How Defrost Cycles Work on Air Source Heat Pumps

Air source heat pumps extract heat from the outside air, even when temperatures are low. During cold and damp UK weather, however, moisture in the air can freeze on the outdoor unit’s heat exchanger. A layer of ice can restrict airflow and reduce the heat pump’s ability to transfer heat. A defrost cycle is the system’s automatic way of removing this ice and maintaining normal operation.

Why Does Ice Form?

When an air source heat pump is heating a property, the outdoor coil can become colder than the surrounding air. If the air contains moisture, condensation can form on the coil and subsequently freeze. This is more likely under certain cold, damp conditions than during very cold but dry weather. A small amount of frost is normal and does not necessarily indicate a fault.

What Happens During Defrost?

When sensors and the heat pump’s controls detect conditions requiring defrosting, the system temporarily changes how it operates. Refrigerant flow is reversed so that heat from inside the system is directed towards the outdoor coil. This warms the coil and melts the accumulated frost or ice.

During this short period, the heat pump may temporarily stop delivering heat to the home. Some systems use stored heat from a buffer or hot water cylinder to help maintain indoor comfort while defrosting takes place. Once the outdoor coil is clear, the system returns to normal heating operation.

Does Defrost Affect Efficiency?

Defrosting uses some additional energy because heat is temporarily directed towards the outdoor unit rather than the property. Modern heat pumps are designed to control defrost cycles automatically and only activate them when necessary. Correct installation, adequate airflow around the outdoor unit and appropriate system design all help the heat pump operate effectively through UK winters.

Occasional frost and defrost cycles are a normal part of air source heat pump operation rather than a sign that the system is malfunctioning.

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