Heat Pump Defrost Cycle  

Frequently Asked Questions and Helpful Tips

What is the Purpose of The Heat Pump Defrost Cycle?

How do I Troubleshoot My Heat Pump Defrost?

How Does Defrost Work?

How Can I Find a Blog for Defrost Discussion and Troubleshooting?

What is the Purpose of the Heat Pump Defrost Cycle?

First of all, heat pump defrost is only used while the heat pump is in heating mode.


We always heat our home or building when it is cold outside. This presents a problem....

In order for the heat pump to pick up heat from outside (for heating inside), the outside refrigerant temperature must be less than the outside air temperature.

However we all know what happens at 32 F or 0 C.

Water freezes....and so does the moisture that is in the air!

Once this takes place, ice begins to build up on the coil surface. The first effect of this.....a less efficient heat pump. The second effect....a damaged coil!

In order to avoid freezing, air source heat pumps are equipped with a defrost cycle.

I Need Troubleshooting Help for My Heat Pump Defrost

Potential solutions to the following four heat pump defrost cycle problems are found in the troubleshooting tables below:

  • "Defrost Won't Shut Off"
  • "Defrost Turns On With No Ice on the Coil"
  • "Outdoor Fan Remains On During Defrost"
  • "Compressor Shuts Off When Defrost Goes On"

Troubleshooting Tables:

"Defrost Won't Shut Off"
Potential Reasons Potential Solutions
Leaking reversing valve Replace
Suction or discharge compressor valves are bad Replace
Wiring problems Trace and repair faulty wiring
Low refrigerant charge Find possible leak, repair, and recharge with more refrigerant
Defrost control timer, thermostat, or air pressure differential switch needs to be adjusted Adjust settings to allow for more appropriate control
Bad defrost control contactors and/or relays Replace
"Defrost Turns On With No Ice on the Coil"
Potential Reasons Potential Solutions
Low refrigerant charge Find possible leak, repair, and recharge with more refrigerant
Defrost control timer, thermostat, sensors, or air pressure differential switch needs to be adjusted or replaced Adjust settings to allow for more appropriate control
Bad defrost control contactors and/or relays Replace
If applicable, the defrost control temperature sensing bulb does not have good contact on the sensing stream and/or piping Increase contact surface by adjusting the location
Outside fan or fan motor problems Check fan belt for adjustment or replacement. Check fan or fan motor for problems
Dirty outside coil Clean
"Outdoor Fan Remains On During Defrost"
Potential Reasons Potential Solutions
Outdoor fan relay is bad Replace
"Compressor Shuts Off When Defrost Goes On"
Potential Reasons Potential Solutions
Defrost control timer, thermostat, or air pressure differential switch needs to be adjusted Adjust settings to allow for more appropriate control
Bad defrost control contactors and/or relays Replace
Bad thermostatic expansion valve or power element Repair valve with new power assembly or replace the entire valve
Outside fan or fan motor problems Check fan belt for adjustment or replacement. Check fan or fan motor for problems
Low refrigerant charge Check the piping for leaks, repair, and replace the heat pump with more refrigerant

If you suspect a bad defrost control board, we need to test for the following:

  • Bad ambient or coil temperature sensor
  • Proper voltage to the control board

Below is a great video from Fritz HVAC in Louisville, Kentucky. It's focus is on an American Standard heat pump, but the principles apply to other heat pumps.

See other great heat pump defrost cycle videos!

How Does Defrost Work?

The outside coil is defrosted by diverting hot refrigerant gas into the coil. This is accomplished in one of two ways:

  1. Switching the heat pump into cooling mode, thereby routing hot gas into the coil.
  2. Some heat pump equipment contain a hot gas or discharge gas bypass valve. This also may be energized allowing hot refrigerant gas to flow through the coil.

In both cases, the outside fan is usually deactivated. This helps to decrease defrost time.

There are a few methods that trigger on the heat pump defrost cycle:

  1. Outside air/coil temperature difference: A special defrost thermostat is used to determine if there is an increase in temperature difference between the coil and the outside air. The defrost cycle is usually deactivated at a preset coil temperature. Many times as high as 50 - 60 F.
  2. Air pressure Differential: As ice begins to form on the coil, the area for air passage is decreased. This results in an increase in air pressure differential. An air pressure differential switch is used to activate the defrost cycle until the pressure differential across the coil returns to a normal and constant differential.
  3. Defrost timer: Using this method, a defrost timer simply activates and deactivates the heat pump defrost cycle at a predetermined time interval. A standard time interval is 90 minutes off/15 minutes on. This is a very simple and "easy to troubleshoot" method, but is very inefficient.
  4. Defrost timer with thermostat: This method uses coil temperature and timing in combination to provide a very effective heat pump defrost cycle. At a preset time interval (typically 90 minutes), a series of contactors allow the coil temperature to be checked. If it is at 32 F or less, the heat pump defrost cycle is activated. It is then deactivated once the coil temperature reaches approximately 60 F.

How Can I Find a Blog For Defrost Discussion?

Heat Pump Defrost Discussion Forum

Feel free to discuss your heat pump defrost problem here. Or help us solve some of the problems presented.

What Other Visitors Have Said

Click below to see contributions from other visitors to this page...

Defrost cycle generates frigid air 
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cold air 
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whirlpool model WHP4430AAE 
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will not defrost 
new defrost board, refrigerant is fine, ice build up and will not defrost its a Carrier heat pump Model #38QN030330

Defrost Icing Problems - American Standard Heat Pump 
This unit is an American Standard heat pump, 2.5 ton 13 seer, R-22. It ices up every time it is 42 degrees or colder and raining outside. The refrigerant …

Trane Heat Pump time delay between OD fan cut on and solenoid deenenergizing Not rated yet
I have a Trane TWR736A100A1 heat pump. At the conclusion of the defrost cycle, the defrost control board has a time delay between the OD fan cut on and …

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will not defrost  Not rated yet
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