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    hvac troubleshooting

    Auxiliary or emergency heat stuck on

    Common symptoms: aux heat always on; emergency heat stuck; high electric bill; em heat indicator on

    Stop and call a pro if:

    • burning odor
    • smoke
    • sparks

    Step-by-step diagnostic flow

    1. Step 1

      Any burning smell, smoke, or sparks from the air handler?

    2. Step 2

      Is the thermostat mode set to 'Emergency Heat' or 'EM Heat' (rather than 'Heat')?

      Some thermostats have a manual emergency-heat mode that locks the heat pump off and runs only the backup heat.

    3. Step 3

      When the system calls for heat, is the outdoor unit (compressor and fan) running?

    4. Step 4

      What is the current outdoor temperature?

    Possible outcomes

    Stop and seek professional help immediately

    high confidence

    Active hazard.

    Electrical or fire hazard present

    Switch thermostat from 'Em Heat' to 'Heat'

    high confidence

    Manual emergency-heat mode bypasses the heat pump and runs only resistance/backup heat. Operating cost is much higher.

    Safe next steps
    • Switch the thermostat mode to 'Heat' (not 'Em Heat' or 'Emergency')
    • Confirm the outdoor unit starts on the next heat call
    • Check whether someone in the household switched it during a recent cold snap

    Outdoor unit isn't running — separate workflow

    high confidence

    If the heat pump itself can't start, the thermostat correctly falls back to auxiliary heat.

    Safe next steps
    • Run the 'Outdoor unit won't start' workflow to diagnose the heat pump
    • Keep aux heat available for warmth in the meantime
    What to document for a pro
    • Outdoor unit make/model
    • Whether outdoor breaker is on
    • Any sounds from outdoor unit on a call

    Aux heat is expected at this temperature

    high confidence

    Below 30 °F most residential heat pumps are at or below their balance point and aux is supposed to assist.

    Safe next steps
    • Run the balance-point calculator with your unit's AHRI 47/17 °F capacities
    • Expect aux to run more in colder weather
    • If aux stays on after weather warms above 40 °F, escalate

    Aux heat shouldn't be running at this temperature — call a pro

    medium confidence

    Above 30 °F a healthy heat pump should carry the load without aux. Possible thermostat config, sensor, or refrigerant-charge issue.

    Safe next steps
    • Check thermostat lockout temperature setting (if accessible)
    • Document indoor temp, setpoint, outdoor temp, and aux indicator state over a day
    • Do not adjust refrigerant charge or wiring
    What to document for a pro
    • Thermostat make/model and settings
    • Equipment make/model
    • Time-stamped temp + aux-state log
    Refrigerant and line-voltage work require certification
    Diagnostic guidance only. If unsure, stop and call a licensed professional — gas, electrical, and refrigerant work is hazardous to untrained users.

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