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I'm trying to figure out where to connect a C wire from a thermostat to my furnace. I found this old post for the same furnace type (Trane XE70), but it seems as though a red wire goes to the terminal the other poster had a white wire go to. This made me too nervous to try it without inquiring with more knowledgeable people.

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The wire bundle with the blue wire is the one coming from the thermostat and the wire bundle with the wrapped unused green is coming from the AC unit (I believe). Any suggestions on which terminal would be a common ground? Is there an easy way I can test with a multimeter?

EDIT: Requested wiring diagram for the model.

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Thanks! - Sincerely, DIY Noob

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Figuring this out is not as hard as it looks

The cable going off to most A/C outdoor units has two wires -- both of them go to the contactor's coil there, but one must connect to the Y wire (call for cooling) from the thermostat and the other must connect to the C terminal from the furnace/air handler (presuming the furnace/air handler is providing 24VAC to the thermostat for cooling). As a result, when the thermostat connects the 24VAC going to it on the R wire to the thermostat to the Y wire going back, the circuit is completed (just like flipping on a lightswitch), and the air conditioner compressor comes on.

As a result, since the white wire going off to your air conditioner connects to the yellow wire going off to the thermostat, and assuming standard thermostat wiring color coding, the red wire going off to your air conditioner must be connected to the C terminal on your furnace, which is where your blue wire going off to the thermostat connects to in order to provide a return path for the 24VAC thermostat power that doesn't go through some load or another in the furnace.

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