The Short Answer
Yes. The big-name models most UK installers fit (Vaillant's aroTHERM plus, Mitsubishi's Ecodan R290 and Daikin's Altherma 3 R) are all rated to keep running down to -25°C, and a British winter rarely gets near that. What cold does is make the heat pump work harder: efficiency drops from around 4 or 5 units of heat per unit of electricity on a mild day to roughly 2.3 to 2.8 on a freezing one. That's still more than twice as efficient as any electric heater, and the government-funded Electrification of Heat trial found exactly that across hundreds of British homes. When a heat pump genuinely leaves a house cold, it's almost never the weather. It's an undersized unit or a system that was never properly designed, and the fix is a proper room-by-room heat loss survey, which any MCS installer does as part of a quote.
Why a Heat Pump Can Pull Heat Out of Freezing Air
It sounds like a trick, so it's worth thirty seconds on how it works. Air at -5°C still holds a lot of heat energy. It's only "cold" compared with your living room. A heat pump runs a refrigerant that boils at a far lower temperature than water (the R290 propane used in many new units boils at around -42°C at atmospheric pressure), so even freezing air is warm enough to evaporate it. The compressor then squeezes that vapour, which heats it up well past the 35 to 55°C your radiators need, and the heat is handed over to your heating water.
The colder the air, the less heat there is to grab and the harder the compressor has to squeeze. That's the whole story of cold-weather performance: it keeps working, it just uses more electricity for each unit of heat it delivers.
What Cold Actually Does to Efficiency
Efficiency is measured as COP (coefficient of performance): units of heat out for every unit of electricity in. A COP of 3 means 3 kWh of heat for 1 kWh of electricity. Here's how one of the most commonly fitted units in the UK behaves, taken straight from Vaillant's own technical data for the aroTHERM plus 7kW:
| Outdoor air | COP at 35°C flow (underfloor or big radiators) | COP at 55°C flow (typical radiators) |
|---|---|---|
| 7°C (a mild winter day) | 4.8 | 2.9 |
| 2°C (a normal British winter day) | 4.1 | not published |
| -7°C (a hard frost) | 2.8 | 2.0 |
Two things jump out. First, even at -7°C with a 35°C flow it's still producing nearly three times as much heat as it uses. Second, the flow temperature matters as much as the weather does. The same unit at the same -7°C is 40% more efficient running at 35°C than at 55°C. That's why good installers fuss about radiator sizes, and it's the single biggest thing you control. More on that in our guide to whether you need new radiators.
One more detail from that datasheet is worth knowing: the 7kW unit is rated to deliver its full 7kW at -7°C with a 35°C flow (6.4kW at 55°C). Heat pumps are sized on cold-day output, not mild-day output, so a correctly chosen unit has already been specified for the frost.
What real British homes did in winter
Lab ratings are one thing. The best evidence we have from actual houses is the government-funded Electrification of Heat trial run by Energy Systems Catapult, which installed 742 heat pump systems in homes across the North East of England, the South East and South East Scotland and monitored them from 2020 to 2023, through three British winters. Its findings on the cold:
- At -3°C outside, the median air source heat pump ran at a COP of 2.37.
- At -5°C, it was 2.30. The trial calls -3°C and -5°C the "common GB design conditions", in other words the coldest weather systems are normally designed around.
- Efficiency peaked at 10°C outside, with a median COP of 3.37.
- Flow temperature mattered just as much. At 35°C the median COP across all heat pumps was 3.50, at 55°C it was 2.38, and above 65°C it fell to 2.02.
Over a full year the median air source system in the trial managed a seasonal efficiency of 2.93. Those were homes fitted in 2020 and 2021 under a trial that installed into a broad spread of house types, and the kit and design practice have moved on since. It's a fair floor, not a ceiling.
If you want a sanity check from somewhere properly cold: Norway has 632 heat pumps for every 1,000 households and Finland 524, according to the European Heat Pump Association. The UK has 19. Nobody in Oslo is worried that it gets a bit chilly in Leeds.
How Low Can They Go? Minimum Operating Temperatures
| Model | Manufacturer's stated operating limit |
|---|---|
| Vaillant aroTHERM plus | Operates down to -25°C |
| Mitsubishi Ecodan R290 | Guaranteed operation down to -25°C ambient |
| Daikin Altherma 3 R | Outdoor unit operates as low as -25°C |
For context, the coldest weather UK systems are designed for is a few degrees below freezing. The operating limit is a safety margin, not a target. What you care about is output and efficiency at your design temperature, which is exactly what the heat loss survey pins down.
