We just recently purchase a house . I went to put a new digilal termostat.finding out to my surprise that we have some sort of air cond system. Upon furter inspection the system is a water cooled of some sort with a condenser inside of heating duct above fan blower.the wires at the thermosat are in colors white black of which were hook up and three wires that were disconnected blue, red ,yellow. How do I determine which wire goes where on the new thermostat. Thanks

Troy :confused:

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.

The terminals on the thermostat should be marked R, B, W, Y, and G. Connect the corresponding wire color. If there is a terminal marked RC, connect it to the red too. Conventional thermostats do not need the blue. If the thermostat doesn't come on at all, make sure the blue is connected at the furnace. Often all its connections are inside the furnace except a pair of wires going to the AC relay.

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