Poor heating performance, higher energy bills, and even system damage can quickly result when your heat pump is not defrosting. During winter, heat pumps go into automatic defrost mode in order to melt ice that forms on the outdoor coil; if that process doesn’t occur, however, frost and ice can begin to build up and impede airflow, reducing efficiency.
In this article, we will explain why your heat pump won’t defrost, how to fix it, and how to keep the same problem from occurring again.
How a Heat Pump Defrost Cycle Works
To understand the problem, it helps to know how the defrost cycle functions.
A heat pump works by transferring heat between the indoor and outdoor units. In heating mode, the outdoor coil becomes cold and can accumulate moisture that freezes when temperatures drop.
To get rid of this ice, the system periodically switches into defrost mode — reversing the flow of refrigerant so that the outdoor coil warms and melts off the ice. This is usually activated by one of the following:
• A defrost sensor or thermostat that monitors a specific coil temperature
• A defrost control board initiates the cycle after a fixed time.
When your heat pump fails to defrost, one or more of these components isn’t operating correctly.
Common Reasons a Heat Pump Won’t Defrost
1. Faulty Defrost Control Board
The defrost control board represents the brain of the defrost system: it determines when it is time to start and stop the cycle based on sensor inputs.
If it’s malfunctioning, the heat pump might:
• Never enter defrost mode
• Stay in defrost too long
• Users switch cycles too often.
Corrected:
Check the board for burnt spots or loose wiring. Usually, if it is defective, replacement is required — which is usually straightforward for an HVAC technician to do.
2. Bad Defrost Sensor or Thermostat
The defrost sensor functions by measuring the outdoor coil’s temperature to identify when it is forming ice. When the sensor is dysfunctional, it may send a wrong reading and fail to trigger the defrost cycle.
Fix:
Measure sensor resistance using a multimeter. If the results are incorrect, the sensor should be replaced. Ensure that it is fixed properly to the coil for correct temperature detection.
3. Low Refrigerant Level
Low refrigerant reduces the pressure of the system, which in turn makes the coil run colder than normal, allowing ice to build up faster than the system can melt it.
Correct:
The HVAC technician will need to find and repair leaks, then charge the refrigerant back to manufacturer specifications.
4. Fan Motor Outdoor Problems
The frosting occurs because the unit cannot dissipate heat effectively when the motor of an outdoor fan does not work or spins too slowly.
Fix:
Check that no debris, snow, or ice is blocking the fan blades. If the fan motor is burned out, it should be replaced.
5. Dirty Outdoor Coil
A dirty coil traps moisture and reduces heat transfer, making it harder for the defrost cycle to work effectively.
Fix:
Switch off the power to the unit and clean the coil gently with a soft brush or coil cleaner. Avoid the use of high-pressure sprays, which may cause damage to the fins.
6. Reversing Valve Malfunction
The reversing valve switches the direction of the refrigerant between heating and cooling, and for defrost. If it’s stuck or defective, the unit can’t switch into defrost mode.
Repairs:
Have a professional test the valve to see that it operates properly. Sometimes the solenoid can be replaced separately; other times, the entire valve may need replacement.
7. Outdoor Temperature Too Low
In temperatures well below freezing-20°F (-6°C)-heat pumps have difficulty extracting heat from the air. Ice may build up more rapidly than the defrost cycle can shed it.
Fix:
Consider installing an auxiliary heat source, such as electric backup heat or a gas furnace hybrid system.
Also read: Mini Split Heat Pump
Signs Your Heat Pump Is Not Defrosting Properly
You can usually tell that your heat pump is not defrosting if you notice:
• Thick ice covering of the entire outdoor coil or fan blades
• Lower heating output or cold air coming from the vents
• Higher energy bills because of longer run times
• Steam not appearing during defrost cycles (normally visible briefly)
• Noises when the unit tries to switch modes
If you see a solid block of ice, immediately turn off the unit. Operation in this condition may damage the compressor.
Step-by-Step Troubleshooting Guide
Follow these steps in order to diagnose a non-defrosting heat pump:
Step 1: Check for Ice Buildup
Turn the unit off and check the outdoor coil. A light frost is normal, but thick or solid ice means defrosting isn’t working.
Step 2: Inspect the Outdoor Fan
Make sure the fan spins freely without obstructions due to ice, leaves, or other debris.
Step 3: Clean the Coils
Clean the dirt or ice accumulation with a coil cleaner or warm water-only; do not use hot water. Avoid using sharp-edged tools which may puncture the coil.
Step 4: Test the Defrost Sensor
Check the resistance with a multimeter. Replace if readings are out of range.
Step 5: Reset the Defrost Control Board
Find the control board, which is normally behind the access panel. Some boards will have a test button on it to manually put the system into defrost mode—press and check.
Step 6: Check Refrigerant Levels
If all the components seem to be working but there is still frosting, low refrigerant might be an issue. A licensed technician can verify and recharge it safely.
Also read: Heat Pump Not Working
How to Prevent Your Heat Pump from Freezing Up
An ounce of prevention is always better (and cheaper) than repairs. Here’s how to keep your heat pump defrost system running smoothly:
• Clean your outdoor unit regularly: remove debris, snow, and leaves.
•Change air filters monthly during heavy use.
• Schedule professional maintenance once a year or more.
• Keep the area around the heat pump clear for good airflow – at least 2 feet.
• Make sure gutter drainage does not drip water onto the unit.
• Check fan blades and coil fins for damage.
Regular maintenance keeps the sensors and control boards in good working condition so the defrost cycle can occur when needed.
When to Call a Professional
If your heat pump isn’t defrosting after basic cleaning, then you need to call an HVAC technician. You should look for immediate help if:
• The entire outdoor coil is frozen solid
• The system shuts off frequently or trips breakers
• You hear loud hissing, buzzing, or grinding noises
• Indoor heating is weak or inconsistent
The more one waits to get the repairs done, the more likelihood of compressor failure-which is among the most expensive parts to replace.
Also read: Low Boiler Pressure
Conclusion: Keep Your Heat Pump Operating Efficiently
Although a heat pump that does not go into defrost mode may seem like a minor inconvenience, ignoring it can cause permanent long-term damage or poor heating performance. More common culprits include defective sensors, dirty coils, or issues with the control board-all of which are fixable either by timely maintenance or professional service.
Clean your outdoor unit regularly, check the airflow frequently, and schedule an annual maintenance inspection for your heat pump to ensure it runs at peak efficiency throughout winter.
