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.
Thanks, Paul
Best Answer
This is your common
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.
https://nest.com/ca/support/article/When-Nest-needs-a-common-C-wire#wire-you-checker