The thermostat fan, controlled by the green wire on the G terminal, is energized whenever the thermostat calls for heating or cooling and when the homeowner selects the fan to run manually.

The G terminal completes the circuit that powers the air handler blower motor through the fan relay inside the air handler or furnace. When the thermostat senses a need for conditioned air it sends twenty four volt AC power to the G terminal which trips the relay and starts the fan, and this happens even while the heating or cooling call is active so the blower runs with the equipment. For manual fan operation most thermostats have a fan on or fan auto setting; selecting fan on supplies continuous power to the G terminal until the setting is changed. Some thermostats and systems support a continuous fan speed or variable fan control which modulates the blower with pulse width modulation or variable frequency drive technology, and you should confirm whether your air handler supports these modes before enabling them to avoid unusual noise or wear. If you want the fan to run after a heating or cooling cycle to use residual heat or coolth, enable the thermostat fan circulation or delay off option if available, or set a short runtime with a fan cycling or circulation feature. Always turn power off at the service switch or breaker before touching terminals, and use a multimeter to verify twenty four volts AC at the G terminal when the thermostat is calling for fan operation.

Common issues include no fan operation when the thermostat calls even though heat or cool runs, a likely loose or broken green wire which you can repair by tightening or replacing the conductor and confirming continuity. Another issue is the fan running continuously when not requested, which is often caused by a stuck fan relay or a thermostat programmed for continuous fan; troubleshoot by swapping the thermostat to fan auto or testing relay operation at the air handler. A third concern is short cycling or inconsistent fan speeds, usually caused by incorrect fan control settings, incompatible thermostat fan type selection, or failing motor capacitors; resolve by matching the thermostat fan type to the blower control method, testing and replacing capacitors if motor current is erratic, and consulting the air handler wiring diagram to ensure correct terminal connections.

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