HVAC Diagnostic Guide MEDIUM URGENCY
Auxiliary heat indicator shows constantly on the thermostat, even in mild weather.
EMERGENCY HEAT mode locks the system into using only the resistance strips, ignoring the heat pump completely. Often set during a heat pump repair and forgotten.
Look at thermostat mode. Should be HEAT or AUTO. EMERGENCY HEAT or EM HEAT = problem.
Switch to HEAT mode. Aux will only engage when actually needed.
Shop ThermostatsThe thermostat (or system control) uses outdoor temperature to decide when to engage aux heat. Failed sensor reads outdoor as colder than reality, triggers aux constantly.
Compare your thermostat's displayed outdoor temp to a thermometer outside. Major discrepancy = bad sensor.
Replace outdoor sensor. Often a small wired probe mounted outside. $30-80 part. Some modern thermostats use internet weather instead — could be a setting issue.
When the heat pump itself fails (no compressor, failed capacitor, tripped breaker), the system defaults to using only aux heat. You stay warm but pay 2-3x more for the electricity.
Look at outdoor unit. Should be running when thermostat calls for heat in mild weather. If silent/fan not spinning, the heat pump has a problem and aux is doing all the work.
Diagnose the heat pump failure — could be tripped breaker, blown fuse, failed capacitor, or compressor issue. Once fixed, aux will return to normal supplemental use only.
Shop CapacitorsThe sequencer controls when each heat strip turns on. A stuck-closed sequencer keeps strips energized continuously.
Indoor air feels very hot (over 110°F at supply vents) — way hotter than a heat pump alone produces (90-100°F). Tech can test sequencer with multimeter.
Replace sequencer. $40-100 part. Match contact rating and pole count.
Shop PartsDuring defrost, aux heat engages to offset the cold air. If defrost gets stuck active, aux runs indefinitely.
Outdoor unit fan stays off while compressor runs (or vice versa). Indoor unit blows warm air from aux strips while heat pump appears confused.
Defrost board replacement. Match by manufacturer part number.
Shop Defrost BoardsWhen you raise the thermostat by 3°F or more all at once, most thermostats engage aux heat to recover quickly. This is normal behavior — but if you do it constantly, aux runs constantly.
Are you setting back thermostat overnight and bumping it 5°F+ in the morning? Or do you have aggressive scheduling?
Smaller setpoint changes (1-2°F max). Or set a smarter schedule that ramps up gradually. Modern thermostats have an 'adaptive recovery' setting that handles this efficiently.
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