Defrosting every 40–90 minutes is normal between about −5 °C and +7 °C when the air is damp, and a cycle should finish in 3–10 minutes. Defrosting every 20–30 minutes, or for longer than 12 minutes, indicates restricted airflow, a fouled coil, low refrigerant charge, too little system water volume, or a unit re-inhaling its own cold discharge. Over a season defrost typically costs 1.5–13% of COP.
How often should an air source heat pump defrost?
Every 40–90 minutes of continuous running in frosting conditions, with each cycle lasting 3–10 minutes. That is normal and is already included in the SCOP figure on the label, because EN 14825 requires defrost losses to be measured as part of seasonal performance testing.
Frosting conditions are narrower than people expect. Frost forms when the evaporator surface falls below both 0 °C and the dew point of the outdoor air, which happens most aggressively between about −5 °C and +7 °C with high humidity — damp, drizzly, foggy weather near freezing. At −12 °C the air holds so little moisture that frosting slows down again, which is why a heat pump often defrosts less in a hard dry frost than in a wet English or German December.
What does defrosting actually cost?
The unit reverses the cycle, pulling heat out of your heating water to melt the ice, so for those minutes the house is being cooled and the electricity is still being paid for. Published studies put the seasonal COP reduction attributable to defrost at roughly 1.5% in cold dry climates and up to 12–13% in cold humid ones — a range that depends almost entirely on local weather, not on brand.
Where it becomes expensive is when defrost goes wrong. Delaying defrost until the coil is heavily iced can cut heating capacity by 30–50% and COP by a similar order, and a unit that defrosts every 25 minutes for 12 minutes at a time is spending a third of its life going backwards.
Why is my unit defrosting every 20–30 minutes?
| Symptom | Likely cause | Fix |
|---|---|---|
| Frequent short defrosts, mild damp weather | Normal frosting conditions | None needed — check it is genuinely 40 min+ apart |
| Defrost every 20–30 min, unit in an alley or corner | Recirculation of cold discharge air | Reposition, remove obstruction, open up clearance |
| Long defrosts (over 12 min), ice at the base | Blocked condensate drain, ice build-up under coil | Clear drain, raise unit, add gravel soakaway |
| Ice persists after defrost | Low refrigerant charge or failing reversing valve | Engineer: pressure and superheat check |
| House goes cold during every defrost | Too little system water volume | Add a volumiser or buffer to the manufacturer's minimum |
| Defrost immediately after a hot water cycle | DHW priority leaving the coil iced | Re-schedule hot water, shorten DHW runs |
| Reduced airflow, dirty coil | Leaves, seeds, cat hair, pollen fouling the fins | Clean the coil with a soft brush and low-pressure water |
The two causes an owner can act on directly are airflow and water volume. Airflow is a siting problem — see where should the heat pump outdoor unit go? — and every degree the intake air is depressed by recirculated discharge brings the frosting threshold closer.
Water volume is the reason so many owners feel the defrost. During a reverse-cycle defrost the machine needs 1–2 kWh of stored heat to draw on; a system with only 30 litres of water in it gives that up in a couple of minutes and the flow temperature collapses, which can abort the defrost before the coil is clear. Manufacturers therefore specify a minimum system volume, often 10–20 litres per kW — the core argument for a buffer tank or volumiser, discussed in do I need a buffer tank with a heat pump?
How do I reduce defrost frequency?
- Give the unit air. Clear 1–2 m in front of the fan; remove screens, hedges and covers that trap the plume.
- Raise it off the ground by 200–300 mm and drain the melt water to a soakaway, so the base never re-freezes into a block.
- Clean the coil at the start of each heating season; a fouled evaporator runs colder and frosts sooner.
- Meet the minimum water volume so each defrost completes properly first time.
- Avoid long resistance-heater backup, which masks the symptom and hides a fault.
Defrost behaviour at low ambient is one of the harder things to engineer, which is why Solimpeks rates its Varm Up Series EVI heat pumps for operation down to −30 °C: frosting conditions, not extreme cold, are what an air source machine actually has to survive.
What you should not do is disable or delay defrost in the controller. Running a heavily iced evaporator destroys efficiency and eventually damages the compressor. A heat pump that defrosts sensibly is doing its job; see do heat pumps work below freezing? for what to expect from the season as a whole, and the defrost cycle entry for the mechanism.
Frequently asked questions
How long should a heat pump defrost cycle last?
Three to ten minutes. Anything beyond about twelve minutes suggests a blocked condensate drain, ice accumulating under the unit, low refrigerant charge or insufficient system water volume for the machine to draw heat from.
Is it normal for a heat pump to blow steam in winter?
Yes. The visible plume during and just after a defrost is water vapour from melting frost, not smoke or a leak. Seeing it every 40–90 minutes in damp weather near freezing is completely normal operation.
Does defrosting mean my heat pump is broken?
No — defrosting is designed behaviour and is already counted in the SCOP on the energy label. It only indicates a fault when cycles come closer than about 30 minutes apart, run longer than 12 minutes, or leave visible ice on the coil afterwards.
Why does my house get cold during a defrost?
Because the cycle reverses and takes heat out of the heating water. With enough system volume — typically 10–20 litres per kW in a buffer or volumiser — that heat comes from stored water rather than from your radiators, and the temperature drop is barely noticeable.