A heat pump defrost cycle is a short, automatic routine in which the outdoor unit melts the frost that builds up on its coil, and yes, it is completely normal. Every air source heat pump does it in cold, damp weather. It usually lasts a few minutes, and you may see a cloud of steam and hear the fan stop and start while it happens.
Defrosting catches a lot of people out in their first winter with a heat pump. This guide explains what the unit is doing, what it costs and when defrosting points to a fault.
Why Does Frost Form on a Heat Pump in Winter?
Frost forms because the outdoor coil runs colder than the air around it, so moisture in the air condenses on the fins and freezes once the coil surface drops below 0°C. An air source heat pump works by pulling heat out of outdoor air. To do that, the refrigerant in the outdoor coil (the evaporator) has to be colder than the air passing over it.
On a mild day that is not a problem. Water may condense on the fins and simply drip away. On a day when the air is a few degrees above freezing, the coil itself can sit several degrees below zero. The condensation then turns to frost and gradually fills the gaps between the fins. Once frost blocks enough of the coil, air cannot flow through it properly and the heat pump loses output. That is when the controls trigger a defrost.
How Does a Heat Pump Defrost Cycle Work?
A heat pump defrost cycle works by briefly sending hot refrigerant through the frozen outdoor coil to melt the ice, then switching back to normal heating once the coil is clear. The controls decide when to start, usually using sensors that read the coil temperature, the outdoor air temperature and how long the unit has been running.
In a typical defrost:
- The heat pump detects frost on the outdoor coil.
- The outdoor fan stops so it is not blowing cold air over the coil.
- The refrigerant flow is reversed or diverted, so hot gas from the compressor goes to the outdoor coil.
- The ice melts and drains out of the bottom of the unit as water.
- The refrigerant flow switches back, the fan restarts and heating resumes.
Grant describes this same sequence for its Aerona units. While the unit is defrosting it is not sending heat to your radiators or underfloor heating.
What Is the Difference Between Reverse-Cycle and Hot-Gas Defrost?
Reverse-cycle defrost runs the heat pump backwards for a few minutes, while hot-gas defrost diverts hot refrigerant straight from the compressor to the frozen coil without reversing the whole system. Reverse-cycle is the method most domestic air-to-water heat pumps use.
In reverse-cycle mode the outdoor coil temporarily becomes the hot side, so a small amount of heat is borrowed from the water in your heating system. A review of defrost methods by Oak Ridge National Laboratory found most of the melting energy in reverse-cycle defrost comes from the indoor side. In hot-gas bypass, hot refrigerant from the compressor goes directly into the frosted coil and releases its heat there.
| Feature | Reverse-cycle defrost | Hot-gas defrost |
|---|---|---|
| How it works | Refrigerant flow reverses, outdoor coil becomes the hot side | Hot compressor gas is diverted to the outdoor coil |
| Where the melting heat comes from | Mostly the heating water and the indoor heat exchanger | Mainly the compressor’s own work |
| Effect on heating water | Flow temperature dips briefly | Heating output pauses briefly |
| Common in UK homes | Yes, most air-to-water units | Less common in domestic units |
How Often Does a Heat Pump Defrost and How Long Does It Take?
In frosty, damp weather a heat pump typically defrosts every 30 to 90 minutes, and each defrost usually lasts about 2 to 10 minutes. On dry or mild days it may not defrost at all.
Most modern units use “demand defrost”, starting only when sensors show frost is building. Simpler controls may defrost on a timer, which a US field study found can trigger defrosts that are not needed.
| Source | How often | How long |
|---|---|---|
| Vaillant (aroTHERM plus homeowner guide) | Most often around 2.5°C, less often below 0°C | A few minutes |
| Grant (Aerona FAQ) | Whenever ice starts to form on the coil | A few minutes |
| Mitsubishi Electric (heat pump air conditioners) | First defrost after about 40 minutes of running, then about every 40 minutes if it stays cold | About 10 minutes at most, depending on model and conditions |
| Lennox (heat pump guidance) | Every 30 to 90 minutes | Not stated |
| US field study of cold-climate heat pumps (2026) | Varies widely by site and humidity | A few minutes up to 20, median around 4 to 6 |
A few defrosts an hour on a damp, near-freezing morning, with normal running in between, is normal. Near-constant defrosting, or defrosts lasting well over 10 to 15 minutes, is worth getting checked.
