When is G wire powered? Want to use it to power duct fan relay.

Old 06-22-18, 05:02 PM
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Join Date: Nov 2016
Location: USA
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When is G wire powered? Want to use it to power duct fan relay.

I am about to install a 110v inline duct fan to improve flow in a room and so I need to create some sort of automatic switch to turn on/off the fan when either heat or the ac is on. It happens that duct is quite close to where my thermostat wire runs to the furnace so my idea was to splice the thermostat wires and install a 24v relay powered by the G thermostat wire... so whenever the furnace fan is on, the duct fan would be on.

So here is my question:

Is the G wire ALWAYS powered when thermostat calls for heat or AC in Auto mode (via the W or Y wire), meaning the G is powered in addition to the W or Y?


The G wire is powered ONLY when the fan is put in Manual mode, meaning the it is the W or Y wire that will also turn on the furnace fan at the furnace board?

Thanks... Daniel
Old 06-22-18, 08:41 PM
Join Date: Jun 2012
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For gas/oil/propane furnace the G is not activated for a heat call (W). For electric or heat pump G is activated. For a/c the G is almost always activated when Y is hot.

Some furnaces have an EAC (electronic air cleaner) terminal on the control board that is live when the blower is running. Usually 120vac, though. They also make sensors that clamp to the blower motor wires that sense when it is on.

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