Air Conditioning Defrost Cycle

During the colder winter months, reverse cycle air conditioners will, at some point during heating operation, will perform a “defrost” cycle. Some owners feel that this is a fault because it isn’t heating as well as it should. Defrosting is normal part of the heating operation and is not a fault with the product. The onus is on both the reseller and the installer to explain the operation of the air conditioner so the customer can understand this prior to experiencing a defrost cycle.

What is a “defrost cycle”?

In heating operation a reverse cycle air conditioner extracts heat from the air outside and transfers it inside your premises to warm it.

Defrosting starts when the temperature of outdoor heat exchanger becomes too low (around 0°C or below) and the moisture in the air starts to freeze on the outdoor unit’s heat exchanger.

The sensors in the outdoor unit detect ice has started forming on the heat exchanger and the system starts trying to remove this ice before it builds up.

The indoor and outdoor fans will stop, but the compressor keeps running to melt the frost away.

This mode continues until the temperature of outdoor heat exchanger rises and the ice melts,.


Why does my unit have to do a defrost cycle?

Any ice building up on the outside heat exchanger will reduce the air flow across the heat exchanger, this reduces the efficiency, the more ice build-up the more it is reduced. In extreme cases this can also cause damage to the outdoor unit.


How do I tell if my unit is in a defrost cycle?

Inside you will notice the unit will stop heating, the indoor fan will stop and depending on the model there will be some form of visual indication such as a message on your wall control indicating heat standby or defrost. Outside, the outdoor fan will also have stopped but compressor will be running.

How often will my unit go in to defrost mode?

There are a number of factors that influence how often a unit will go in to defrost mode. Some of these include, but is not limited to:

  • The outdoor temperature and humidity
  • The amount of heating required
  • The condition and cleanliness of the system.
  • The system has programmed logic to control how often defrosting can occur. Generally a unit could run for around 40 minutes after starting up before a defrost can be initiated. From there defrosts could occur approximately every 40 minutes if the temperature hasn’t risen.

    Once my unit is defrosting how long will it take?

    There are many variables that affect the duration of a defrost cycle.

    As a general guide only, these factors could bring the unit out of a defrost cycle.

  • If the sensors on the outdoor unit detect that the heat exchanger temperature has risen enough, the unit will stop defrosting and resume heating. 
  • If the sensors have not already stopped the defrost, the time a unit might be in defrost cycle could be approximately 10 minutes depending on conditions and model. This may be a little longer depending on whether all the ice was removed during the previous defrost cycle.
  • The mode has changed out of heat. A mode other than “heating” has been selected
  • It is important not to stop the unit before the defrost cycle has ended, because if the unit is restarted shortly afterwards, efficiency will be reduced and you may cause damage to the unit.

    My unit is defrosting frequently / not delivering enough heat – what could be wrong?

    Regular defrosting, or a lack of heat could be caused by a number of factors.

  • If the unit has operated like this since it was first installed you may be operating it incorrectly or it may be inadequately sized for the space it is trying to heat. 
  • The outdoor unit has been installed in a location with insufficient ventilation. 
  • The sensor may be in the wrong location.
  • Initially you should consult your instruction manual to ensure you are operating the unit correctly.

    If this doesn’t remedy the problem you should then consult your installer prior to contacting the manufacturer.

    They can assist you to ensure correct operation.

  • A recently developed problem may be an indication of a fault or maintenance 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 remedy the problem you should consult your installer prior to contacting the manufacturer.
  • PLEASE NOTE: If the unit is inadequately sized for the space it is trying to heat, this is not a fault with the product. The responsibility for correctly sizing the unit initially rests with the installing company – they will need to remedy the situation.


    Is there any way I can help to reduce defrosting?

    Yes there is.

  • Keep your unit well maintained (as above) and ensure you are operating it correctly. 
  • Don’t have the thermostat set too high. 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.
  • Make sure there has been no barriers erected around the outdoor unit since it was installed.
  • Raise the outdoor unit off the ground by at least 100 mm, by doing this the outdoor unit isn’t sitting in a pool of cold moist air.