I'm replacing two White-Rodgers thermostats, one heating and one cooling with a Honeywell T8624D heating/cooling thermostat. I have three wires coming from the cooling thermostat (W, R & G) and 2 wires for heating coming from the transformer located in the wall at the thermostat which are both blue. My question is which wire do I use to go to the "C" common terminal to power this thermostat or must I go to the TC8602 thermostat.

Thanks,

Bob

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 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. It may be wired to have the 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.

Check the wiring at the furnace. Does the red wire come off the transformer? Does the white go to the air conditioner and the green to the fan relay? If so, then you need to determine which blue wire goes from the gas valve to the thermostat. If there are 2 transformers, disconnect the one. Usually the contacts on the thermostat are marked G, Y, W, and R or common. Connect the red to R or common, green to G, the blue to the gas valve to W, and the white to the air to Y. To prevent future confusion, it might be best to run a new standard 4 conductor thermostat wire from the furnace to the thermostat. Then connect everything as I explained in the first paragraph.

However you do it, you must have one contact of the transformer connected to the fan, gas valve, and air conditioner relay. The other to the common on the thermostat. Then the proper contacts of the thermostat must be connected to the other terminal of the gas valve, fan, and air conditioner. I you retain the old wiring, I would make a diagram of the wire colors perhaps putting it on or inside the furnace.