When the heat pump aux heat not working issue appears, homes can lose reliable backup warmth during cold weather. This guide explains the common causes, step-by-step troubleshooting, repair options, and preventive tips to restore auxiliary heat and keep the system operating efficiently.
| Symptom | Likely Cause | Quick Fix |
|---|---|---|
| Aux heat won’t turn on | Faulty thermostat setting or control board | Check thermostat mode and wiring; reset controls |
| Aux heat not producing heat | Defective electric heat strips or relay | Test elements and replace bad components |
| Frequent short cycling | Incorrect thermostat staging | Adjust staging settings or replace thermostat |
| Aux and heat pump run simultaneously | Sensor or wiring fault | Inspect sensors, thermostatic settings, and wiring |
Content Navigation
- How Auxiliary Heat Works In A Heat Pump System
- Common Causes When Aux Heat Is Not Working
- Step-By-Step Troubleshooting For Aux Heat Not Working
- When To Call A Professional HVAC Technician
- Common Repairs And Replacement Options
- Preventive Maintenance To Keep Aux Heat Working
- Energy And Cost Considerations For Auxiliary Heat
- When Replacement Is The Best Option
- Useful Diagnostic Tools And Safety Tips
- Frequently Asked Questions About Aux Heat Not Working
- Key Takeaways And Next Steps
How Auxiliary Heat Works In A Heat Pump System
Auxiliary heat, often called aux heat or secondary heat, provides backup warming when the heat pump cannot meet demand, such as during very low outdoor temperatures or defrost cycles. In most residential systems, aux heat consists of electric resistance heating strips, a gas furnace integrated as a dual-fuel system, or a separate boiler. The thermostat monitors indoor temperature and outdoor conditions, staging the system to call aux heat when required.
Common Causes When Aux Heat Is Not Working
Diagnosing why the aux heat not working begins with understanding the typical failure points. Problems generally fall into electrical, mechanical, control, or sensor categories. Identifying the category narrows troubleshooting steps and speeds repair.
Thermostat Settings And Staging Issues
Many aux heat problems are due to incorrect thermostat programming or improper stage setup. If the thermostat is set to emergency heat or single-stage incorrectly, the system may bypass normal aux functions or fail to energize them when needed.
Blown Fuses Or Tripped Breakers
Auxiliary heat elements draw high current. A blown fuse or tripped breaker in the HVAC subpanel can prevent power reaching the heat strips, making the aux heat appear dead even when controls are correct.
Failed Electric Heat Strips Or Elements
Electric resistance elements will degrade and fail over time. If one or more strips are open or shorted, the aux system will not generate the required heat or may operate at reduced capacity.
Contactors, Relays, And Control Board Failures
Contactors and relays switch high-voltage power to the aux elements. A stuck or burned contactor, or a malfunctioning control board, will prevent aux heat from engaging even if the thermostat signals a call for heat.
Wiring And Connector Problems
Loose, damaged, or corroded wiring between the thermostat, outdoor unit, and indoor auxiliary components can interrupt signals or power. Rodent damage and mechanical strain are common causes of wiring failures.
Outdoor Temperature Sensor Or Defrost Control Issues
Some systems rely on outdoor sensors to prevent aux heat from operating unnecessarily. A faulty sensor can stop the system from recognizing when aux heat should be engaged. Defrost controls can also temporarily disable the heat pump and trigger aux heat—if those controls misbehave, aux heat may not respond properly.
Step-By-Step Troubleshooting For Aux Heat Not Working
Follow these safe, logical steps to isolate the problem. If any step requires opening electrical panels or handling wiring and the homeowner is not comfortable, contact a licensed HVAC technician.
1. Confirm Thermostat Mode And Settings
Ensure the thermostat is set to heat and the system type is correct (heat pump). Check staging settings; typical thermostats have an option for heat pump with auxiliary heat and emergency heat. Set the thermostat to a temperature significantly higher than room temperature and observe whether the aux heat engages after a short delay.
2. Check Breakers And Fuses
Locate the HVAC breakers and any fuses on the furnace or air handler. Reset tripped breakers and replace blown fuses with the correct rating. If a breaker trips again immediately, stop and call a technician—repeated trips indicate a short or overload.
3. Inspect The Air Handler And Electric Elements
Turn off power at the breaker, then remove the air handler access panel to visually inspect the heating elements for discoloration or breaks. Use a multimeter to test element continuity; open elements need replacement.
4. Test Contactor And Relay Operation
With the thermostat calling for aux heat, observe whether the contactor or relay clicks. A lack of clicking suggests a control signal or coil problem. Measure coil voltage at the contactor to confirm whether the relay is receiving a signal.
5. Check Wiring And Terminal Connections
Inspect terminal connections at the thermostat, control board, and air handler. Tighten loose screws and look for burnt terminals or melted insulation. Replace any damaged wires and secure connectors to restore reliable signaling and power flow.
