Heat Pump Troubleshooting: A Technician's Guide
Published by HVACProSales Team on Mar 16th 2026
Quick Answer Box
Why is my heat pump not working?
Common heat pump problems include incorrect thermostat settings, dirty air filters, or a tripped breaker. For more complex issues, it could be a refrigerant leak, a failed reversing valve, or a problem with the defrost control board. Always start with the simplest solutions before moving to major component diagnosis.
Heat pumps are complex systems, and when they fail, it can be for a variety of reasons. For an HVAC technician, a systematic approach to troubleshooting is key to quickly and accurately identifying the root cause of the problem. Whether the unit is not heating, not cooling, or stuck in defrost, this guide will walk you through the most common heat pump problems and how to solve them.
Problem 1: Heat Pump Not Heating
This is a common complaint during the first cold snap of the season. If the system is blowing air, but it's not warm, follow these steps:
- Check the Thermostat: Ensure it is set to "Heat" mode and the temperature is set above the current room temperature.
- Inspect the Air Filter: A clogged air filter can severely restrict airflow and reduce heating performance. Replace it if it's dirty.
- Check the Outdoor Unit: Make sure the unit is not covered in snow or ice. A heat pump needs good airflow to extract heat from the outdoor air.
- Diagnose the Reversing Valve: The reversing valve is what switches the unit between heating and cooling. If it's stuck in the cooling position, the system won't produce heat. You can check for 24V at the reversing valve solenoid.
- Check Refrigerant Charge: A low refrigerant charge will cause a significant drop in heating capacity.
Problem 2: Heat Pump Not Cooling
If the heat pump works in heating mode but not in cooling, the problem is often related to the same components, just in reverse.
- Check the Thermostat: Verify it is set to "Cool" and the setpoint is below the room temperature.
- Clean the Coils: Both the indoor and outdoor coils must be clean to effectively transfer heat. Dirty coils are a very common cause of poor cooling performance.
- Check for a Failed Contactor: The contactor sends power to the outdoor unit. If it has failed, the compressor and fan won't run.
- Test the Run Capacitor: A weak or failed run capacitor can prevent the compressor or fan motor from starting.
A clean outdoor coil is essential for proper heat pump operation in both heating and cooling modes.
Problem 3: Heat Pump is Stuck in Defrost Mode
In the winter, the outdoor coil can frost over. The heat pump will periodically run a defrost cycle to melt this ice. If the unit seems to be running but not heating, and the outdoor fan is off, it may be stuck in defrost.
- Faulty Defrost Control Board: This is the most common cause. The board may not be correctly sensing the coil temperature or the timer may be faulty.
- Bad Defrost Thermostat or Sensor: The sensor that tells the board when to start and stop the defrost cycle may have failed.
| Troubleshooting Step | What to Look For | | :--- | :--- | | Check for 24V at Reversing Valve | In heat mode, the reversing valve is typically energized. In cooling, it is not. | | Measure Superheat and Subcooling | This is the definitive way to diagnose a refrigerant charge issue. | | Inspect Defrost Board LEDs | Many defrost boards have diagnostic LEDs that can indicate a specific fault. |
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Expert Summary: Key Takeaways for Technicians
- Start with the Basics: Always check the thermostat, filter, and breakers before diving into complex diagnostics.
- The Reversing Valve is Key: This component is unique to heat pumps and is a common point of failure that can affect both heating and cooling.
- Understand the Defrost Cycle: Knowing how the defrost cycle works is essential for troubleshooting winter heating problems.
- A Systemic Approach is Best: Don't guess. Follow a logical sequence of diagnostic steps to isolate the problem.
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FAQ Section
Q: Why is my heat pump blowing cold air in heat mode? A: This is often a sign that the unit is in its defrost cycle, which is normal. However, if it persists, it could be a sign of low refrigerant or a problem with the reversing valve.
Q: How do I know if my reversing valve is bad? A: A common test is to feel the copper lines connected to the valve. In heat mode, the suction line should be hot. If it's not, and you have 24V at the solenoid, the valve is likely stuck.
Q: Can I fix a heat pump myself? A: While homeowners can perform basic tasks like changing the filter and checking the thermostat, most heat pump repairs require a licensed and certified HVAC technician due to the high voltages and high-pressure refrigerants involved.
Blog Tags: * HVAC Repair * Heat Pump * Troubleshooting * HVAC Technician * Heating and Cooling