Nest Fan Only Operation / Failed Ignition or Heat Source

Nest thermostat shows heating, but only the fan runs and no warm air comes out

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Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

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Possible Causes

Incorrect wiring of G and W1 at Nest base, Furnace ignition failure (flame sensor, igniter, gas valve), Heat pump in defrost or fault mode, Misconfigured system type in Nest settings

How to Fix / Troubleshooting

Safety: Turn off power and gas (if applicable) before opening furnace panels. Gas and combustion issues should be handled by a professional.

  • Check wiring: Confirm that G (fan) and W1 (heat) wires are on the correct Nest terminals and not swapped.
  • System type: In Nest settings, verify the correct system type (gas furnace, electric furnace, or heat pump) is selected.
  • Furnace diagnostics: Observe the furnace control board LED for error codes indicating ignition or flame issues. Refer to the furnace manual.
  • Filter and airflow: Ensure the air filter is clean and vents are open; some furnaces will shut off burners if overheating is detected.
  • Professional service: If the blower runs but burners or heat strips do not energize, a technician should inspect the igniter, flame sensor, gas valve, or heat strips.
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Repair Difficulty

Professional Required 5/5

Required Part

Furnace Ignition System or Heat Source (not Nest)
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