Why Is My Heat Pump Steaming and Making Noise?
The steam is warm water vapour coming off the melting ice on the outdoor coil, and it is normal. Vaillant’s operating instructions say that if you see a cloud of vapour on the unit, you do not have to do anything, because it can happen during the thawing process.
The steam can look dramatic on a still, cold morning and can be mistaken for smoke. It is only water. It usually clears within a minute or two of the fan restarting.
The sound changes too. The fan stops, a valve clicks or hisses as the refrigerant flow switches, meltwater drips from the base and the fan restarts at the end. None of that means something is wrong. Grinding, screeching or loud rattling are different and should be checked by an engineer.
At What Temperatures Does a Heat Pump Defrost the Most?
A heat pump defrosts most when the outdoor air is just above freezing, roughly 0 to 5°C, and the air is damp. Vaillant’s homeowner guide says de-icing happens more often around 2.5°C and less often below 0°C, because colder air holds less water. Research from Oak Ridge National Laboratory identifies the most troublesome frosting conditions as air between about -7°C and 5°C with relative humidity above 65%.
This matters in the UK because our winters sit right in that band. Using Met Office data, the government’s long-term mean temperatures for 1991 to 2020 are 5.2°C in December, 4.9°C in January and 5.2°C in February. Add frequent drizzle, fog and damp air, and a UK heat pump can defrost more often than one in a colder but drier climate.
| Outdoor conditions | What to expect from defrosting |
|---|---|
| Above about 6°C | Little or no frost, rare defrosts |
| 0 to 5°C and damp, foggy or drizzly | Most frequent defrosts, often several an hour |
| 0 to 5°C and dry | Some defrosts, less often |
| Below 0°C and dry | Fewer defrosts than you might expect, as the air holds less moisture |
| Freezing fog or wet snow | Frequent defrosts, and snow can block the coil |
How Much Energy Does Defrosting Use?
Defrosting typically adds roughly 5 to 11% to a heat pump’s seasonal electricity use in published modelling, depending mainly on how much time the unit spends in damp, near-freezing air. It is a real cost, and a well-designed system is sized to allow for it.
The clearest published figures come from a 2021 simulation study using eight years of real weather data from three Italian towns. Defrosting increased electricity use by an average of 10.7% in Milan and 9.5% in San Benedetto del Tronto, both mild and humid, but only 4.6% in Livigno, a colder and drier Alpine town. The UK’s damp winters are closer to the milder, humid end of that range than to the Alpine one.
Manufacturers allow for this in their design guidance. Vaillant’s installer information for the aroTHERM plus suggests allowing a 12 to 15% drop in output at design conditions for most parts of the UK. The MCS design standard says an air source system should be able to maintain the design indoor temperatures across multiple defrost cycles.
Real UK results include defrosting. The government-funded Electrification of Heat project measured a median seasonal efficiency (SPF H4) of 2.78 across 428 air source heat pumps in British homes.
Will Defrosting Make My House Go Cold?
No, a correctly sized heat pump should not let your home go noticeably cold during defrosts. Radiators may cool slightly for a few minutes, but heat stored in the water and the building covers the gap.
If rooms cool every time the unit defrosts, that usually points to a design issue, such as an undersized heat pump or too little water in the system. Some systems use a buffer tank or volumiser to give the heat pump more water to draw on.
How Can I Tell Normal Defrosting From a Fault?
