Heat pump defrost cycle: what’s normal and what isn’t?

Your heat pump’s defrost cycle is an automatic, self-cleaning process where the system briefly reverses its operation to melt any ice or frost that has built up on the outdoor unit. This happens mainly in cold, damp weather—common in West Sussex winters—to maintain optimal efficiency and prevent damage to the heat pump. By clearing the ice, the defrost cycle ensures your system keeps running smoothly, and your home stays warm without unnecessary energy use.

Why is my heat pump steaming?

Seeing clouds of “smoke” billowing from the outdoor unit can be alarming, but in most cases, it is simply steam. During defrost, the outdoor coil warms up and melts surface frost, which then evaporates into the cold air.

In West Sussex’s damp winter conditions, frost forms readily on the coil, so you may see more frequent, heavier steaming around Billingshurst, Horsham and Pulborough. As long as the steam clears within a few minutes and the unit restarts normally, it is generally nothing to worry about.

If you notice a burning smell, visible flames or the casing discolouring, that is not normal, and you should switch the system off and call an engineer straight away.

Why did it switch off or blow cooler air?

During defrost, most air source heat pumps temporarily stop providing heating to the house. You may hear fans and pumps change speed, feel cooler air at the emitters or notice the outdoor unit pause while it runs a different cycle.

Some systems blow slightly cooler air through radiators or underfloor heating for a short time as the refrigerant circuit changes. Rooms might feel a touch cooler for a few minutes, but they should recover quickly once normal heating resumes.

If the system does not come back on by itself, keeps tripping the breaker or shows a fault code on the controller, that points to an issue that needs professional diagnosis.

How long should defrost last?

On most domestic air source heat pumps, a normal defrost cycle lasts around 3 to 10 minutes. In very cold, damp weather, it can occasionally run a little longer while heavier ice clears from the coil.

During this time, it is normal for the outdoor fan to stop, for steam to appear and for the compressor sound to change. Once the coil temperature rises and the control board is satisfied that the ice has melted, the unit should switch back to heating.

• Typical duration: 3–10 minutes per cycle
• Occasional longer cycles in freezing fog or sleet
• Heating should resume automatically afterwards

If your heat pump seems stuck in defrost for 20 minutes or more, or it keeps entering defrost without properly heating the property in between, that is a sign it needs checking.

How often is too often?

Frequency depends on outdoor temperature, humidity and the specific make of the heat pump. In cold, damp weather, it is common to see a defrost roughly every 30 to 90 minutes of run time, especially overnight.

On milder, dry days, your system may run for many hours without needing to defrost at all. Modern controls aim to defrost only when sensors detect it is genuinely needed, rather than at rigid time intervals.

If your heat pump is defrosting every 10–15 minutes, or almost as soon as it restarts, efficiency and comfort will suffer. That pattern often points to restricted airflow, dirty coils, low refrigerant charge or a faulty sensor that should be checked by an engineer.

When is ice build-up a problem?

A thin, even layer of frost on the outdoor coil before defrost is completely normal. The system is designed to work in freezing conditions and to manage that frost automatically.

Ice becomes a concern when it is thick, uneven or building up on parts of the unit that should stay clear, such as the fan blades, grille or base tray. Solid blocks of ice, or a unit that looks “entombed”, can damage components and prevent proper airflow.

Water from defrost should drain away freely. If it re-freezes below the unit or forms an ice “skirt” that grows up into the coil, that also needs attention.

Normal behaviour vs calling an engineer

Use this quick guide to judge what you are seeing:

Normal operation
Steam during cold, damp weather that clears within a few minutes; outdoor fan stopping briefly; a slight dip in indoor temperature, then recovery; thin, even frost on the coil between cycles; defrost every 30–90 minutes in poor weather.

Call an engineer
Defrost cycles every 10–15 minutes; stuck in defrost for 20+ minutes; no heat restored afterwards; loud new noises or vibration; thick or uneven ice on fan, grille or casing; constant fault codes or tripping electrics.

Simple safety notes

Never chip or scrape ice off your heat pump with tools. You can easily damage the delicate fins or coil, which will be costly to repair. Let the system and an engineer deal with heavy icing.

Keep the area around the outdoor unit clear so it can breathe. Avoid stacking items against it, and trim plants back so they do not restrict airflow or block the grille.

It is also wise to glance at the condensate and defrost water route. If the drain is blocked or running across a path where it re-freezes, it can cause both icing and slipping hazards.

Quick homeowner checklist (no tools needed)

Before calling for help, you can safely check a few basics:

• Confirm the heat pump has power, and any isolator switches are on
• Look for obvious obstructions: leaves, bags, garden items around the unit
• Check vents and grilles are not blocked by snow, debris or overgrown shrubs
• Glance at the controller for error messages or low set temperatures
• Observe a full cycle: does it defrost then come back to heating as normal?

If everything above looks fine yet comfort is poor, the unit is very noisy or ice build-up keeps returning, it is time for a professional visit.

What a professional heat pump service covers

A proper air source heat pump service goes well beyond a quick visual check. The engineer will assess airflow by inspecting the outdoor fan, grilles and clearances, and ensuring there is no build-up of dirt or debris that could trigger excessive frosting.

Coil cleanliness is checked and gently cleaned if needed so heat can transfer efficiently. A dirty coil can cause longer, more frequent defrost cycles and higher running costs.

The refrigerant circuit is inspected for correct pressures and signs of leaks. Low charge affects performance, can lead to icing issues and should only be handled by qualified engineers.

Controls and sensors are tested to confirm that the defrost strategy is functioning correctly. That includes temperature sensors on the coil, an outdoor ambient sensor, and the logic that decides when to start and stop defrost.

Indoors, the engineer will also look at emitters such as radiators or underfloor heating if fitted, ensuring flow temperatures and settings are appropriate for efficient, comfortable operation.

Book expert ASHP servicing in West Sussex

If your heat pump is steaming, icing up or defrosting more than you expect in Billingshurst, Horsham, Pulborough or the surrounding West Sussex area, it is worth getting it checked. Regular servicing keeps the defrost cycle working as intended, protects the unit and maintains efficiency.

For friendly, expert support with air source heat pumps, including diagnostics and ongoing maintenance, contact PIPELINE PLUMBING SERVICES LTD on 01903740538. We can assess whether what you are seeing is normal, resolve any faults and help keep your home warm and comfortable all winter.

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