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I recently got a new Honeywell WiFi thermostat, which requires a C-wire. I have 5-wires going to my current thermostat, with only 4 being used. So I thought adding the 5th as my C-wire was going to be easy, but when I opened the furnace panel I saw there was already a wire on C.

I have searched the net for hours, and I have not been able to find anyone with an installation like mine, so I thought I'd ask for some help. I've done my best to trace all the wires, and have created a crude drawing (attached). There are splices in a few places, due to the humidifier, and the outside unit is spliced in between the furnace C and the thermostat Y in between the joists above the furnace. This is the part that is really throwing me for a loop, along with the Y terminal on my furnace being empty. Based on what I have been learning while surfing around, I'm not understanding how it's currently working (which makes me hesitant to change anything). And yes, it currently works!

I would appreciate any thoughts you can share.

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If it really is wired like your diagram, there's nothing wrong with the current wiring.

To add your C wire, simply connect your extra wire to the C terminal in the furnace. Depending on where the splice of the blue wires from the outdoor unit and the humidifier is, it might be convenient to splice your new C wire in there.

Warning: You'll want to make sure the transformer in the furnace can handle the additional load of the thermostat. It's not commonly a problem, but sometimes can be depending on how the humidifier is connected.

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I agree with Tester101 on how to connect a new "C" terminal.

As for how the current setup works, the existing thermostat does not require any external power to function (it is an old mechanical contact type or is battery powered). There is no need for the "C" terminal because control is achieved by closing contacts between control power (Rh, +24v) and Y, W and G to turn on the compressor, furnace, and blower respectively. Since the yellow (Y) wire has nothing to do with the furnace, it is not connected in that location.

Your arrangement is typical for many residential installations. The "C" terminal is frequently only connected at the thermostat if the thermostat itself separately needs external power to be supplied.

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