The defrost cycle and how it affects your air conditioner

Jul 21, 2021

During the colder months leading into winter, there is a defrost mode that will occur within your air conditioner when it is in heating operation, especially in the Canberra climate where the temperature can drop significantly overnight.

It is a common occurrence for most reverse cycle air conditioning systems and is considered a normal part of your system’s life cycle but can often be misconstrued as a system fault when the air conditioner’s heating is not performing as well as it usually does.

So, what does a defrost cycle look, sound, and feel like?

Defrosting usually begins when the outdoor ambient temperature gets very cold, and the moisture in the air has begun to freeze on the outdoor unit’s heat exchanger. A sensor in the outdoor machine will recognise ice starting to form on the heat exchanger and trigger the defrost cycle to remove the ice before it builds up.

When the system goes into the defrost cycle, you will notice the indoor and outdoor fans will pause while the compressor continues running to melt the frost away.

If an air conditioner does not go through the defrost cycle, ice will build up on the heat exchanger and reduce airflow moving across it, which in turn reduces the efficiency of the machine. The more ice that builds up, the less efficient the system will be. In extreme cases, this can also cause damage to the outdoor unit.

Signs your air conditioner is going through the defrost cycle

  • The indoor and outdoor fans will pause for a period of time.
  • The compressor will keep running and can appear louder than normal.
  • Some models of air conditioning units display an icon on the controller indicating the heat is on standby or in defrost mode during this time.
  • Heavier pools of water from ice melting will appear around the outdoor machine.
  • You may notice a “smoke like” appearance coming from the outdoor unit. This is steam and is normal.



How often will my air conditioner go through a defrost cycle?

  • There are many variables to how often an air conditioner will go through the defrost cycle, including outdoor temperature and humidity, the amount of heat load the air conditioner is trying to deliver as well as the condition of the machine. However generally speaking, it can be approximately 40 minutes after initial start-up and could then occur every 40 minutes thereafter if the temperature has not risen.
  • The defrost cycle can last approximately 10 – 45 minutes depending on the conditions ie. outdoor temperature, age and condition of the machine and model of unit.
  • It is important not to stop the air conditioner before the defrost cycle has completed. If the system is restarted shortly afterwards, it will run inefficiently and may cause harm to the machine.

Tips to ensure your home is kept warm in winter

If you find your home is dropping to a very low temperature overnight and struggling to heat up in the morning, you can try:

  • Setting a timer 1.5 – 2 hours prior to getting up in the morning.
  • Running your air conditioner on 18-19 degrees at a low speed overnight. If you don’t like the air conditioner on in your bedroom/s and you have zoning installed, you can switch these areas off and just have the main living areas running overnight.
  • Setting the timer for the air conditioner to turn on 1 hour before you arrive home.
  • Insulation of walls, under floor or ceiling space, keeping doors shut and curtains closed will help to reduce your homes’ heating requirements.

Ways to reduce the defrost cycle

  • Ensure your air conditioner is well maintained, including regular cleaning of filters.
  • Do not set the thermostat too high. A lower thermostat setting will place less load on your air conditioner and prevent it from having to go into the defrost cycle as often.

    Have some more questions about the defrost cycle and your air conditioner? Our industry experts can help answer all them all!