How do I determine which is my common C wire?

Question

I am trying to install a Nest Thermostat and I need some help locating which of the wires would be the common "C" wire. I have traced all the wires going from the thermostat, condensate removal pump, ac compressor, and furnace. Below are some pictures of what I am seeing and working with. I was able to also make a wiring diagram that I know is correct to how my furnace is currently hooked up, but I just can't seem to see which one would be the "C" wire. The only wire coming out from the control board from "C" goes to the compressor. So unless I am wrong, if I follow the path of the electricity, would my potential common wire be the Y?
My thought process.

"C" travels to the compressor on the red wire
Returns from the compressor to "Y1"
Travels to the Condensate Removal Pump from "Y1" on the white wire
Returns from Condensate Removal Pump on the red wire to the Wire Nut connection
Travels from Wire Nut connection on the yellow wire to "Y" on the thermostat.

Doesn't that technically make my Y the 24v common?
I tried testing the thermostat with a voltmeter for 24v by connecting to the red and testing all the wires one by one but they all gave about 26 volts.
What am I doing wrong? I appreciate any help I can get.

ThreePhaseEel · Answer

Your C wire is indeed the one that's labeled C
While your setup with the condensate pump is a bit more complicated than most, it doesn't change the fundamental way things work.  Your thermostat still puts 24V on Y to call for cooling, which gets passed on by the condensate pump to the Y1 terminal on your control board.  Your AC compressor is connected to that Y1 terminal, so it sees that signal and starts the compressor; we then know that the other wire coming from the AC is a C wire, and lo and behold, it lands on the terminal labeled "C" on the control board.
As to getting a spare wire, you could use an "add-a-wire" gadget of some sort, convert your furnace to single stage operation, or simply run a fatter thermostat cable, the choice is yours.

How do I determine which is my common C wire?

One Answer

Your C wire is indeed the one that's labeled C

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