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Discussion Starter · #1 ·
I'm installing a digital wifi thermostat that requires a 'c' or common wire.

Situation: Two separate heating and cooling units feed my thermostat. One set of wires comes from my boiler and the other set from my central air handler.

My current thermostat wire from the air handler is a 3-wire. I don't have a problem re-running a 4-wire as the run is pretty short (which I heard it should be as the thermostat should be located near the return).

Attached is a picture of the wiring in the RUUD air handler. Also attached is the wiring diagram.

From what I could decipher, The 3-wire to/from the thermostat has the following going on currently.

RED wire is connected to Rc on thermostat and connected to a yellow wire in the air handler that goes directly to the 24v side of the transformer.

WHITE wire is connected to Y on the thermostat and connects to the white wire going out to the compressor.

GREEN wire is connected to a brown wire labeled "G" that goes to other connections and terminals within the air handler.

I'm looking for help locating the common and running this correctly.
 

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Trace the 2 wires from the outside AC backwards to the air handler. One of them will connect to Y from the tstat and the other goes to C which goes to one side of the transformer which is on the left next to the fan relay and sequencer. Run a C wire from there to your new tstat and yer Golden.

You have a isolated connection in the tsat where R or RH is to the boiler and using W as well.

RC is your cooling and using Y as well. You MUST remove the jumper on the subbase of the new tstat or it will blow everything. Jumper is between R and RH or may say RH to RC. R is always Hot and W is heat, Y is cool, G is fan. You need to keep RH and RC seperate. Should tell you that in the install info.
 

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Discussion Starter · #3 ·
Thank you. Tested voltage across from the non-yellow wire from the compressor to the R and 24V!

Thank you, thank you.
 
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