6. Verify Outdoor Sensor And Defrost Function
If the system uses an outdoor sensor or intelligent defrost control, check sensor resistance or readings and confirm the defrost cycle is functioning. Replace a failing sensor to ensure proper staging between the heat pump and aux heat.
7. Confirm Thermostat Compatibility And Calibration
Older thermostats may not properly control modern heat pumps and auxiliary systems. Verify compatibility, update thermostat firmware if available, or test with a known working thermostat. Calibration errors can be corrected in the thermostat settings.
When To Call A Professional HVAC Technician
Some issues require specialized tools and safety precautions. Call a licensed HVAC technician if: the homeowner must handle high-voltage wiring, breakers continue to trip, heating elements need replacement, or the control board shows faults. Professionals carry diagnostics for control boards, staged systems, and refrigerant-related problems that can mimic aux heat failures.
Common Repairs And Replacement Options
Repair choices vary by cause and system age. Cost and complexity range from simple thermostat reprogramming to replacement of the air handler or control board. The table below outlines typical fixes and considerations.
| Issue | Repair | Estimated Complexity |
|---|---|---|
| Thermostat misconfiguration | Reprogram or replace thermostat | Low |
| Tripped breaker or blown fuse | Replace fuses, diagnose short | Low to Medium |
| Failed heat strips | Replace electric elements | Medium |
| Bad contactor/relay | Replace contactor or relay | Medium |
| Control board failure | Replace control board | High |
Preventive Maintenance To Keep Aux Heat Working
Regular maintenance reduces the likelihood of aux heat failure. Recommended actions include annual professional tune-ups, visual inspections of wiring and elements, changing filters, and keeping the outdoor unit clear of debris. Schedule an HVAC inspection before cold seasons to verify staging, sensors, and control logic are functioning properly.
Recommended Maintenance Checklist
- Annual HVAC system inspection and tune-up by a licensed technician
- Visual check of heat strips and connectors for signs of wear
- Test operation of thermostat and verify firmware updates
- Replace air filters every 1–3 months to prevent restricted airflow
- Keep outdoor unit free from snow, ice, and vegetation
Energy And Cost Considerations For Auxiliary Heat
Auxiliary electric heat is significantly more expensive per kilowatt-hour than heat pump operation. Overreliance on aux heat raises energy bills. Homeowners should balance comfort and cost by using proper thermostat setback strategies, maintaining the heat pump to maximize its efficiency, and considering a dual-fuel setup with a gas furnace if that reduces operational costs in a given region.
Tips To Minimize Aux Heat Use
- Set a moderate thermostat differential to allow the heat pump to run before aux engages
- Use programmable or smart thermostats to avoid unnecessary backup heat
- Improve home insulation and air sealing to reduce heating load
- Consider supplemental zoned heating for frequently used areas
When Replacement Is The Best Option
Replacement may be appropriate when the air handler is older, the control board is obsolete, repair costs approach the value of a new unit, or the system is undersized. Replacing an old heat pump with a modern high-efficiency unit and an integrated staging controller can eliminate frequent aux heat calls and reduce long-term energy costs.
Useful Diagnostic Tools And Safety Tips
Essential tools for troubleshooting include a multimeter, clamp meter, screwdrivers, and a thermometer. Safety tips: always shut off power before accessing the air handler, avoid touching live terminals, and never bypass safety controls. For high-voltage or refrigerant-related tasks, call a licensed technician.
Frequently Asked Questions About Aux Heat Not Working
Why Does Aux Heat Engage Frequently?
Aux heat may engage frequently when the heat pump cannot keep up with demand during extreme cold, when airflow is restricted, or if the thermostat staging is too aggressive. Improving system efficiency and correcting thermostat staging can reduce unnecessary aux calls.
How Long Should Aux Heat Run During A Call?
Aux heat typically runs until the indoor temperature reaches the thermostat setpoint or until the heat pump recovers. Continuous aux operation could indicate a defective heat pump, failed reversing valve, or incorrect thermostat configuration.
Can A Homeowner Replace Heat Strips?
Replacing heat strips requires electrical experience and proper safety measures. Homeowners with electrical skills may perform the replacement after cutting power and following manufacturer instructions. Otherwise, hire a qualified technician to ensure safe installation and correct wiring.
Key Takeaways And Next Steps
When the heat pump aux heat not working problem arises, a systematic approach helps identify whether the issue is a simple thermostat setting, a power supply problem, a failed element, or a control failure. Homeowners should perform basic checks—thermostat mode, breakers, and visible wiring—then engage an HVAC professional for high-voltage, refrigerant, or control-board work. Regular maintenance and correct thermostat configuration are the best defenses against future aux heat failures.
Energy Star and local HVAC professionals provide additional resources for efficient heat pump operation and professional service recommendations.
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