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My thermostat is in the basement, connected to a Honeywell R845A to run a circulating pump for heat. I bought an Nest E for the main house and it works great. I want to replace the thermostat in the basement also with a Nest E.

Someone suggested to use a plug in transformer to power the thermostat, which requires 2 wires, but I called Nest and they told me you can only use 1 wire to the "C" terminal.

In the R845A I have the two wires connected to the old thermostat.

Is there any way I could connect the plug in transformer by making it into a single wire "C" or is there any connection I could make in the R845A?

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    Can you post photos of the wiring configuration at both ends? – ThreePhaseEel Mar 30 '18 at 1:26
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The Honeywell device does not provide a C wire out. There is fairly open access to it; perhaps you can pull a rabbit out of a hat.

enter image description here

Otherwise add a second transformer (2 wires from this new transformer). Phase it correctly, I discuss how to do that here.

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I'm sure you've found a solution to your problem in the last 9 months, and I'd be curious to know what you ended up doing, but I just came through the same problem (wiring a Nest E to a Honeywell R845a).

I ended up using a plug in 24vac transformer that I connected to the C and R terminals on the Nest, and everything appears to be functioning correctly so far.

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