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My thermostat is a Honeywell that goes to an old furnace with a Honeywell furnace controller (the board that provides the spark for my gas furnace, among other things like 24V, TH-W, etc.). The thermostat is using the red (R), white (W), and green (G) wires. R and W seem to be connected to the furnace controller correctly, because my furnace and fan work fine when heat is needed. The G wire is connected in the thermostat, but then goes no where in the furnace. I would really like to be able to turn the fan to ON without having a heating cycle activated (fan ON is readily available in the thermostat, it just does nothing because the G wire is not hooked up in my setup).

Where do I connect the G wire in the furnace?

Photos of furnace: dangling G wire circled in red close up of furnace controller furnace label fan limiter fan

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    What's the make and model of the furnace? – Tester101 May 22 '17 at 11:24
  • im not sure unfortunately. i think its from the late '70s. i will try to figure it out later, but does this mean its not dependent on that honeywell control board and rather something directly wired to the blower? thanks. – tau May 22 '17 at 16:58
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    The silver label to the far right in the picture above the Honeywell control appears to have the model number, I can't read it in the picture tho. That Honeywell control is a pilot light controller. – Tyson May 22 '17 at 21:59
  • @Tyson ive posted more photos, including the fan limiter control and fan itself. i also took the time to look at how its wired, and it looks like the hot 120v line goes into the left-most terminal in the fan limiter, the 24v transformer hot comes out of the bottom right terminal, and the fan hot comes out of the upper right terminal. im guessing i would need some sort of relay between the hot 120v and fan hot, controlled by the thermostat g wire, although im not sure how this is typically done. thanks! – tau May 23 '17 at 0:47
  • @Tester101 i took photos of the only labels i could find, and it does not seem like it is easily identifiable (ie, its not trane, lennox, etc.). thank you. – tau May 23 '17 at 0:49
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After looking at all the photos, it looks like the fan is controlled by a fan and limit switch (the black thing with a silver dial and three black wires).

So basically, the thermostat calls for heat by energizing the TH W terminal on the gas and ignition controller (Black Honeywell box with all the wires connected to it). This opens the pilot, turns on the spark, and ignites the main burners. Once the temperature in the heat exchanger gets up to the FAN ON temp., set on the fan and limit switch, the blower comes on. When the thermostat is satisfied, it removes power from the TH W terminal. This tells the ignition and gas controller to turn off the main burner. When the temperature in the heat exchanger falls below the FAN OFF temperature setting, the blower turns off.

To control the fan via the G wire from the thermostat, you're going to have to use a relay to turn on the blower. You'll energize the coil of the relay using the G wire from the thermostat, and the 24V common terminal from the transformer. You'll have line voltage (120V or 240V) connected to the common terminal on the relay, and a wire to the blower connected to the normally open (NO) terminal on the relay.

If you don't understand the information provided in this answer, or don't feel comfortable making this type of modification to the furnace. Please don't hesitate to contact a local licensed HVAC technician.

Side Note: The gas and ignition controller is installed upside down

  • thank you so much for the help. note that i recently read through some honeywell manuals and the just learned the ignition controller is upside down. im not sure who installed it though (it was before i owned the house). :) – tau May 23 '17 at 17:20

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