You have heard the line in the pub: heat pumps are fine in Spain but useless in a British January. It is the biggest worry homeowners raise before spending £10,000 or more on one. The short version is that heat pump cold weather UK performance is better than most people expect, because modern units are built to run well below freezing.
Yes, heat pumps work in a UK winter. A correctly sized air source heat pump in a reasonably insulated home will keep you warm at -10°C outside, though it will use more electricity on the coldest days than on a mild one. The real risk is not the weather. It is a unit that was sized badly for a leaky house.
How a heat pump works when it is freezing
A heat pump does not make heat by burning anything. It moves heat from the outside air into your home, much like a fridge in reverse. Air can still hold usable heat well below zero, so the unit keeps working. Most modern air source models are rated to run down to around -15°C to -20°C. The lowest temperatures recorded in most of England in a typical winter sit well inside that range, even if a rare cold snap goes lower.
What changes is efficiency. The colder it is outside, the harder the unit works for each unit of heat delivered. That is normal, and it is why the efficiency number matters.
What the efficiency figures actually mean
You will see two numbers. The coefficient of performance (COP) is a snapshot: how many units of heat you get for each unit of electricity at one outside temperature. The seasonal performance factor (SPF) is the average across the whole year, and it is the one that predicts your bill.
A well-installed system might deliver a COP of around 3 on a mild day and closer to 2 on a freezing one. Over a full year, a typical SPF for a good UK installation is often quoted at around 3 to 3.5. An SPF of 3 means every £1 of electricity buys about £3 of heat. A modern gas boiler converts roughly 90p of every £1 of gas into heat. The heat pump wins on efficiency even in winter. Whether it wins on cost depends on the gap between your electricity and gas unit prices.
What happens on the coldest days
The defrost cycle
In damp, near-zero weather, frost can build up on the outdoor unit’s coil. The pump runs a short defrost cycle, a few minutes every so often, melting the ice. You may see steam from the fan and notice a brief dip in output. This is normal and not a fault.
Running costs in a cold spell
On a frozen week, expect the electricity use to climb. A system that averages 3 units of heat per unit of electricity over the year may drop to around 2 during the worst days. That is why a winter bill can be double a summer bill, which is true of a gas boiler too. If you are on a time-of-use tariff, running the heating early in cheaper periods and letting the house coast can trim the cost.
The sizing problem that actually causes complaints
When a homeowner says a heat pump “doesn’t work in winter”, the cause is almost always a poor survey, not a poor product. A unit sized for an average day will struggle on a freezing one. A unit fitted to a house with no loft insulation and single-glazed windows will run flat out and still feel cool.
A proper installer carries out a room-by-room heat loss calculation and sizes the unit for the coldest design day, not the average. Be wary of any quote delivered without one, or one that simply swaps the boiler’s kilowatt rating for a heat pump of the same number. Radiators matter too. Heat pumps run the water cooler than a boiler, typically 35°C to 50°C rather than 70°C or more, so some radiators need to be larger to give out the same warmth.
Will it work in your house?
The signs that a home is a good fit are:
A decent EPC rating, ideally band C or better, or insulation you can cheaply improve.
Loft insulation of 270mm or more and no obvious draughts.
Enough outside space for the unit, with clearance from boundaries and windows.
Radiators that are large for the room, or a plan to upgrade them.
None of these is a hard rule. Many Victorian and 1930s houses run heat pumps happily once draughts and loft insulation are sorted. A leaky solid-wall home with a poor EPC is where you should insulate first or get a firm opinion before committing. Your EPC’s recommendations list is a good place to start.
What a cold snap feels like in a real house
Picture a 1930s semi in the West Midlands with 270mm of loft insulation, cavity walls filled and a heat pump sized by a proper survey. On a -5°C morning the unit runs almost continuously at a low, steady output. The radiators feel warm rather than hot to the touch, which surprises people used to a boiler, but the rooms hold 20°C. That gentle, constant running is how a heat pump is designed to work, and it is why turning the heating up and down in short bursts tends to waste energy.
Compare that with a similar house where the loft has 100mm of old insulation and the windows leak. The same unit has to work much harder, the bills climb and the rooms feel cool. The weather was the same. The difference was the house, which is why the fabric of your home matters as much as the machine.
Backup heat and extreme cold
Most systems include an immersion heater element as a safety net. It tops up the hot water and, on a few extreme days, the space heating. If you find the backup running often, something is wrong: the unit is undersized, the settings are off or the house is losing too much heat. Ask your installer to review the settings after the first winter, because a few tweaks to the heating curve can cut usage noticeably.
Noise, frost and siting
Outdoor units make a low hum, comparable to a fridge, and a defrost cycle can add a brief change in sound. Position the unit away from bedroom windows and neighbours’ boundaries, and keep it clear of leaves and snow drifts so air can flow through. Never box it in.
What a heat pump costs and what help exists
A typical air source installation costs somewhere around £10,000 to £14,000 before grants. In England and Wales, the Boiler Upgrade Scheme currently offers a grant towards the cost, which comes off the installer’s quote. Always check that your installer is MCS-certified, since that is a condition of the grant.
What to do in the next 30 minutes
Find your latest EPC and read the recommendations. Loft and wall insulation advice there tells you how ready your home is.
Measure your loft insulation depth. Under 270mm means topping it up is probably worth doing whatever heating you choose.
Check where an outdoor unit could sit: somewhere with air flow, not squeezed into a tight alley.
Note what you pay per kWh for gas and electricity on your latest bills.
Ask two MCS-certified installers for a quote that includes a room-by-room heat loss calculation.
If you are comparing options, log your current boiler, its service history and your EPC in your free Home+ account, so you have everything to hand when installers ask.
FAQ
At what temperature does a heat pump stop working? Most modern air source heat pumps keep running down to about -15°C or lower. Efficiency falls as it gets colder, but the unit does not simply switch off at freezing.
Will a heat pump keep my house warm at -10°C? Yes, if it is sized properly for your house and the house is reasonably insulated. It will use more electricity on that day, and some systems include a backup immersion heater element for extreme cold.
Why is there steam coming off my heat pump? It is almost certainly the defrost cycle. The unit melts frost from its coil and the vapour is harmless. If it ices over completely or stops heating, call your installer.
Are heat pumps cheaper to run than gas boilers in winter? Per unit of heat they are more efficient, but electricity costs more per kWh than gas. The saving depends on the efficiency you achieve and your tariff, so ask installers for a running cost estimate based on your home.
Do I need to keep a heat pump running all day? Usually it works best on steady, lower-temperature heating rather than short, hot bursts. Your installer will set the heating curve, and you can fine-tune the schedule over the first winter.
Do I need new radiators for a heat pump? Often some, not always. Because the water runs cooler, small radiators may need replacing or upsizing. The heat loss calculation will show which rooms need it.
