Wire letter guide
G Wire on a Thermostat: Fan Control Explained
G (sometimes F) turns on the indoor blower by itself, which lets you use the thermostat’s "Fan On" setting. It is usually green.
If you install a C-wire adapter (ecobee PEK, Honeywell wire saver, or Amazon’s adapter), the G wire at the thermostat typically becomes the new C wire, and the adapter sends fan and cooling signals over the Y wire.
G wire troubleshooting
- Fan won’t stop running. Check the thermostat fan setting is on Auto, not On. If it keeps running with the thermostat removed, the fan setting or a relay on the furnace control board is the cause.
- Fan doesn’t run in cooling. Most thermostats turn on G together with Y. If G is missing or loose, the AC can freeze the indoor coil because no air moves over it. Turn the system off and let ice melt before testing again.
- No G wire at all. Some heat-only boilers and a few older furnaces don’t use G. The furnace runs its own fan with heat, so that’s normal.
What G is used for on a smart thermostat
Besides “Fan On”, smart thermostats use G for features such as running the fan a set number of minutes per hour to even out temperatures, running it after a cooling cycle, or circulating air through a filter or air cleaner. Each of those uses more electricity, so check the fan settings if the blower runs more than you expect.
G on electric heat and heat pumps
A gas furnace starts its own blower when it heats. An electric furnace, air handler with heat strips or heat pump usually does not: the thermostat has to turn on G with the heat call. That is why the thermostat asks whether your heat is gas or electric. If it is set wrong, the heat can come on with no airflow until a safety limit trips.