Defrost Cycles: The Steam Is Normal
The first frosty morning with a new heat pump catches a lot of people out. In damp air between roughly 0 and 5°C (which describes most of a British winter), moisture freezes onto the outdoor unit's fins. Left alone, that ice would block the airflow, so every so often the heat pump reverses itself for a few minutes, borrowing a little heat from your heating water to warm the outdoor coil and melt the ice.
What you'll notice:
- A cloud of steam pouring off the unit. It looks alarming and it's completely harmless. Neighbours occasionally mistake it for smoke.
- The fan stops and there's a hiss or a whoosh as the refrigerant changes direction, then a trickle of meltwater from underneath.
- Radiators cool slightly for a few minutes, then pick straight back up.
That meltwater needs somewhere to go. A good installer mounts the unit on feet or a bracket with a clear drain path, because meltwater that refreezes into a sheet of ice under the unit (or across your path) is the one genuinely annoying defrost problem. Vaillant's datasheet even lists a minimum volume of water the heating system must hold for defrost to work properly, which is one of the details a proper design covers and a rushed one skips.
Defrost is also why a heat pump can sound different in winter. If you're worried about noise, our heat pump noise guide covers what's normal and what isn't.
What a Freezing Day Actually Costs
Here's the honest version, using the October 2026 price cap (electricity 26.32p/kWh, gas 7.97p/kWh). A gas boiler at 90% efficiency delivers heat at 8.86p per kWh. A heat pump's cost per kWh of heat is the electricity price divided by its COP:
| Conditions | Heat pump COP | Cost per kWh of heat, standard tariff | On a heat pump tariff (about 20p) |
|---|---|---|---|
| Across the year (SCOP 3.5) | 3.5 | 7.52p | 5.71p |
| -3°C (trial median) | 2.37 | 11.11p | 8.44p |
| -5°C (trial median) | 2.30 | 11.44p | 8.70p |
| Gas boiler at 90%, any weather | n/a | 8.86p | n/a |
So yes, on the very coldest days a kWh of heat from a heat pump on a standard tariff costs more than a kWh from a boiler. Take a home that needs 100 kWh of heat on a freezing day: that's about £11.11 through the heat pump at a COP of 2.37, against £8.86 of gas. On a heat pump tariff the same day comes to roughly £8.44. The "about 20p" is our estimate of the blended rate a heat pump realistically gets on Cosy Octopus, which prices its cheap windows regionally rather than at one national figure.
The thing that matters is the whole winter, not the worst day. Most heating-season days in most of the UK are above freezing, where the heat pump runs well above its cold-day figure, and across the year a well-designed system averages out at around 7.5p a kWh of heat against the boiler's 8.86p. Break-even is a seasonal efficiency of 2.97. Be honest with yourself about what that means: the trial's median system, at 2.93, came out roughly level with gas at today's prices rather than ahead. The gap between that and a well-designed 3.5 is almost entirely design, which is why the survey matters more than the brand. Our running costs guide goes through the annual numbers in detail, or try the calculator below with your own bill.
1. What heats your home today?
Pick whichever your main heating uses.
Add up last year's bills, or use the typical figure for your home below.
3. What size is your home?
We use this to estimate the size of heat pump you'd need.
4. Do you have solar panels?
Free electricity from your roof cuts what the heat pump costs to run.
5. Your options
Most homes qualify for both. Leave them on unless you know they don't apply.
Your estimated saving
£531a year
Over 20 years that's around £10,621 off your heating bills.
Now: Gas boiler
£1,300 / year
With heat pump
£769 / year
You save
£531 / year
What about the upfront cost?
- Heat pump install (typical)
- £11,000
- Less government grant
- -£7,500
- You pay
- £3,500
- A new gas boiler would cost
- ~£2,750
If your boiler needs replacing anyway, the heat pump costs about £750 more upfront - earned back in roughly 1.4 years from running cost savings.
Like the look of £531 a year?
These are estimates. The only way to know your real saving is a free quote from an MCS-certified installer. Takes 2 minutes, no obligation, no pushy sales calls.
How we worked this out
Energy prices: Ofgem price cap for 1 October to 31 December 2026 (Direct Debit, GB average): gas 7.97p/kWh, electricity 26.32p/kWh. There is no VAT on domestic electricity from 1 October 2026 to 31 March 2027; the gas figure includes 5% VAT. Heat pump tariff blended at 20p/kWh (an estimate for Cosy Octopus, E.ON Next Pumped or EDF Heat Pump Tracker on a heat pump load profile, not a quoted rate). Heating oil 10.97p/kWh (113.52p/litre ÷ 10.35 kWh/litre, BoilerJuice UK average, September 2026). LPG 11p/kWh, indicative midpoint of the typical UK domestic range.
