Frost on the unit? Defrost, explained

Frost is normal in cold, damp conditions. The outdoor coil can be colder than the surrounding air, so moisture condenses on it, and during cold weather frost can build on the coil.

The heat pump periodically runs a defrost cycle to warm the outdoor coil and melt the ice. You may see steam or a sudden flow of water beneath the unit during this process.

Air-source heat pumps are designed to keep operating below freezing, and many models are rated for outdoor temperatures around -15°C to -20°C. Efficiency normally drops as outdoor temperature falls. That does not mean the unit stops working.

The water should be managed so it does not create damp, ice or a slip hazard. Common solutions include a condensate tray, drain, gully or soakaway. A refrigerant leak will not normally look like a puddle of clear water under the outdoor unit.

Persistent heavy ice, repeated faults or a unit buried in ice should be checked. Get the system checked too if water is entering the building, ice builds up around the unit, or you see an error code or loss of heating performance.