A defrost cycle is the routine by which an air-source heat pump melts frost from its outdoor coil, usually by reversing the refrigerant circuit for 2–10 minutes and drawing heat from the heating water. Frost forms mainly between +5 and -7 °C in humid weather. Defrosting typically consumes 5–10% of winter heat output, and manufacturers require a minimum system volume — often 10–20 litres per kW — to supply it.
Why do heat pumps need to defrost?
Frost forms on the outdoor coil whenever its surface sits below both the dew point and 0 °C — conditions met most often between about +5 and -7 °C outdoor temperature in humid weather, and most intensely around 0 °C. The evaporator runs several degrees colder than the air passing it, so moisture freezes onto the fins.
A frosted coil fails twice over: the ice layer insulates the fins and chokes the airflow, so heating capacity and COP sag together. Left unmanaged, the coil would block completely. Every air-source unit therefore schedules defrost events; ground-source units, whose source never frosts, skip the entire subject.
How does a reverse-cycle defrost work?
The controller flips the four-way valve, running the heat pump backwards for a few minutes: the outdoor coil becomes the condenser, hot refrigerant melts the frost from inside, and the fan stops so the heat stays in the metal. The energy comes from the heating circuit — the same water the unit spent the last hour warming. After typically 2–10 minutes, sensors confirm the coil is clear and normal heating resumes.
Two visible side effects are normal and regularly mistaken for faults: a cloud of water vapour rising from the unit during defrost, and meltwater draining from the base pan — which needs a clear path away from the unit, plus a pan heater in hard-frost climates, so the melt does not refreeze into an ice pedestal.
How much energy does defrosting cost?
In cold, humid climates defrosting typically consumes 5–10% of winter heat output — a real but bounded tax, and one already included in seasonal ratings, since EN 14825 test points behind SCOP capture frosting conditions. The bigger practical issue is system volume: because defrost borrows heat from the heating water, manufacturers specify a minimum volume, commonly 10–20 litres per kW. Too little water and rooms cool noticeably at every defrost, or the unit trips on low flow temperature. A buffer tank is the standard guarantee — the reasoning is laid out in Do I need a buffer tank with a heat pump?.
What defrost problems should installers watch for?
| Observation | Likely cause | Remedy |
|---|---|---|
| Defrosts every 20–30 minutes | Wind-exposed siting, recirculating cold air, low refrigerant charge | Re-site or shield the unit, verify charge |
| Rooms cool during defrost | System volume below manufacturer minimum | Add buffer volume |
| Ice sheet under the unit | Blocked condensate path, no pan heater | Clear drainage, fit pan heater |
| Coil never fully clears | Faulty sensors or valve, undersized defrost duration | Service the unit |
Modern demand-based defrost — triggered by measured coil conditions rather than a fixed timer — cuts unnecessary cycles substantially, one of several controls refinements behind the efficiency of current units. Cold-climate designs pair it with EVI compressors, as in the Solimpeks Varm Up Series rated for operation down to -30 °C, so capacity survives the weather that causes frosting in the first place. Europe installed 2.88 million heat pumps in 2025 (EHPA, +13%), the majority of them air-source units for which well-managed defrost is simply part of normal winter operation.
Frequently asked questions
Is steam coming from my heat pump normal?
Yes. The visible cloud during a defrost cycle is water vapour from melting frost, not smoke or refrigerant. It appears for a few minutes in cold weather and stops when normal heating resumes.
How often should a heat pump defrost?
In frosting conditions, an interval of roughly 30–90 minutes between defrosts is typical, each lasting 2–10 minutes. Much more frequent defrosting suggests poor siting, airflow recirculation or a refrigerant fault worth investigating.
Does the defrost cycle use the backup heater?
Usually not — reverse-cycle defrost draws heat from the heating water. Some controls temporarily enable the backup heater to protect comfort during defrost, which is a configuration choice, not a necessity, when system volume is adequate.
Can defrost cycles freeze my heating circuit?
No. The energy borrowed per event is small; with the manufacturer's minimum system volume — often secured by a buffer tank — flow temperature dips only a few degrees before the unit switches back to heating.