What is a “defrost cycle” or “deice mode”?
In heating mode a heat pump extracts heat from the outside air and transfers it inside your premises to warm it. When the ambient temperature outside gets very cold (close to 0 degrees or below) the moisture in the air freezes on the outdoor unit’s heat exchanger as the fan blows the air across it. A defrost cycle is simply the system recognising that ice has formed or begun to form and automatically fixing this.
It is important not to stop the unit before the defrost cycle has ended, because if the unit is restarted shortly afterwards it will run very inefficiently and may cause damage to itself.
Why does my unit have to do a defrost cycle?
Any ice building up on the outside heat exchanger reduces the airflow across it, which will effect the efficiency, sometimes reducing it dramatically. 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. Outside, the outdoor fan will also have stopped and the compressor will be running. There will be a puddle of water under and around the outdoor unit.
A couple of things you may notice your air conditioning doing now you have switched it to heating
A loud whooshing sound just before the indoor fan stops. This is the system cycling off on thermostat or going into de-ice mode
Seeing what appears to be smoke coming from the outdoor unit. This is actually steam coming off in de-ice mode. It is quite a common occurrence during the colder weather
The outdoor unit (condensing unit) will create a lot of water. The outdoor unit can get quite icy as well, even to the point that it is totally white with frost.