Understanding Defrost Cycle

What is a "defrost cycle?"

In heating mode a heat pump extracts heat from outside air and transfers it to inside your home to warm it.  When the ambient temperature outside gets very cold, 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 recognizing that ice has formed or begun to form and automatically fixes this.

Why does my unit have to do a defrost cycle?

Any ice that builds up on the outside heat exchanger reduces the airflow across it, which will affect 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 the defrost cycle?

Inside you will notice the unit will temporarily stop heating, the indoor fan will stop and depending on the model, there will usually be some form of visual indication on the unit, such as a light or a blinking "run" light.  Outside, the outdoor fan will also have stopped and the compressor will be running.

How often will my system go into defrost mode?

There are a number of factors that influence how often a unit will go into the defrost mode, including:

  • The outdoor temperature and humidity
  • The amount of heating load the system is trying to deliver
  • The condition of the heat pump

There are timers built into the computer control of the system that restrict how often defrosting can occur.  Generally, a unit must run for a minimum of about 35 minutes after starting up before completing its first defrost.   From there, defrosts should occur no more often than approximately every 40 minutes.

How long does the defrosting take?

Either of two factors can bring the unit out of a defrost cycle.  Firstly, if the sensors on the outdoor section detect the heat exchanger's temperature has risen enough, the unit will stop defrosting.  Secondly, if the sensors do not stop the cycle beforehand, the maximum time a system will be in the defrost cycle is around 10 minutes.  It is important not to stop the defrost cycle because the system will run inefficiently and could cause harm to the whole system.

My unit is defrosting frequently and isn't delivering enough heat--what could be wrong?

Regular defrosting, or a lack of heat could be caused by a number of factors.  A recently developed problem may be an indication of a fault and require maintenance.  You can perform some basic maintenance yourself by cleaning the filters on your indoor unit and making sure the outdoor section is clear of foliage or debris, keeping the heat exchanger unblocked.  If this doesn't remedy the problem, your heat pump may need serviced.

Is there any way I can help to reduce defrosting?

There certainly is.  Keep your system well maintained by having seasonal maintenance checks performed.  Cleaning or changing your filters regularly is an easy, proactive measure you can take to keep your heater running smoothly.

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