So i purchased a nest gen 3 and proceeded to install it. This is on a dual transformer system, separate boiler and AC system. The good news was i had a unused G wire on the Heat side that I was going to use to supply constant voltage. I wire everything up turn on the Nest and get E79 error. I checked between C/Rh at the thermostat and around 26 volts. I call up Nest and they tell me that when you have the heat and ac connected to thermostat on a dual transformer system, the nest wants to see power from the AC side. The problem is the AC thermostat wire coming for the air handler only has 3 wires, G/R/Y - in my case the Y is white wire.

Nest tells me i can jump/group the Rc to C on the AC transformer, thats what i understood, or run and new wire and connect to the C wire on the AC transformer.

I have added pictures of my system, the wire will the R/G/Y is going to the thermostat, can I just take the R wire and jump it to the C, basically wire nut a small piece of wire from the R bundle to the Common? If so which one is the common, the blue???

If i run new wire that has an extra wire for the common, where on the transformer/air handler to i connect to. The brown wires look like they go to the compressor. I would basically connect everything as it currently is but where would of tap/connect the new C wire to? Directly to the Transformer where the blue wire is? or could i wire nut the new C wire to the blue/white bundle that is currently wire nutted? Again Im assuming that the blue is the Common.

I guess i can always run a voltmeter from the Rc to the blue to see if i get constant 24 volts.

If someone can point me in the right direction i would really a appreciate it.

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Thanks, Paul

1 Answer 1


This is your common enter image description here You connect here and bring it up to the stat with the spare wire from the boiler.

Nest is also capable of power stealing in many cases. Here's a link to check before running a new wire.


  • 1st off thanks Joe for your reply, really appreciate it. So I can just use the extra wire on the new thermostat wire that I will be running for the AC to the stat? and connect everything else like it is now correct? This AC is in the attic the boiler is in the basement, 2 separate wires. I would then disconnect the C wire from the boiler and use the spare from the AC? Also what is that extra wire twisted with the blue, is that going to the compressor? i dont want to mess anything up. I really appreciate you taking the time. Paul
    – Da P.A
    Mar 18, 2019 at 1:23
  • Yes if you do run a new cable for the a/c, reconnect as is and add the common to the blue wire. The white wire tied in is for the contactor in the condensor(compressor) I added a link to the answer. I didn't read it, I just sent it to a customer last night and it helped him get his nest working without a common wire.
    – Joe Fala
    Mar 18, 2019 at 1:33
  • Joe thank you very much, going to run wires this weekend, I will look at the link also. Btw i know if i take out the c wire from boiler the nest will work, it will pluse of the rh and rc but i want to do it right. Nest wants power from ac side. Paul
    – Da P.A
    Mar 18, 2019 at 1:36

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