One of the most common concerns people raise before installing an air source heat pump is: what happens in winter? It’s a fair question, and the answer is more nuanced than either “heat pumps don’t work in cold weather” (false) or “heat pumps work perfectly fine at any temperature” (also misleading).
Here’s what actually happens when the temperature drops.
How Heat Pumps Work at Low Temperatures
An air source heat pump extracts heat from outdoor air and transfers it inside. Crucially, there’s usable heat energy in air even at temperatures well below zero. A heat pump can extract heat from air at -15°C, though it’s doing so with less efficiency than at 7°C.
The measure of efficiency is the Coefficient of Performance (COP). At 7°C outside and a flow temperature of 35°C, a modern heat pump might achieve a COP of 4.0 — meaning it delivers 4kWh of heat for every 1kWh of electricity consumed. At -3°C outside, the same heat pump running at the same flow temperature might achieve a COP of 2.5–3.0. Still more efficient than electric resistance heating (COP of 1.0), but noticeably lower.
UK winters are mild by the standards of heat pump design. The average outdoor temperature across the UK heating season is around 5–7°C, with genuinely cold periods — below -5°C — being uncommon and short-lived in most of England. Scotland, northern England, and higher elevation sites see more extreme cold, but even there the average heating season temperature doesn’t approach the levels where heat pumps struggle significantly.
Your heat pump installer should design the system for the “design outdoor temperature” for your location — in most of England this is around -3°C to -5°C. This is the temperature at which the heat pump should still deliver enough output to heat your home, though at reduced efficiency.
What Defrost Cycles Are and Why They Happen
When outdoor air passes over the heat pump’s evaporator coil, moisture in the air can freeze onto the coil surface. This is the same process that frosts up a freezer’s evaporator — and just like a freezer needs defrosting, so does a heat pump when conditions are right.
Frosting conditions vary by temperature and humidity. You’re most likely to see frost on the outdoor unit when the temperature is between roughly -5°C and +5°C combined with high humidity — so damp, mild-but-cold UK winter days, not the driest coldest days. At very low temperatures, there’s often less moisture in the air and less frosting occurs.
The heat pump manages this automatically. When the controller detects that the evaporator is frosting up (by monitoring coil temperature), it initiates a defrost cycle: typically reversing the refrigerant flow so warm refrigerant passes through the outdoor coil, melting the ice. This takes 5–10 minutes. During this period, the heat pump isn’t heating your home — it’s briefly drawing heat from the house to melt the ice.
You’ll know a defrost cycle is happening because:
- The outdoor unit fans stop and you may hear different sounds from the unit
- Steam or water may come from the outdoor unit as ice melts
- Your heating system may temporarily cool slightly
- Modern systems may show a defrost indicator on the controller
This is completely normal and expected. A well-commissioned heat pump will have a buffer tank or thermal mass in the system (your radiators, underfloor heating, or a dedicated buffer vessel) that absorbs the brief interruption without you noticing a temperature drop indoors.
How Often Defrost Cycles Happen
In a UK winter, a heat pump might defrost several times per day during particularly humid conditions, or not at all during cold, dry periods. The frequency depends heavily on your local climate. Coastal locations with high humidity tend to see more frequent defrosting than inland sites.
If your heat pump is defrosting very frequently — more than once per hour — this can indicate a problem with the system rather than normal operation. Possible causes include inadequate airflow around the outdoor unit, a unit sited where it receives drips of water from above, or refrigerant charge issues. If you’re seeing very frequent defrost cycles, contact your installer.
Supplementary Heating
Some heat pump systems include an immersion heater or electric resistance heater as a backup — often called an “auxiliary heater” or “backup heater.” This kicks in when the heat pump’s output isn’t sufficient to meet demand, typically during very cold weather.
The presence of backup resistance heating is normal in a well-designed system. The question is how much it runs. A well-sized heat pump in a reasonably insulated UK home should rely on backup heating for only a small fraction of annual energy use — perhaps 5–15% of heating energy in a typical year, mostly during the coldest weeks.
If your backup heater is running constantly throughout winter, that suggests the heat pump is undersized for your home’s heat loss. This can happen if the installer didn’t carry out a proper heat loss calculation, or if the home has higher heat losses than assumed (common in poorly insulated solid wall properties).
Managing Your Heat Pump in Cold Weather
A few practical points:
Don’t turn the heat pump off overnight. Unlike a gas boiler, a heat pump is designed to run continuously at a lower output rather than cycling on and off. Turning it off and reheating from cold uses more energy than maintaining temperature.
Use weather compensation if your system has it. This automatically adjusts the flow temperature based on outdoor temperature — raising it when it’s cold outside, lowering it when mild. It optimises efficiency across the winter without you needing to adjust settings.
Don’t panic when you see steam from the outdoor unit. This is the defrost cycle working as intended. It’s steam from melting ice, not a sign of a problem.
Give the outdoor unit clear airflow. Snow or leaves blocking the unit will cause problems. Clear snow from around the unit after heavy falls, but don’t cover it.
The bottom line is that heat pumps do work in UK winters — the UK’s climate is well-suited to heat pump technology. The seasonal performance figures across real-world UK installations are consistently positive, and the mild, damp winter conditions are actually more representative of typical use than the extreme cold that sometimes dominates discussions of heat pump limits.