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Why will a fan keep running even when the furnace thermostat is off ?

If a fan continues to run even when the furnace thermostat is off, several possible reasons could be contributing to this behavior:

  1. Fan Setting on Thermostat:
    • Check the thermostat settings. Some thermostats have a fan setting that allows the fan to operate independently of the heating or cooling system. If the fan setting is set to “On” instead of “Auto,” the fan will run continuously.
  2. Limit Switch Issues:
    • The furnace contains a limit switch that regulates the fan operation based on the temperature inside the furnace. If the limit switch is faulty or incorrectly set, the fan may continue to run even when the thermostat is turned off.
  3. Fan Relay Malfunction:
    • The furnace’s control board uses a relay to turn the fan on and off. If the fan relay is malfunctioning or stuck in the “on” position, the fan may continue to run irrespective of the thermostat setting.
  4. Thermostat Wiring Issues:
    • Faulty or incorrect wiring in the thermostat or the furnace can lead to continuous fan operation. Check for any loose or disconnected wires.
  5. Faulty Thermostat:
    • The thermostat itself may be faulty. If the thermostat is not sending the correct signals to the furnace, it could result in continuous fan operation.
  6. Shorted Wires or Control Board Issues:
    • Shorted wires in the furnace’s control system or a malfunctioning control board can also cause the fan to run continuously.
  7. Delayed Fan Feature:
    • Some furnaces have a feature where the fan continues to run for a short period after the heating cycle to extract any remaining heat from the system. This is normal behavior and not a cause for concern.

To troubleshoot and address the issue, it’s recommended to:

  • Verify the thermostat settings.
  • Inspect and test the limit switch, fan relay, and thermostat for proper functionality.
  • Check for any loose or damaged wiring.
  • If the issue persists, it may be necessary to consult with a professional HVAC technician to diagnose and resolve the problem.

Identifying the specific cause will help in implementing the appropriate solution to stop the fan from running continuously.

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