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I have installed a Nest thermostat into a system with hot water heat and separate a/c system each with its own transformer. To run a C wire to gain constant power, does the wire connect to the boiler, a/c, or to both. Additionally, I would like to be able to disconnect the opposite system depending upon the season. Is this possible? Thanks

2 on boiler side; R to Rh and W1 back to zone valves. 3 from a/c; R to Rc, Y to Y1, and G to G. There are no designations on a/c terminal block but I am attaching schematic from documentation. I assume my terminal marked with arrow would correspond to C.enter image description here

Easy. It is in the attic right above the thermo. I can go either way, also, since I can fish from boiler to attic, to thermo as well. Nest has never confirmed that with both Rc and Rh wired, the common wire connection for constant power is only between Rc and C which makes connecting to a/c the logical answer.

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    What wires do you have running to the thermostat at the moment? – ThreePhaseEel Feb 13 '17 at 0:03
  • How feasible is it for you to run new thermostat cable from the 'stat to the air handler? – ThreePhaseEel Feb 14 '17 at 4:19
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Run it from the A/C air handler

According to this tech support article from Nest, the Nest prefers Rc and C over Rh and C when all three are connected.

So, I'd run a new thermostat cable (18/8 is trivial to find, although even 18/4 or 18/5 will do for what you're doing) in place of the old one from the air handler -- this gives you spare wires so you can use one for a C wire from the Nest to your air handler. Once you have the wire in place (blue is the standard color), you can connect it to the C terminal on the Nest and to the #6 terminal (as you have identified in the diagram) on your air handler's terminal block.

From there, Rh and W1 on the Nest go to the red and white wires from the boiler cable, while Rc, Y1, and G go to the red, yellow, and green wires in the new cable to the air handler. At the air handler end, the red wire goes to R (#8 terminal), the green wire goes to G (#7 terminal), and the yellow wire goes to Y (#2 terminal).

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In my experience, it's typically easier to get the C wire from the A/C. Boilers tend to have more complex control systems, that make it more difficult to provide a C wire.

However, you might want to contact NEST directly, as the thermostat itself might dictate where the C has to come from. I Think Honeywell thermostats expect the C from the A/C, when Rh and Rc are used separately. I'm not sure about NEST, but it might say in the documentation. You'll also want to check to to see if the Nest even needs a C wire, as it's not always required (from what I've found).

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