Why Is My Heat Pump Steaming? Defrost Cycles Explained

The short answer

If your heat pump is steaming on a cold morning, it’s almost certainly defrosting. The outdoor unit collects frost in cold, damp weather and every so often melts it off. The steam is that melting frost. It’s normal and takes a few minutes.

Why frost forms

The outdoor unit pulls heat out of the air by keeping its metal coil colder than the air. In winter, that coil drops below freezing. Any moisture in the air freezes onto it, like frost on a car. Leicester’s inland, East Midlands winters bring plenty of cold, damp mornings — the nearby Sutton Bonington weather station averages around nine frosty nights in a typical January — so expect regular defrosts from November to March.

What happens during a defrost

The controller spots frost building up.
The fan stops and the heat pump briefly sends warmth to the outdoor coil.
The frost melts. You may see steam and hear a gentle whoosh.
Water drains from the bottom of the unit.
After a few minutes, normal heating resumes.

Will I notice it indoors?

Not usually. Your house stays warm for the few minutes it takes. If you happen to be watching the radiator temperature closely, you might see a small dip.

How often, and does it cost much?

On a damp morning around freezing, a defrost every 30–90 minutes is normal. The energy used is small and is already included in realistic yearly efficiency figures — around 3.0–3.5 for a good installation in Leicester. See running costs.

Back yard checks

Most Leicester heat pumps sit in a back yard, so a few things matter:

Drainage. Meltwater should run to a drain or gravel, not across the yard to the back door or onto the shared alley (the jitty), where it can freeze.
Clearance. Keep bins, bikes and garden furniture at least 30 cm away. A blocked unit frosts up faster and defrosts more often.
Raised feet. The unit should sit up off the ground so ice can’t build up underneath.
Neighbours. Don’t let water drain over the boundary onto a neighbour’s yard.

When it’s a fault

Call an engineer if:

ice covers the coil and doesn’t melt after a defrost
a solid lump of ice builds up under the unit
it seems to defrost constantly, every few minutes, for hours
the fan isn’t turning when it should be

Switch it off at the isolator and wait for help. Don’t chip at the ice or pour hot water on it — you can damage the fins. See heat pump repair.

See what a heat pump would look like in your Leicester home

The mechanism is the same in every UK home; the design is specific to yours. Our team carries out a free in-home survey including a full room-by-room heat-loss calculation, identifies the system size and configuration that fits your property, and provides a written quote with the £7,500 BUS grant already deducted.

You'll see the actual figure you'd pay — not an estimate — and the projected efficiency for your specific install.