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Mustangannie · Oct 20, 2006, 12:09 PM

My furnace thermostat was taken off the wall while moving furniture now I have 3 wires hanging out of a whole in the wall. I would like to hook those wires up to Honeywell RTH110B but I’m not sure which wire goes where. On the red wire I have 24vac, on my white wire I have 12vac and I have a blue wire that has nothing on it. Any help I can get with this would be fantastic. :rolleyes:

labman · Oct 20, 2006, 05:02 PM

24 volts to what? 12 on the white wire?

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.

omzig · Oct 21, 2006, 10:04 PM

Maybe the manual would help. If you don't have one, here is a link to it: RTH110B