Do Heat Pumps Work in Cold Weather? The Truth About Winter Performance

Modern air source heat pumps work efficiently in temperatures well below freezing. The Vaillant aroTHERM Plus we install operates down to -22°C and still produces hot water at up to 75°C. The coldest temperature ever recorded in Leicester was -15.9°C in December 1981 — well within the unit’s operating range.

The confusion comes from older heat pump technology, which genuinely did struggle in cold weather. The current generation — particularly R290 (propane) refrigerant models — is a completely different proposition.

A heat pump’s efficiency does decrease as the outdoor temperature drops — this is basic physics. But the decrease is much smaller than people think:

• At 7°C (mild): COP of 4.0–4.5 • At 2°C (chilly): COP of 3.5–4.0 • At -5°C (cold): COP of 2.8–3.2 • At -10°C (very cold): COP of 2.2–2.8

Even at -10°C, a COP of 2.5 means you are getting 2.5 kWh of heat for every 1 kWh of electricity — still far more efficient than a direct electric heater and comparable to gas when you factor in the gas boiler’s real-world efficiency.

At Greenmotion, we design every system to meet your home’s full heating demand on the coldest day of the year (the “design day”). For the East Midlands, this is typically -3°C to -5°C.

This means on the 95% of days that are warmer than the design day, your heat pump has capacity to spare. It runs at partial load, which is actually more efficient than full-load operation.

This design approach is fundamental — and it is where poorly designed systems fail. A system sized for “average” conditions will struggle on cold days, giving heat pumps an undeserved bad reputation.

When the outdoor temperature drops below about 5°C and humidity is high, frost can form on the heat pump’s outdoor unit. The unit periodically runs a defrost cycle — briefly reversing its operation to melt the frost.

This is completely normal and lasts only a few minutes. Modern units like the aroTHERM Plus use intelligent defrost algorithms that only run when actually needed, minimising any impact on efficiency. You should not notice any change in comfort inside your home during a defrost cycle.