Heat Pump Auxiliary Heat Keeps Coming On: Causes and Fixes

When a heat pump’s auxiliary heat keeps coming on it can spike energy bills and signal a problem. This article explains why aux heat activates, how to troubleshoot common causes, and when professional service is needed to restore efficient operation.

Symptom	Likely Cause	First Action
Aux heat cycles frequently	Incorrect thermostat settings or balance point	Check thermostat mode and temperature differential
Aux heat runs even when outdoor temp is mild	Sensor, wiring, or control board fault	Inspect thermostat wiring and error codes
Aux heat during defrost only	Normal defrost operation	Verify brief, periodic operation
Continuous aux heat	Compressor failure or refrigerant low	Turn off aux heat, call HVAC pro

How Heat Pumps And Auxiliary Heat Work

Heat pumps move heat rather than generate it, using a compressor and refrigerant to transfer warmth from outside to inside. When outdoor temperatures fall or the heat pump cannot keep up, the system engages auxiliary (aux) heat—usually electric resistance strips or a gas furnace—to maintain setpoint temperature.

The thermostat controls when aux heat starts by monitoring indoor temperature, outdoor conditions, and system feedback. Understanding this interplay helps diagnose why the aux heat keeps coming on.

Common Reasons The Aux Heat Keeps Coming On

1. Low Outdoor Temperature And The Balance Point

Every heat pump has a balance point, the outdoor temperature below which the heat pump alone cannot provide enough heat. Below this point the thermostat calls for aux heat to supplement the system.

If the balance point is higher than expected, aux heat may engage frequently. This can be due to system sizing, insulation deficits, or incorrect thermostat settings that raise the threshold for aux activation.

2. Thermostat Settings And Emergency Heat

Incorrect thermostat configuration is a frequent cause. If the thermostat is set to emergency heat the heat pump will be bypassed and the aux heat will run continuously.

Other settings such as stage switching, temperature differential, or adaptive recovery features may also cause premature aux activation if misconfigured.

3. Defrost Cycle And Short Aux Bursts

During defrost, the outdoor coil is heated to remove frost. The heat pump may reverse and temporarily stop heating; the thermostat may call for aux heat briefly to maintain indoor temperature. This is normal if the aux runs only for short, intermittent periods during defrost.

4. Faulty Reversing Valve Or Compressor Issues

If the reversing valve or compressor is malfunctioning the heat pump cannot produce heat effectively. The thermostat senses lack of heat and activates the aux heat as backup. Signs include cold air from registers, high energy use, and frequent aux cycles.

5. Low Refrigerant Charge

Low refrigerant reduces heat transfer capacity, causing the heat pump to struggle and aux heat to cycle on. This usually accompanies reduced heating performance and may be accompanied by ice on the outdoor unit.

6. Sensor, Wiring Or Control Board Faults

Thermostat sensor failures, loose wiring, or a defective control board can falsely signal that the heat pump isn’t producing heat, triggering the aux heat. Error codes on the thermostat or outdoor unit can point to control faults.

7. Staged Heat Pump Operation And Short Cycling

Multi-stage systems use aux heat as a second stage. If staging logic is incorrect or the heat pump short cycles, the system can rely on aux heat more than necessary. Improper staging increases wear and energy consumption.

How To Troubleshoot Aux Heat Continuously Turning On

Initial Quick Checks

• Confirm the thermostat is not set to Emergency Heat.
• Check thermostat fan and mode settings; ensure it is set to Heat and Heat Pump.
• Verify the setpoint: a large gap between indoor temperature and setpoint can trigger aux heat.
• Look at the outdoor unit for obvious damage, ice buildup, or error flashing lights.

Thermostat Inspection And Reset

Inspect thermostat wiring for loose or miswired connections (especially the O/B, Y, W, and Aux terminals). Reset the thermostat by removing power or using the reset function to clear transient errors.

If available, review thermostat logs or error messages for diagnostic clues. Modern thermostats sometimes report heat pump or auxiliary calls and system faults.

Check For Defrost Versus Continuous Aux Heat

Observe the system for one full defrost cycle. Defrost-related aux heat calls are short and periodic. Continuous long runs indicate a deeper problem such as refrigerant issues or compressor failure.

