Hi! My furnace's control board recently blew and the repair guy replaced it with a different kind. Aparently the new board requires a wire from the thermostat to the G terminal to work the A/C fan, and the old one didn't. The repair guy would need to make another trip for this repair.

I am wondering if I could fix this myself. Since replacing the wire to the thermostat would be a big job (the wire is stapled and runs a long long way behind walls and under floors) I hope to fix it without a new wire.

- Can I add a jumper in the furnace from the Y terminal to the G terminal? Y seems to be used to turn on the A/C, so it seems to make sense to power G whenever Y is powered. Is this safe?

- Lots of schematics I found show that the C terminal wire is optional. Can I move that wire from the furnace and thermostat C terminals to the G terminals? What does C do? All I can determine is that is possibly powers the thermostat? Why is it optional then?

- Why does the fan turn on fine when heat is called for when there is no G wire?

Thanks for any insights :)

OK, thermostat wiring 101:

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.

I think in some systems the signal for the blower is connected on the board. In other systems, the thermostat powers both the yellow and green wires. If you jumper the Y and G at the board, the A/C will come on if you select fan only at the thermostat. If you seldom use the fan only setting, not much loss. If the thermostat can be set to power the G terminal when calling for air, and you have an unused blue wire and no green, you could use it in place of the green.

The white wire usually supplies power on the board to the blower relay, often a different relay for a slower speed coil in the motor. Thus, the blower runs with the heat.