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The Heat Pump Defrost Cycle Explained

Posted by Ed Miller on Tue, Dec 01, 2020

As winter approaches and temps start to drop, your heat pump will occasionally need to go through a defrost cycle. The defrost cycle helps your heat pump, and ultimately your HVAC system, operate more efficiently.


There is often confusion surrounding the defrost cycle, which can lead homeowners to believe their heat pump is malfunctioning. Read on to learn everything you need to know about your heat pump’s defrost cycle.

What Is the Defrost Cycle?

In heating mode, a heat pump pulls heat from the outside air and transfers it inside to warm it. The outdoor air is cool, so the outdoor coil acts as an evaporator. Under certain ambient temperature and humidity conditions (when the 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, and frost can form on the outdoor coil. This layer of frost will ultimately make the heat pump work harder, aka inefficiently, so it needs to be removed. The defrost cycle is what removes the frost.

How Does the Defrost Cycle Work?

During the defrost cycle, the heat pump is operated in reverse. A defrost control tells the reversing valve when to send hot refrigerant outdoors to thaw the outdoor coil. When the heat pump switches over, the outdoor fan is prevented from turning on and the temperature increase of the coil is accelerated. The time it takes to thaw the outdoor coil will vary, but heat pumps will typically be in defrost cycle until the coil reaches around 58 degrees. Once the unit is free of frost, the internal heater will stop, the valve will reverse, and the unit will resume the heating cycle.

How Often Will My Unit Switch to the Defrost Cycle?

There are a number of factors that influence when a heat pump switches over to the defrost cycle. The main ones are: outdoor temperature and humidity, the amount of heat load the system is trying to deliver, and the condition of the pump system. Generally, heat pumps will defrost regularly when frost conditions are occurring. However, the frequency of defrosts should be no more than roughly every 35 minutes. The length of time the heat pump will defrost will vary, but ordinarily it should not take longer than 10 minutes. The defrost cycle is meant to be long enough to melt frost or ice, but short enough to be energy efficient.

How Can I Tell if My Unit Is in the Defrost Cycle?

You can tell from the inside once the system stops heating and the fan will shut off. With many units, there is also usually a visual indicator, like a blinking light. You can tell from the outside once the outdoor fan has also stopped and the compressor is on.

What Are Some Indicators of Heat Pump Problems?

If your heat pump is defrosting too frequently or not delivering enough heat, you may have other issues with your heat pump. 

  • If this has been happening since installation, it could be operator error, in which case you should consult the user manual. If you’ve consulted the manual and ruled out user error, then it may be a contractor installation issue, and the unit may be the wrong size for the space it’s trying to heat. You either need to contact the installer, or another reputable HVAC technician, to replace the wrong sized unit with the correctly sized one. A certified professional can assist you with correct operation and sizing.
  • If this problem is new, it may mean that the equipment is faulty or that maintenance is required. Try some easy at-home maintenance yourself first: change your air filters, clear your outdoor unit of debris and foliage, and make sure your heat exchanger isn’t blocked. If you’ve done those things and are still having trouble, it’s time to call in the pros!

If you are experiencing issues with your heat pump, or any other part of your HVAC system, Snyder Heating & Air Conditioning can help. The last thing you want is to be caught with faulty heating in the middle of a cold snap! Contact us today for more information, or schedule a service appointment online.

 

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