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.
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.