Originally Posted By: K447
Can you measure (using a multi-meter) the actual AC voltage at the Nest terminals? Is it indeed 37 volts?


Can't check right now since I'm out of the house for a week, but that would be a good check. Not sure which wires to put the circult tester probes on, though, since I don't clearly understand which wires I'm supposed to be testing.

Right now it has connected:
W1 heat
G fan
Rc power
C 24v common

Until I did the hack described at the top of the thread, that C common wire was not there.

What I don't understand is how that can work since none of the above are a ground wire or neutral wire. So I don't understand how thermostat wiring works in general. So I wouldn't know where to put the test probes.

Quote:
If yes, does the AC transformer itself (usually located within the heating or cooling system unit) have multiple taps for AC supply voltage?


Don't know how to tell? All I know is, that I can open the panel on the furnace and see where the wires go in, and there's a little circuit board in there with screw terminals labeled with the same letter codes as the ones on the inside of the Nest. That's how I could easily add the "C" wire because it was clearly labeled, and there was a blue wire free on the wire bundle.
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Tony Fabris