Wiring Thermostat SchematicHello! Trying to add a smart thermostat to an old system (currently running a mercury thermostat on it).

There is an extra wire (orange) which you can see both inside the thermostat and also in the image of the furnace. I should be able to use that for the C wire.

You'll notice that the wiring of the furnace is done with wire nuts. All of the instructions to add the C Wire that I can find are assuming that I have a C port on the furnace, which I don't, since this is all done with nuts.

My question is this: on the furnace side, into which wire nut should the orange be joined to get the correct power supplied?

Thanks so much!


You already have a C wire, it's just not called by that name

From a close examination of your furnace's wiring diagram, the fat brown wire it supplies in the thermostat wiring compartment is the C-wire feed from the transformer. The way your system is wired connects the brown wire in the outdoor-unit cable and the brown wire in the thermostat cable to that brown C-wire feed, so the brown wire that connects to the X terminal on your thermostat is your C wire! As a result of this, just hook that brown wire up to the C terminal on your new thermostat and call it a day. (If you're wondering what terminal the blue wire goes to on the new thermostat, it went to the B terminal on the old thermostat, so it goes to the O/B terminal on the new thermostat, by the way.)

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  • Thank you so much for this thoughtful response. Trying it shortly. I greatly appreciate the help. – BenC111 Jan 6 at 1:40

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