Inspect The Outdoor Unit And Airflow

Ensure the outdoor coil is clear of debris, and airflow around the unit is unobstructed. Restricted airflow can reduce heat pump capacity and cause more aux heat usage.

Ice on the coil that does not clear in defrost may suggest a refrigerant leak or expansion valve problem.

Measure Temperatures And Pressure

Technicians measure refrigerant pressures and temperatures to verify system capacity and diagnose leaks. Homeowners can check supply and return air temperatures; a small delta T in heating mode indicates reduced heat output.

Test And Replace Faulty Components

If the thermostat, reversing valve, contactors, or heat strips are suspect, testing and replacing these components may resolve the issue. Wiring problems should be corrected by a qualified technician if the homeowner is not comfortable with electrical work.

When To Call A Professional HVAC Technician

Call a licensed HVAC technician when there is continuous aux heat, loss of heating capacity, iced outdoor coils that persist, abnormal noises, or electrical issues. Professionals can safely diagnose refrigerant leaks, compressor and reversing valve faults, and control board errors.

Safety note: Turn off power to the unit before inspecting components and never attempt refrigerant repairs without certification.

Energy And Cost Implications

Auxiliary resistance heat is much less efficient than a functioning heat pump. Electric strip heat converts electricity to heat at about 100% efficiency, but heat pumps typically deliver 200-400% efficiency in mild conditions.

Frequent aux heat use can substantially increase utility bills. Identifying and fixing the root cause usually reduces operating costs and extends equipment life.

Preventive Maintenance To Reduce Unnecessary Aux Heat

• Schedule annual professional maintenance to check refrigerant charge, electrical connections, and controls.
• Keep outdoor units clear of debris and ensure proper airflow.
• Change indoor air filters regularly to improve heat pump performance.
• Insulate the home and seal air leaks to reduce heating demand and lower the balance point.
• Upgrade thermostats to intelligent models that better manage staging and balance point settings.

Thermostat Settings To Optimize Aux Heat Use

Adjusting thermostat settings can reduce unnecessary aux heat activation. Use heat pump mode with proper staging disabled for emergency heat only. If the thermostat supports an adaptive balance point or outdoor sensor input, configure these features to delay aux activation until truly necessary.

Set reasonable temperature setbacks to avoid calling for large rapid temperature increases that trigger aux heat. Use gradual recovery settings when available.

Replacement And Upgrade Considerations

If the heat pump is old, repeatedly relying on aux heat may signal it is time for replacement. Modern heat pumps are more efficient at lower temperatures and can reduce aux heat dependence.

Consider a cold-climate heat pump or a properly sized two-stage system for better performance. Upgrading to a thermostat that supports outdoor temperature sensors and optimized staging can also improve efficiency.

Common Myths And Misconceptions

• Myth: Aux heat always means the heat pump is broken. Fact: Aux heat can be normal during very cold weather or during defrost cycles.
• Myth: Turning off aux heat saves money. Fact: Disabling aux heat can cause comfort issues and may force emergency heat use if the heat pump can’t maintain setpoint.
• Myth: Thermostat replacement always fixes the issue. Fact: Thermostats help diagnose but underlying mechanical or refrigerant issues may persist.

Checklist: Quick Steps To Take Right Now

1. Verify thermostat is in Heat (Heat Pump) mode and not Emergency Heat.
2. Check and replace indoor filters and clear outdoor unit of debris.
3. Observe whether aux heat runs only during defrost cycles.
4. Reset the thermostat and inspect wiring for loose connections.
5. Schedule professional service if aux heat runs continuously or heating performance is poor.

When Continuous Aux Heat Signals A Major Problem

Continuous or long-duration aux heat usage often indicates serious issues like compressor failure, major refrigerant loss, or a failed reversing valve. These require immediate professional assessment to prevent further damage and high utility costs.

Homeowners should document symptoms, thermostat readouts, and any error codes to provide technicians clear information for faster diagnosis.

Resources And Helpful Links

For detailed manufacturer-specific troubleshooting consult the heat pump and thermostat manuals. The U.S. Department of Energy provides guidance on heat pump operation and efficiency, and the Air Conditioning Contractors of America (ACCA) offers maintenance standards for HVAC professionals.

When in doubt, contact a licensed HVAC technician for a diagnostic check and written estimate for repairs or replacement.