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Heat Pump Defrost Cycle

Heat Pump Defrost Cycle

We recieve many calls about this issue, particularly over the winter months; Auto Defrost Cycle.

What is it? Defrost Cycle occurs during heating mode; when the heat pump extracts heat from the outside air and transfers it inside your home. When the outside ambient temperature is very cold (near 0°C or below) the moisture in the outside air freezes on the outdoor unit’s heat exchanger coil as the fan blows air across it. Defrost cycle is simply the heat pump recognising that ice has formed, or begun to form, on the heat exchange coil and automatically fixing this.

Defrost mode is needed as any ice build up on the outside heat exchange coil reduces the airflow across it, effecting the efficiency. In extreme cases this can damage to the outdoor unit.

When the heat pump goes to defrost mode you'll notice the unit inside will stop heating, the indoor fan will stop or slow right down, and depending on the model there will can be some blinking lights on the indoor unit. The outdoor unit's fan will stop though the compressor will be running.

There are a number of factors that influence how often a unit will go in to defrost mode. Some of these include:

  • Temperature and humidity outside
  • Amount of heat the unit is trying to delivery
  • General condition of the heat pump

There are timers built in to the heat pump's control board that restrict the frequency of defrost cycles. Usually a unit must run for a minimum of 35 minutes before completing the first defrost. From there they shouldn't occur any more frequently than 30 - 40 mins.

Defrost cycle can last between 5 - 10 minutes depending on the reason the heat pump goes to defrost cycle. The fastest is usually when the the outdoor unit sensors unit detect the heat exchange temperature has risen enough and the unit will stop defrost cycle. Otherwise a timer usually stops defrost after around 10 minutes

NOTE: It is important not to stop the unit before the defrost cycle has ended; if the unit is restarted shortly afterwards it will run very inefficiently and may cause damage to itself.

If your heat pump is regularly defrosting, or has a lack of heat, there could be a number of causes.

  1. If the unit has operated like this since it was first installed (first cold snap), you may be operating it incorrectly or it may be undersized for the space it is trying to heat. Initially you should consult your instruction manual to ensure you are operating the unit correctly. If this doesn’t help please call us on 0800 47 48 97.
  2. If the unit is undersized for the space it is not faulty. The responsibility for correctly sizing the unit initially rests with the installing company – they will need to remedy the situation if the unit is too small.
  3. If this heat pump suddenly starts defrost cycle more frequenty it may indicate a fault, or that maintenance is required. You can perform some basic maintenance yourself by cleaning the filters on your indoor unit, and ensuring that your outdoor unit is clear of foliage and the heat exchanger is not blocked. If this doesn’t fix the problem please call us on 0800 47 48 97 to book a service technician to take a look.

You can minimuse your heat pump going into defrost mode by keeping your unit well maintained and ensure you are operating it correctly.

Obviously the less load you place on the unit the less frequently it will need to defrost in cold conditions. Ultimately permanent fixes such as installing insulation in ceilings, walls and under floors will help reduce your heating requirement (and ultimately save you money). More immediately, keeping doors closed and curtains drawn will also help to reduce your heating required.