Heat pump efficiency:Seasonal performance factor (SCOP) of 3.5. The real-world average across 252 monitored UK heat pumps on heatpumpmonitor.org is 3.87 (January 2026), so we've picked a deliberately conservative figure.
Typical bill ranges: Gas 3-bed £1,000-£1,400, oil £1,500-£2,000, LPG £1,400-£2,200, electric storage heaters £1,500-£2,500 (Uswitch, retrofitplanner.co.uk, Checkatrade 2026). Oil and LPG homes pay roughly twice as much per unit of heat as gas, which is why the saving from switching to a heat pump is bigger.
Solar: A typical UK 4kW system generates around 3,800 kWh/yr. Without a battery the heat pump can self-consume around 1,000 kWh of that; with a battery shifting midday surplus into morning and evening, around 1,700 kWh.
Install costs: Energy Saving Trust 2026 typical range £10,000-£13,000 by home size. New gas boiler ~£2,750 (UK average, Checkatrade 2026). The £7,500 grant is the Boiler Upgrade Scheme (gov.uk) for England and Wales.
Estimates only. Your actual saving depends on the installer, flow temperatures, your home's heat loss and your electricity tariff. Always get a heat loss survey from an MCS installer for an accurate quote.
Winter Settings That Actually Help
Use weather compensation and leave it alone
Weather compensation reads the outdoor temperature and sets the flow temperature to match: cooler water on mild days, warmer on cold ones. It's the single most effective efficiency setting on any heat pump. Ask your installer to set the curve so the house is just warm enough on the coldest days, then leave it running.
Don't turn it off overnight
Boiler habits die hard. A boiler blasts the house back up to temperature in twenty minutes, so switching it off at night makes sense. A heat pump works best running steadily at a low flow temperature. Turn it off on a freezing night and it has to recover a cold house in the morning at a high flow temperature, which is its least efficient way of working. A small overnight setback of a degree or two is fine. Off is not.
Keep the outdoor unit breathing
Clear fallen leaves in autumn and brush off drifting snow so the fins and the drain stay clear. Don't build a box around it to "protect" it: it needs a free flow of air, and the manufacturer sets minimum clearances for a reason. A light cover over the top against heavy snow is fine if it doesn't restrict airflow.
Know how your system is protected from freezing
A monobloc heat pump has water pipes running outside, so it needs freeze protection. There are two common approaches. Some systems run a glycol (antifreeze) mix through the circuit, which protects it even in a power cut but costs a little efficiency. Others use plain water with freeze-protection valves that dump water from the outdoor pipework if it gets close to freezing during a power cut, and the unit's own controls run the pump in cold weather to stop water sitting still. Neither is wrong. Just ask which one you have, because it changes what you'd do after a long power cut.
Check the immersion and backup heater settings
Most systems have a backup electric heater that can kick in during the coldest weather. A little use on the worst days is expected. If your electricity use jumps and the house still isn't warm, the backup may be doing work the heat pump should be doing, which is a sign the system is undersized or set up badly.
When Cold Really Is a Problem
Every heat pump horror story you've heard about cold houses traces back to one of these, not to the weather itself:
- No proper heat loss survey. MCS requires installers to do a room-by-room heat loss calculation to BS EN 12831-1:2017, and MCS provides its own calculator for exactly that. It's what tells the installer how much heat each room needs on the coldest day. A quote based on the size of your old boiler is a guess.
- An undersized unit. If the heat pump can't meet the home's heat loss at design temperature, it'll hold on through mild weather and fall behind in a cold snap, leaning on the backup heater to catch up.
- A high flow temperature. Small radiators forcing the system to run at 55°C or above push efficiency down towards 2 (as the trial showed) and make every cold day more expensive.
- A leaky house. You don't need a passivhaus, but obvious gaps (no loft insulation, open chimneys, draughty doors) raise the heat loss and the size of unit you need. Our guide to insulating before a heat pump covers what's worth doing first and what isn't.
All four are design problems, and all four are sorted before installation by an installer who does the job properly. That's the real answer to "will it keep me warm in January": get at least two or three MCS installers to survey the house and compare how they've sized it. If one quotes a much smaller unit than the others without explaining why, that's your warning. The Boiler Upgrade Scheme only works through MCS installers anyway, so you'll need one either way. We've written up how to compare heat pump quotes, and you can get quotes from MCS installers near you in a couple of minutes.
If you only do one thing: ask every installer to show you the heat loss figure for your home and the unit's output at your design temperature. If the output covers the heat loss, the cold isn't your problem.
