How to add a C wire with this unusual installation

hvacthermostat-c-wire

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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Best Answer

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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