Do heat pumps work below freezing in the UK?

Yes. Modern UK-spec air source heat pumps are designed to run through ordinary UK winters, including days below 0°C, and most models are tested down to around -15°C to -20°C. Sub-zero operation is routine, factored-in behaviour, not an edge case the equipment struggles to cope with.

Do heat pumps get less efficient in cold weather?

Yes — this is the real trade-off, distinct from whether they work at all. As the gap between outdoor and indoor temperature widens, the compressor has to work harder to move the same amount of heat, so COP (the ratio of heat delivered to electricity used) falls. A heat pump might show a COP around 4 at a mild +7°C and closer to 2 on a genuinely cold day — still meaning more heat out than electricity in, just by a smaller margin.

What is a defrost cycle, and is it a problem?

In cold, damp conditions, frost can build on the outdoor unit's coil. The heat pump periodically reverses briefly to melt it off, using a small burst of energy before resuming normal heating. It happens several times an hour in the right conditions, lasts a few minutes, and is accounted for in the manufacturer's seasonal efficiency figures — not a fault or a sign of undersizing.

Will I need a backup heater for cold snaps?

Many systems include a small immersion element in the hot water cylinder as backup, mainly for very high hot water demand or emergencies, and it should represent a tiny fraction of annual energy use if the heat pump itself is correctly sized. If backup heat is running frequently or for long periods, that points to the heat pump or radiators being undersized for the property, not an inherent cold-weather limitation.

Do I need a bigger heat pump if I live somewhere particularly cold, like the Scottish Highlands or the Pennines?

Your installer's heat loss calculation should use a "design temperature" appropriate to your specific location — colder in parts of Scotland and at altitude than in most of England — and size the system to that figure, not a generic national average. If you're in an unusually exposed or cold location, ask specifically whether the proposed model and sizing account for your local design condition, and whether a cold-climate-rated model is worth the difference.

Is my heating bill higher in winter with a heat pump than it was with a gas boiler?

Winter is always the highest-demand, lowest-efficiency period for a heat pump, exactly as it is for a gas boiler — so a higher winter bill on its own tells you little. What matters is the annual total against your specific tariff, which the cost calculator models using SCOP (the seasonal average) rather than a single cold-day snapshot.

Do Heat Pumps Work in Cold Weather? UK Winter Performance Explained

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Air source heat pump outdoor unit running normally in light snow on a cold UK winter morning, visible frost on surrounding garden

Sub-zero mornings are routine operating conditions for a correctly sized UK heat pump, not an exception it struggles with.

Yes, heat pumps work in cold weather, including below freezing — modern UK-spec air source units are built and tested for exactly this climate. What changes as it gets colder isn't whether the system runs, but how efficiently: the COP (the ratio of heat delivered to electricity used) falls as the temperature gap widens, and the unit occasionally runs brief defrost cycles. Neither of those is a malfunction — both are normal, accounted-for parts of how the technology behaves.

Why "does it work" and "how efficiently" are different questions

A heat pump extracts heat from outdoor air using a refrigerant that boils at a very low temperature — typically -25°C to -40°C, depending on the refrigerant used (see our how heat pumps work guide for the full refrigerant cycle). As long as the outdoor air is warmer than the refrigerant's boiling point — which UK winters, however cold they feel, virtually always are — there's physically some heat available to extract. The question that actually varies with temperature isn't "is there heat to move" but "how hard does the compressor have to work to move it, and at what electrical cost."

COP versus outdoor temperature

COP falls roughly in line with the widening gap between outdoor and indoor (or flow) temperature. A well-installed UK air source heat pump might show a COP of around 4 at a mild +7°C test condition, and something closer to 2 on a genuinely cold day around -10°C to -15°C — still meaning it delivers roughly twice as much heat as the electricity it consumes, just with a smaller margin than in milder weather.

SCOP (Seasonal COP) is the figure that actually determines your running costs, because it's averaged across a full year of real UK weather rather than one lab condition or one cold snap. UK air source heat pumps typically deliver SCOP 2.8–3.5 across a year — and the reason that seasonal average stays reasonably high despite cold-day dips is that most of a UK winter sits well above the coldest extremes: genuinely sub-zero days are a minority of the heating season, not the norm. Ground source systems, drawing from underground temperatures that stay stable year-round, typically deliver a higher SCOP of 3.5–4.5 — see our ground source guide for that comparison in full.

