rules it out for many homes, especially small urban plots. For most people the choice is really air source or nothing, and air source is more than capable in the British climate; the deeper comparison is in ground versus air source heat pumps.
What I would actually do
Do not let the cold-weather worry put you off, because it is the wrong worry. A heat pump will keep an old British house warm through the frostiest week of the year. What decides whether you are glad you fitted one is the quality of the install: a proper heat loss survey, radiators sized for a low flow temperature, and the fabric of the house tightened up first. Insist on the survey, ask what flow temperature the system is designed around, and be wary of any quote that skips straight to swapping the box without touching the radiators. Sort the emitters and the insulation, run it at a low, steady temperature rather than blasting it, and an old house on a heat pump is warm, quiet and sensible to run. Cut corners on the design and the machine gets the blame for the installer's shortcuts.
Common questions
At what temperature do heat pumps stop working?
Air source heat pumps keep producing heat down to roughly minus 15 to minus 25 degrees, depending on the model, which is far colder than a normal UK winter. They do not stop working in British conditions; they simply become less efficient as the air gets colder, drawing more electricity for each unit of heat.
Do heat pumps work in old houses?
Yes. The building's age is not the barrier; the radiators and insulation are. Because a heat pump works best at a lower water temperature, older homes usually need some radiators upsized and the worst draughts and loft losses dealt with. Do that and a Victorian or interwar house heats comfortably.
Why do people say heat pumps are expensive to run?
Almost always because the system was set up to run too hot, which drops its efficiency toward that of a plain electric heater. A properly designed install runs at a low flow temperature with correctly sized radiators, which keeps the running cost competitive with gas and well below oil, LPG or old electric heating.
What is a defrost cycle?
In cold, damp weather frost forms on the outdoor coil. Every so often the pump briefly reverses to melt it, sometimes with a visible puff of vapour, then carries on heating. It is normal and uses only a little energy, which is already included in the seasonal efficiency figures.
Do I need underfloor heating for a heat pump?
No. Underfloor heating suits a heat pump well because it works at a low temperature over a large area, but correctly sized radiators do the job in most homes. The key is that the emitters, whatever type, are big enough to warm the room at the pump's lower flow temperature.