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    hvac troubleshooting

    Indoor blower runs constantly

    Common symptoms: blower never stops; fan always on; air handler runs nonstop; furnace fan won't shut off

    Stop and call a pro if:

    • burning odor
    • smoke

    Step-by-step diagnostic flow

    1. Step 1

      Any burning smell or smoke from the air handler or supply registers?

    2. Step 2

      What is the thermostat fan setting?

      'On' runs the blower continuously by design. 'Circulate' or 'Smart Fan' on Ecobee/Nest runs the fan for a percentage of each hour even without heat/cool demand.

    3. Step 3

      When you set the thermostat to Off (or change mode), does the blower stop?

    Possible outcomes

    Stop and seek professional help immediately

    high confidence

    Active hazard.

    Smoke or burning smell from the air handler

    Fan is set to 'On' — switch to 'Auto'

    high confidence

    The thermostat is doing exactly what you told it: run continuously.

    Safe next steps
    • Change the thermostat fan setting from 'On' to 'Auto'
    • Continuous fan in humid weather can re-evaporate condensate off the coil and increase indoor humidity — Auto is usually better in summer
    • If you specifically want air mixing without continuous run, try 'Circulate' instead

    Circulate / smart-fan is doing its job

    high confidence

    Smart-fan modes run the blower a configurable percentage of each hour (often 20–50 %) even with no heat/cool demand, to mix air and equalize temps.

    Safe next steps
    • Reduce the circulate percentage in the thermostat settings if it's running more than you want
    • Or switch to 'Auto' to have the fan run only during heat/cool calls
    • On Ecobee, this is 'Fan: 20 minutes per hour' style; on Nest it's 'Schedule fan to run X minutes per hour'

    Find and document the thermostat fan setting first

    low confidence

    Without knowing the fan setting, we can't distinguish 'working as configured' from 'fault'.

    Safe next steps
    • Open the thermostat fan menu — most have 'On', 'Auto', and sometimes 'Circulate'
    • Photograph the screen
    • Return to this workflow with the setting identified

    Fan-off delay — normal behavior

    high confidence

    After a heat or cool call ends, the blower runs an extra 30–180 seconds to pull residual conditioning off the heat exchanger or coil. This is by design and is good — it improves efficiency.

    Safe next steps
    • If the delay is excessive (>3 minutes), check the furnace control board for a configurable 'fan delay' jumper or dip switch
    • Otherwise, no action needed — this is working correctly

    Blower won't stop even with thermostat off — stuck fan relay

    medium confidence

    If the thermostat is calling for nothing and the blower still won't stop, the fan relay (or its control board output) is stuck closed.

    Safe next steps
    • Cut power at the breaker to stop the blower (do not let it run indefinitely)
    • Call a pro — fan-relay or control-board replacement involves line-voltage work
    • Avoid running the breaker on/off repeatedly — that compounds the wear
    What to document for a pro
    • Furnace/air-handler make and model
    • Thermostat make/model
    • Whether the blower runs at one speed or varies
    Stuck relays involve line-voltage work and a real risk of further short

    Blower restarts on its own — heat-exchanger limit switch tripping

    medium confidence

    On gas furnaces, an over-temperature limit switch keeps the blower running until the heat exchanger cools — typical after a clogged filter, closed registers, or a failed blower motor caused overheating. The pattern of 'stops then restarts' often indicates limit-switch cycling.

    Safe next steps
    • Replace the filter if it's dirty
    • Open all closed registers
    • If the cycling continues with a clean filter and open registers, the heat exchanger may be sooting or the blower motor is dragging — call a pro
    • Continuous limit-switch trips can indicate a cracked heat exchanger — get a CO alarm in the area as a safety measure
    What to document for a pro
    • Furnace make/model
    • Last service date
    • Whether you have a CO alarm in the building
    Cracked heat exchanger can introduce CO into supply air
    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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