The number to ask your installer for

Ask for the modelled SCOP for your specific property, not a generic brochure figure — it depends on your insulation, radiator sizing, and target flow temperature, not just the heat pump model. That figure, not a single cold-day COP, is what should feed your running cost expectations.

Design temperature: how installers size for your coldest realistic day

A competent MCS installer sizes your system against a design temperature — the coldest outdoor temperature the heat loss calculation assumes your home needs to be heated against continuously, not the very lowest temperature ever recorded locally. Design temperatures are colder in parts of Scotland, at higher altitude, and in more exposed locations than in most of lowland England. The system should be sized to meet that design condition; short, exceptional cold snaps below it rely on the heat pump running harder and longer, and any backup heat, rather than failing to heat the home at all.

This is also why a heat loss survey matters more than the heat pump brand: an accurately sized system copes comfortably with its design condition, while an undersized one — regardless of which manufacturer built it — struggles in exactly the same cold snap.

Defrost cycles, explained properly

In cold, humid conditions — often just above and just below freezing, when moisture in the air readily forms ice — frost can build up on the outdoor unit's coil, which reduces its ability to absorb heat from the air. The heat pump's controller detects this and periodically reverses the refrigerant cycle for a short period, using a small burst of energy to melt the ice off the coil, before switching back to normal heating.

Typically lasts a few minutes at a time
Can happen several times an hour in the specific conditions that favour frost formation
Is factored into the manufacturer's seasonal efficiency (SCOP) figures already — it's not an unaccounted-for drain

If you notice steam or a brief burst of what looks like the unit "reversing" on a cold, damp morning, that's very likely a normal defrost cycle rather than a fault.

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Air source heat pump outdoor unit venting visible steam during a defrost cycle on a cold damp UK morning, normal operation

Brief steam or visible venting during a cold, damp morning is typically a normal defrost cycle — a routine, accounted-for part of operation, not a fault.

Backup heat: what it's actually for

Some systems include a small immersion heating element in the hot water cylinder, used as backup — mainly for periods of unusually high hot water demand, or as a safety fallback rather than a primary heat source. In a correctly sized system, this should account for a small fraction of total annual energy use. If it's engaging frequently or running for extended periods through winter, that's a signal the heat pump or radiators are undersized for the property's actual heat loss — worth raising with your installer as a sizing issue, not something to just accept as "what heat pumps do in winter."

Cold-climate-rated models

Some manufacturers offer specific cold-climate variants, engineered and validated to maintain a higher proportion of their rated heating capacity at lower outdoor temperatures than a standard model — relevant if you're in a particularly cold or exposed UK location. If your property sits somewhere like the Scottish Highlands, the Pennines, or another location with a notably colder design temperature than the national average, it's worth asking your installer specifically whether a cold-climate-rated model changes the sizing or the economics for your address.

Where the "heat pumps don't work in winter" myth actually comes from

Two separate things feed this belief, and neither is really about cold weather itself:

Older, non-inverter heat pump technology — particularly older US split systems without cold-climate refrigerant blends or variable-speed compressors — genuinely did struggle below around 0°C to -5°C and leaned heavily on expensive resistive backup heating. Modern UK and European inverter-driven units with cold-climate refrigerant blends are a different generation of equipment, and the datasets behind the myth largely predate them.
Undersizing, not temperature, causes most real UK complaints. A heat pump or radiator set sized against a rule of thumb rather than a proper room-by-room heat loss calculation — see our radiators guide for why this matters — will underperform in cold weather regardless of how capable the unit itself is. The fix is a correct heat loss survey at install, not a different heating technology.

What to ask your installer specifically about cold-weather performance

What's the modelled SCOP for this specific property, not a generic figure?
What design temperature has the heat loss calculation used for this location?
What's the flow temperature on the coldest design day, and are the radiators sized for it?
Is backup heat included, and under what conditions does it engage?
Is this model rated for the coldest temperatures this location can realistically see?

The bottom line

A correctly sized UK heat pump works through winter, including hard frosts and snow, as a matter of routine — the technology, sizing methodology and refrigerants used in modern installations are built around exactly this climate. What genuinely varies with temperature is efficiency, not function: expect COP to fall on the coldest days while SCOP — the figure that actually governs your annual bill — stays in a predictable, well-understood range for a property sized correctly in the first place.

Model your own expected running costs, including a realistic winter efficiency dip, in the cost calculator, and check your eligibility for the £7,500 Boiler Upgrade Scheme with the 60-second checker before comparing installers.