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What is Heat Pump Defrost Cycle?


During the winter, many heat pumps go through a “defrost” cycle while operating. Often, this is not explained well before installation, leading to confusion about why the heat pump is not working. This article aims to help users understand the defrost cycle and address any concerns.


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

The defrost cycle can be confusing and may cause homeowners to think that their heat pump is malfunctioning. Keep reading to learn all the information you need to know about the heat pump defrost cycle.


What is the heat pump defrost cycle?

In heating mode, the heat pump absorbs heat from the outside air and transfers it indoors for heating. The outdoor air is cool, so the outdoor coil acts as an evaporator. Under certain environmental temperature and humidity conditions (when the outdoor temperature becomes very cold), moisture in the air will freeze on the heat exchanger of the outdoor unit, forming frost on the outdoor coil. This frost layer eventually makes it more difficult for the heat pump to operate efficiently, leading to decreased efficiency, and therefore, it needs to be removed. The defrost cycle is the process of removing the frost.


Why does my unit need to go through a defrost cycle?

Any ice buildup on the external heat exchanger reduces the airflow passing through the heat exchanger, which affects efficiency and sometimes significantly reduces it. In extreme cases, it can even lead to damage to the outdoor unit.


How does the defrost cycle work?

During the defrost cycle, the heat pump operates in reverse. The defrost control tells the reversing valve when to send hot refrigerant to the outdoor unit to thaw the outdoor coil. When the heat pump switches, the outdoor fan cannot operate, and the coil temperature rises rapidly. The time required for the outdoor coil to defrost may vary, but the heat pump typically remains in the defrost cycle until the coil reaches around 58 degrees. Once the unit is free of frost, the internal heater stops, the valve reverses, and the unit resumes the heating cycle.


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How often does my unit switch to the defrost cycle?

Several factors affect when the heat pump switches to the defrost cycle. The primary factors are outdoor temperature and humidity, the thermal load the system is attempting to deliver, and the condition of the pump system. Generally, when frost conditions occur, the heat pump defrosts periodically. However, the defrost frequency should not exceed approximately once every 35 minutes. The duration of the defrost cycle may vary, but it should typically not exceed 10 minutes. The defrost cycle should be long enough to melt frost or ice but short enough to improve energy efficiency.


How can I tell if my unit is in a defrost cycle?

Once the system stops heating, and the fan shuts off, you can tell from the inside. For many units, there is usually a visual indicator such as a flashing light. Once the outdoor fan also stops, and the compressor is running, you can tell from the outside.


How long does my unit take after defrosting?

Either of two factors can cause the unit to exit the defrost cycle. First, if sensors on the outdoor unit detect that its heat exchanger temperature has risen sufficiently, the unit will stop defrosting. Secondly, if the sensor does not stop it prematurely, the unit remains in the defrost cycle for a maximum of about 10 minutes.


It is important not to shut off the unit before the defrost cycle is complete because if the unit restarts shortly after the defrost cycle ends, it will operate at very low efficiency and may damage itself.


My unit defrosts frequently/cannot provide enough heat – what could be the issue?

Regular defrosting or lack of heat may be caused by various factors.


If your unit has been operating in this manner since installation (first cold spell), you may be operating it incorrectly, or it may be undersized for the space you are trying to heat. Initially, you should consult the user manual to ensure correct operation of the unit. If that doesn’t resolve the issue, you should consult your installer or another reputable heat pump professional. They can help ensure proper operation and sizing.


If the unit is undersized for the space, it is not a malfunction. The responsibility for correctly sizing the unit lies initially with the installation company – if the unit is too small, they will need to rectify the situation.


If the issue has recently arisen, it may indicate a malfunction or the need for maintenance. You can perform some basic maintenance yourself, such as cleaning the filters on the indoor unit and ensuring the outdoor unit is free of leaves and debris, and the heat exchanger is not blocked. If that doesn’t solve the problem, you should consult your installer or another reputable heat pump professional.

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Are there any methods to help reduce defrosting?

Yes, there are. Keeping your unit well-maintained (as mentioned above) and ensuring correct operation will be of great help.


Of course, the less load you impose on the unit, the less frequently it will need to defrost under cold conditions. Permanent fixes, such as installing insulation in ceilings, walls, and under floors, will help reduce your heating demand (and ultimately save you money). More directly, closing doors and drawing curtains will also help reduce heating demands.


What are some indicators of heat pump issues?

If your heat pump defrosts too frequently or cannot provide sufficient heat, there may be other issues with your heat pump.


If this has been the case since installation, it may be due to 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 an installation issue by the contractor, and the unit may be incorrectly sized for the space it is intended to heat. You need to contact the installer or another reputable HVAC technician to replace the improperly sized unit with the correctly sized one. Certified professionals can help with correct operation and sizing.


If this issue is new, it may indicate a malfunction or the need for maintenance. Start by trying some simple home maintenance: replace the air filters, clear debris and leaves from the outdoor unit, and ensure the heat exchanger is not blocked. If you’ve done these steps and still encounter issues, it’s time to call in the professionals!

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