Guide · Troubleshooting
Heat pump defrost cycles explained
The steam cloud that makes people think their heat pump is broken.
A heat pump defrost cycle briefly reverses the refrigerant flow to melt frost off the outdoor coil, typically lasting two to ten minutes and occurring most often between -5°C and +7°C in damp UK weather. Visible steam, a change in fan noise and a short dip in indoor flow temperature are all normal.
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Why frost forms at all
The outdoor coil sits below the outside air temperature so it can absorb heat. When air is humid and close to freezing, moisture condenses on the coil and freezes. Frost insulates the coil and blocks airflow, so it has to be cleared.
What is normal
- One to four defrosts an hour in damp near-freezing conditions.
- Steam rising from the unit as melted frost evaporates.
- A puddle or ice patch under the unit if the condensate drain is short.
- A brief drop in indoor radiator temperature during the cycle.
When to call the installer
- Defrosting constantly in mild, dry weather.
- Thick ice building up on the base pan and not clearing.
- Water pooling and freezing across a path — a drainage design issue.
- Repeated fault codes after defrost cycles.
Frequently asked questions
- Why is my heat pump steaming?
- It is running a defrost cycle — melted frost evaporating off the coil. This is normal in cold, damp weather.
- How long does a defrost cycle last?
- Usually two to ten minutes, after which normal heating resumes automatically.
- Do defrost cycles waste a lot of energy?
- They cost a small amount of efficiency, which is already accounted for in the SCOP figure on your MCS performance estimate.
- Energy Saving Trust — Heat pump efficiency, running cost and SCOP benchmarks (UK).
- MCS Certified — Installer certification scheme required for grant eligibility.
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