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Heat pumps in winter: cold weather, frost and defrost explained

James Welford

3 hours ago

3 min read

UK homeowner guide. Checked 30 October 2026.

An air-source heat pump can work in cold weather, but its output and efficiency change with outdoor and water temperature. Frost on the outdoor coil can be normal. The important distinction is between a system completing its intended defrost cycle and ice or poor heating that persists because something is wrong.

Three winter questions that matter

Can the unit supply enough heat at your local design temperature? Can the emitters deliver that heat at the selected water temperature? Can condensate and defrost water drain without freezing into a hazard? A catalogue minimum operating temperature answers none of those questions on its own.

The MCS design-condition reference uses location and altitude when establishing design conditions. A generic national “winter temperature” should not replace your project calculation.

Why frost appears

The outdoor heat exchanger can be colder than the surrounding air. Moisture can freeze on its surface, restricting airflow and heat transfer. The heat pump periodically removes that frost using its designed defrost process. A temporary change in fan behaviour, sound or visible water vapour may accompany it.

Frequency depends on the weather and system operation. A damp day near freezing can create a different frosting duty from a colder, dry day. There is no universal correct number of defrosts per hour that diagnoses every model.

What to watch and record

Observation	Useful action
Frost clears and heating resumes	Record normal behaviour; follow the manual
Ice remains after repeated cycles	Photograph it and contact the installer
Water freezes below the unit	Have drainage and the base arrangement assessed
Cold rooms or long recovery	Record room/outdoor/flow temperatures and operating schedule
Repeated faults or backup use	Keep fault codes and electricity records for the engineer

Do not chip the coil, pour boiling water onto equipment or open panels. Keep the authorised airflow space clear and follow the manufacturer’s safe owner checks. A sustained fault or loss of heating belongs with the installer or service provider.

Defrost needs a system, not just a compressor

The manufacturer specifies minimum flow and water-volume requirements. Closing too many emitter circuits or changing valves can leave the system outside those requirements. Ask the engineer to explain how defrost is supported when room thermostats or underfloor actuators close.

A volumiser, buffer or always-open circuit may form part of an approved design, but the correct arrangement is model-specific. Keep the commissioning record; avoid replacing it with a generic online plumbing diagram.

High flow temperature makes winter harder

As the required water temperature rises, the compressor has a larger temperature lift to provide. A maximum 70°C or 75°C claim is useful information about the operating envelope, but it is not a guarantee of cheap operation at that temperature throughout winter.

Ask for capacity and COP at your design condition and for an annual estimate that includes hot water, pumps, backup and defrost as appropriate. Our running-cost guide explains the measurement boundary.

Example: compare the actual duty

Suppose a house requires 7 kW at its design weather and a 50°C flow. A product called “8 kW” is suitable only if the exact selection can meet the required duty at that condition, including the agreed treatment of defrost and any acoustic restriction. The same label on another product may use a different rating point.

The Grant Aerona 290 review explains a range explicitly labelled at A-5/W55. The Cosy review uses published cold-air outputs at two water temperatures. These are more useful comparisons than the lowest advertised ambient limit.

Prepare before the cold spell

Keep the drainage and airflow space clear, understand the normal heating schedule, and know the fault-reporting route. Check the commissioned frost strategy if a property may be left unoccupied or lose power. Do not turn off essential protections to chase a lower app reading.

For persistent high bills or cold rooms, use the problem-diagnosis guide and provide records to your installer.