Defrost Cycle
A heat pump pulls heat from the outside air and transfers it inside so you get warm. When the temperature outside gets cold (in the mid 30F) the moisture in the air freezes on the heat exchanger. The defrost cycle kicks in to rid the exchanger of this ice. If the exchanger is not defrosted, then its efficiency will be compromised and the unit could be damaged. You will know the unit is in defrost mode when the indoor fan stops, the unit will stop heating, and an indicator light may blink. The outdoor fan will also stop and the compressor will continue running.The number of times your unit goes into a defrost cycle is based on several factors. The three primary ones are the overall condition of your unit, the amount of heat the unit is attempting to deliver, and the outdoor temperature and humidity. There is a timer built into the unit that controls how often the unit goes into a defrost cycle. When the temperatures outside are approaching the mid 30sF range, then defrosting cycles will occur in either 30, 60, or 90 minute increments.Here is a step by step basic explanation of what is happening during a defrost cycle.

  1. A sensor measures the temperature of the outside coils. If the coils drop below freezing, an internal clock will send the unit into a defrost cycle.
  2. During the cycle, the outside fan is stopped. The compressor continues running and the unit switches to air conditioning mode. The outdoor coils will heat up and melt the buildup of ice.
  3. The back-up heating coils in the furnace will turn on to maintain the temperature of your home.
  4. The cycle will last until the coils reach a correct temperature and should do this within 10 minutes.

If a unit is going into a defrost mode frequently, then there may be some things wrong. One thing to consider is whether or not the unit is undersized for the area in need of heat. Consult with the installer and include the insights of a professional HVAC person to help with this. Another consideration is whether or not regular maintenance is needed. You can clean the filters and make sure the outdoor unit is clear of brush, limbs, leaves, excessive winds, and has a clear heat exchanger. In addition, seek to reduce the overall demand being placed on the air source heat pump by insulating well, reducing air leaks in walls, ceilings, and floors, and using weather stripping and other “green” ideas for your home.

One of the reasons frequent defrost cycles are a cause of concern is they may be a strong indicator of something else wrong with the unit in your home. The causes of frost buildup may be one other problem or a combination of things. Other reasons for frost buildup can be:

  • A damaged outdoor coil
  • A bad reversing valve
  • A wiring problem
  • A bad thermostat
  • A leak in the refrigerant
  • A dirty coil
  • A fan that will not turn on
  • A fan installed backwards
  • A motor operating in the wrong direction
  • A fan motor spinning at a low rpm.

Many of these issues can only be diagnosed, repaired and/or maintained by a professional installer or HVAC company.

The essential issue to be concerned about with an air source heat pump defrost cycle is that excessive cycles can be harmful to the parts and mechanics of the system. Remember also that each time the defrost cycle begins, the auxiliary heat source is fired up and brought in use to keep your home temperature stable. This causes an excessive use of electrical or other energy source you are using to power your home.

The goal is to keep the entire unit working at peak performance so that defrost cycles are initiated by the system when needed. This requires a balance between the size of the unit for your home, the performance of all the parts, and the overall demand performance of the unit for your climate and geographical area.

Want more?  Check out these great Heat Pump Resources