Heat Pump Going Into Defrost — Is It Normal?
If you've recently installed an air source heat pump, you may have noticed it occasionally stops heating and releases steam from the outdoor unit — especially during cold, damp British winters. This is the defrost cycle at work, and it's one of the most misunderstood aspects of heat pump ownership. When temperatures drop below 5°C and humidity is high, frost naturally builds up on the outdoor coil, reducing efficiency. Your heat pump has been designed to handle this automatically by briefly reversing operation to melt the ice away. This guide explains what's happening, what you'll observe, and — most importantly — when defrost is normal behaviour versus a genuine fault. Most homeowners worry unnecessarily during their first winter; understanding the cycle will give you confidence that your system is working exactly as it should.
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In cold and damp weather (typically below 5°C), frost can form on the outdoor unit's heat exchanger coil. When this happens, the heat pump automatically enters a defrost cycle — it temporarily reverses its operation to melt the ice. This is completely normal and not a fault.
During defrost you may notice: the outdoor unit fan stops spinning, steam or water vapour rises from the outdoor unit, the indoor heating temporarily pauses for 3–10 minutes, and the unit may make gurgling or hissing sounds. All of this is expected behaviour.
Defrost cycles typically last 3–10 minutes and happen more frequently in very cold, damp weather (0–5°C and high humidity). In dry cold weather (below -5°C and low humidity) defrost cycles are actually less frequent.
After defrost the heat pump resumes normal operation automatically — no user action is required. If your heat pump stays in defrost for more than 20 minutes or goes in and out of defrost repeatedly without returning to heating mode, there may be a fault — note any error codes on the controller and call your heat pump installer.
If the outdoor unit is heavily iced over and not defrosting itself — for example if the unit has been covered in compacted snow — carefully remove snow from around the unit (not from the coil itself) to allow airflow. Never use a sharp implement near the fins.
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Frequently asked questions
Why does my heat pump blow cold air during defrost?
During defrost, the heat pump reverses its heating cycle to melt ice from the outdoor coil — this means warm air is temporarily sent outside instead of into your home. This pause typically lasts 3–10 minutes and is completely normal. Your heating will resume automatically once the coil is clear.
How often should my heat pump go into defrost?
Defrost cycles happen more frequently in temperatures between 0–5°C with high humidity — you might see them every 30 minutes to an hour during damp winter weather. In very cold, dry conditions (below -5°C), they're actually less frequent because frost forms more slowly. The frequency varies depending on your local climate and outdoor humidity.
Is it normal to see steam coming from the outdoor unit?
Yes, the steam or water vapour you see is simply frost melting on the coil — it's a sign defrost is working properly. You may also hear gurgling or hissing sounds, and the fan may stop briefly. All of these are expected during a defrost cycle.
When should I call my heat pump installer about defrost problems?
Contact your installer if defrost lasts longer than 20 minutes, the unit repeatedly cycles in and out of defrost without returning to heating mode, or the outdoor coil remains heavily iced over. Note any error codes displayed on your controller to give the engineer. Snow compacted around the unit can also prevent proper defrost — gently clear snow from around the unit (not the coil itself) to restore airflow.