Normal defrosting is short, regular and leaves the coil clear afterwards, while a fault usually shows as ice that keeps building, defrosts that never seem to finish or error messages on the controller. The table below covers the signs we see most often.
| What you notice | Normal | Possible fault |
|---|---|---|
| Frost on the coil | Thin, even layer that clears after each defrost | Thick ice that does not clear, or the whole unit encased in ice |
| Steam | Short cloud at the end of a defrost | Constant steam or vapour with no break |
| Defrost length | A few minutes, occasionally up to about 10 | Regularly much longer, or the unit seems stuck in defrost |
| Defrost frequency | Several an hour in damp, near-freezing weather | Almost non-stop, or frequent defrosts on mild, dry days |
| Ice under the unit | Wet ground, a little ice in very cold spells | Ice building up from the base into the coil |
| Fan | Stops during defrost, restarts after | Does not restart, or is blocked by ice |
| Controller | Shows a defrost or de-icing icon | Shows a fault or error code |
| Home temperature | Stays comfortable | Rooms cool noticeably and do not recover |
Common causes of defrost problems include a faulty sensor, a sticking reversing valve, low refrigerant charge, restricted airflow from leaves or snow, and a blocked or frozen condensate drain. Lennox lists all of these as causes of ice build-up on outdoor units. Anything involving the sealed refrigerant circuit needs a qualified refrigeration engineer.
Where Should the Defrost Water Go?
Defrost meltwater must drain away safely to a drain, gully or soakaway, and not onto a path or doorstep where it can freeze. The MCS installation standard (MIS 3005-I) requires that condensate from an air source heat pump is discharged safely to a suitable drain or soakaway, and installers must confirm the drainage is suitable at commissioning.
If meltwater pools under the unit and freezes, ice can build up into the bottom of the coil and stop the defrost working. Many units have a heated base tray to prevent this. Grant, for example, fits an element that switches on when the air falls to 2.5°C.
Good siting and drainage usually includes:
- A drain pipe from the base of the unit to a gully, drain or soakaway.
- A gravel or stone bed under ground-mounted units where no drain is close.
- Enough height off the ground to keep the unit above snow and standing water, following the manufacturer’s instructions.
- Clear space for airflow, following the manufacturer’s clearance distances.
- Meltwater kept well away from paths, steps, doors and drives.
If ice sheets form on a path near your unit, ask your installer to reroute the drainage.
What Should I Do and Not Do When My Heat Pump Is Icing Up?
In most cases you should leave the heat pump alone and let the defrost cycle do its job. Your role is mainly to keep the unit clear and powered.
Do:
- Keep leaves, plants and clutter away from the unit and its grilles.
- Gently brush snow off the air inlet and outlet grilles, as Vaillant recommends.
- Leave the heat pump switched on. Vaillant says it should stay on for frost protection unless separate frost protection is fitted.
- Note any error codes and tell your installer.
- Check the drain outlet is not blocked by leaves or ice.
Do not:
- Pour hot or boiling water on the coil. Lennox warns the sudden temperature change can crack components.
- Chip or scrape ice off the fins with tools. The aluminium fins bend and puncture easily.
- Cover the unit to keep frost off. This blocks airflow.
- Switch the heat pump off at the isolator to “let it thaw”. Without power, its frost protection cannot run.
- Use sprays, solvents or chlorine-based cleaners on it. Vaillant’s instructions say not to.
When Should I Call an Engineer About Defrosting?
You should call an engineer if ice keeps building on the coil after several defrost cycles, if the unit shows an error code, or if your home stops reaching temperature. Other reasons to book a visit include:
- The unit seems to be in defrost most of the time.
- Defrosts happen often on mild, dry days above about 6°C.
- You hear grinding, scraping or loud rattling.
Your annual service is also a good time to raise defrost concerns, so the engineer can check the sensors, drainage and base tray heater.
If you are planning a heat pump and want to know how our engineers will site the unit and handle drainage at your property, we are happy to talk it through as part of a survey.
