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.