The auxiliary heat, or emergency heat, is a crucial backup for heat pumps when outside temperatures drop or when the system needs extra warmth. If the auxiliary heat isn’t coming on, the home may rely too heavily on less efficient modes, leading to higher energy bills and uneven comfort. This guide explains common causes, practical troubleshooting steps, and when to contact a professional to restore reliable heating.
Common Causes Of Auxiliary Heat Not Activating
Understanding why auxiliary heat fails to engage helps homeowners focus on the most likely issues. Thermostat settings are a frequent culprit: if the thermostat is set to heat pump mode with a low heat demand, auxiliary heat may not activate as intended. Defrost cycles can temporarily delay auxiliary heat activation, since the system prioritizes defrosting the outdoor coil. A faulty outdoor sensor or reversing valve can also prevent auxiliary heat from engaging. Additionally, electrical problems like a tripped breaker or a blown fuse can cut power to the heat strips used for auxiliary heating.
Other mechanical and system-related causes include a low refrigerant charge that reduces heat pump efficiency, a malfunctioning contactor or control board, or an aging electric resistance strip that no longer provides adequate heat. A clogged air filter or restricted ductwork can also make it feel as though auxiliary heat isn’t working because airflow is insufficient to deliver warmth.
Troubleshooting Steps To Diagnose The Issue
Follow these steps to narrow down causes and potentially restore auxiliary heat without immediate service calls. Always prioritize safety when inspecting electrical components.
1) Check Thermostat Settings And Indicators
Verify the thermostat is set to heat mode and that auxiliary or emergency heat is enabled if required by the model. Look for a status indicator on the thermostat screen showing aux or emergency heat. If the thermostat displays a fault or error code, consult the user manual for interpretation and recommended actions.
2) Review Temperature And Demand
Setpoint temperature should require additional warming beyond the heat pump’s capacity. If the home is only moderately cold, the system may rely on the heat pump without engaging aux heat. Increase the setpoint briefly to observe if auxiliary heat engages, then revert to a comfortable level.
3) Inspect Outdoor Defrost And System Run
During cold, humid weather, the outdoor unit may enter defrost mode, temporarily delaying auxiliary heat. Listen for changes in fan operation and observe the outdoor unit if safe. If defrost is active for an extended period, allow it to complete; if it persists, a sensor or control issue may be present.
4) Check Electrical Power And Breakers
Ensure the indoor and outdoor units have power. Look for tripped breakers or blown fuses protecting heat strips and control circuits. Reset any tripped breakers and replace fuses if necessary. If a breaker frequently trips, the system may have a short or overload requiring professional inspection.
5) Inspect Airflow And Filtration
Replace dirty air filters and ensure registers and ductwork are unobstructed. Poor airflow can hinder heat delivery, making auxiliary heat less effective or appear as if it’s not turning on. After replacement, run the system to see if performance improves.
6) Check For Refrigerant And Component Health
A professional should examine refrigerant levels if the unit seems to run but provides little heat. Low refrigerant can reduce heat pump efficiency and affect auxiliary heat operation. In addition, inspect the contactor, relays, and control board for signs of wear or damage.
7) Test The Reversing Valve And Sensors
A malfunctioning reversing valve or outdoor temperature sensor can prevent auxiliary heat from engaging correctly. If the valve is stuck or sensor signals are inaccurate, the system may fail to switch to emergency heat when needed.
What To Do If Auxiliary Heat Still Won’t Engage
If the above steps do not restore auxiliary heat, the problem likely involves electrical components, refrigerant, or control logic that requires professional service. Do not attempt to replace high-voltage parts without proper training. A qualified HVAC technician can diagnose with tools such as digital multimeters, refrigerant gauges, and system diagnostic software.
Maintenance Tips To Prevent Future Failures
Regular maintenance reduces the odds of auxiliary heat failure. Key practices include seasonal HVAC inspection, cleaning or replacing air filters every 1–3 months, and ensuring ductwork is sealed and unobstructed. Schedule professional tune-ups twice a year to keep heat pumps, including auxiliary heat systems, operating efficiently. Keeping the outdoor unit clear of debris and ensuring proper clearance improves airflow and system reliability.
When To Call A Pro
Call a licensed HVAC technician if any of the following occur: the thermostat shows persistent error codes, breakers repeatedly trip, you detect hissing, burning, or unusual smells, the system runs continuously without reaching comfort, or refrigerant leaks are suspected. A pro can safely test electrical components, verify proper refrigerant charge, and diagnose sensor or valve malfunctions.
Key Takeaways For Homeowners
- Thermostat settings and demand are frequent culprits; confirm correct mode and enable auxiliary heat when required.
- Electrical power and breakers should be checked first if the auxiliary heat does not engage.
- Airflow and filtration significantly impact perceived heating performance; replace filters and clear obstructions.
- Defrost cycles can delay auxiliary heat temporarily; monitor outdoor unit behavior during cold, humid conditions.
- Professional diagnostics are recommended for refrigerant issues, sensor faults, or control-board problems.
