Heat Pumps and Defrost Cycle

A heat pump is an electrical device that extracts heat from one place and transfers it to another in order to maintain a constant temperature.  It can be used for both heating and cooling.

  • A heat pump transfers heat by circulating a refrigerant through a cycle of evaporation and condensation.
  • A compressor pumps the refrigerant through two heat exchanger coils.
  • In one coil, the refrigerant is evaporated at low pressure and absorbs heat from its surroundings.
  • The refrigerant is then compressed on the way to the other coil where it condenses at high pressure.

The cycle is fully reversible, meaning a heat pump can provide year 'round climate control for your home.

During winter months, many heat pumps will need to go through a defrost cycle during operation.  When the heat pump defrosts, it may appear as if heater isn't working, but in most cases, it is doing exactly what it should be doing.

What is a "defrost cycle?"  In heating mode, a heat pump extracts heat from the outside air and transfers it inside your home to warm it.  When the ambient temperature outside gets very cold, moisture in the air can freeze on the outdoor component's heat exchanger from the fan blowing the air across it.  A defrost cycle is simply the system recognizing ice has formed or is beginning to form and automatically fixes this.

Any ice build 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 can I tell if my unit is in the defrost cycle?  Inside your home, you will notice the unit will stop heating, the indoor fan will stop and depending on the model, there will usually be some sort of indication, like a blinking light on the unit.  Outside, the outdoor fan will also have stopped and the compressor will be running.

How often will my unit go into the defrost cycle?  There are a number of factors that influence how often a unit will go into a defrost mode:

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

There are timers built into the computer control of the unit that restrict how often defrosting can occur.  Depending on the unit, defrost cycle should happen no more than every 2-3 hours.

How long does the defrost cycle take?  Either of two factors can end a defrost cycle.  Firstly, if the sensors on the outdoor unit detect that it's heat exchanger temperature has risen enough, the unit will stop defrosting.  Secondly, if the sensors do not stop it beforehand, the maximum time a unit will be in the defrost cycle is around 10 minutes.  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.

My system is defrosting frequently and/or not delivering enough heat.  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 or required maintenance.  You can perform some basic maintenance by cleaning or replacing the filters on your indoor unit.  Make sure the outdoor unit is free from foliage or another blockage.

Can I help to reduce defrosting?  Yes.  Keep your system well-maintained by having seasonal maintenance checks on the complete system to ensure it is operating efficiently.  Cleaning or changing your filters regularly is the easiest, proactive measure you can take to keep your heater running smoothly.


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