Defrost Cycles on Frosty Cambridge Nights — A Heat Pump Owner's Guide

Why Cambridge owners notice defrosting

The Met Office station at NIAB records around 52 air-frost days a year in Cambridge, far more than western cities such as Bristol. On many of those nights, the outdoor coil of a heat pump collects frost and the unit has to clear it. If you’re new to heat pumps, the first frosty spell can be surprising: steam, dripping water, the fan stopping. Almost all of it is normal.

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What’s happening, briefly

The coil outside works at a temperature below the air around it so it can absorb heat. When that’s below freezing and the air has moisture in it, frost forms on the fins. Every so often the heat pump runs a defrost cycle, briefly sending warmth back to the coil to melt the frost, then returning to heating.

Interestingly, the very dry, clear, cold nights Cambridge often gets cause less frosting than milder, damp, misty ones, because there’s less moisture in the air to freeze.

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Normal vs not normal

You see…	Normal?
A cloud of steam for a minute or two	Yes
Water running from the base of the unit	Yes
The fan stopping briefly, then restarting	Yes
A light frost on the fins that comes and goes	Yes
A thick layer of ice that stays after defrosting	No — call an engineer
Ice building up into a block beneath the unit	No — usually a drainage problem
Defrosts every few minutes for hours	No — likely a sensor or refrigerant fault
The fan not moving at all while the unit runs	No

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Will the house cool down?

Not noticeably. A defrost lasts a few minutes and your home’s thermal mass carries it through. Solid-walled gault-brick houses, common across Cambridge’s Victorian streets, hold heat particularly well.

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Drainage matters more in a frosty city

Meltwater from each defrost needs to drain away before the next frost. If it pools, it freezes, builds up under the unit and can eventually restrict it. Cambridge also gets very little rain, so pooled water doesn’t wash away on its own. A good installation:

• sits the unit on raised feet or a wall bracket
• drains water to gravel, a soakaway or a gully
• keeps water off paths, doorsteps and shared alleys, where ice is a slip hazard
• avoids low spots where cold air and water collect

If you rent out a property, check this before tenants move in. An icy path outside the back door is an easy complaint to prevent.

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Winter checklist for owners

1. Before winter, make sure the drain or gravel under the unit is clear.
2. After snow, brush the top and sides of the unit gently.
3. Keep leaves away from the coil — Riverside, Newnham and De Freville’s tree-lined streets drop a lot in autumn.
4. After a run of frosts, glance at the coil: light frost is fine; solid ice isn’t.
5. Never chip, scrape or pour hot water on the unit.

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Does defrosting cost much?

A little energy, already accounted for in realistic efficiency figures. A well-designed system in Cambridge typically averages around 3–3.5 units of heat per unit of electricity across the year, frosty nights included. See SCOP and COP explained.

More: heat pumps on a frosty Cambridge morning · repairs by symptom