This is a followup to an earlier question I posted where it appeared that the previously unused Common wire on my HVAC system was damaged. All of the other wires read 28V between them and R, and the Common wire alone was only reading phantom voltage at the thermostat. Now that I need the Common wire for my new Nest thermostats, I swapped the terminals the G and C wires were hooked up to at the furnace since I don't care for manual fan control. Now the green wire was Common, and the blue wire (broken) was hooked up to the fan.
After turning the power back on at the breaker though, I was reading 0V on all of the wires at the two thermostats (the furnace still read 28V across all of them). I turned the power off again, reverted the furnace to its original configuration, flipped the power back on, and am still getting 0V across all wires at the thermostats.
(in the image above, I had temporarily hooked up Y/R alone to the old thermostat so the AC would work while I figured this out. G/C were not connected to it at all)
Everything is flipped on at the breaker:
(granted, I haven't checked the left side of the breaker, but it seems locked, so I assume that it does not have anything I need to access)
I also checked the voltage across the wires at the thermostat at the same time as the furnace with a helper to verify that it wasn't an issue where e.g. there was power for a few seconds & by the time I had moved from the furnace to the thermostat it had already shut off, but that wasn't the issue. Even at the exact same time the furnace is reading 28V, the thermostat wires are reading 0V.
My understanding is that if a continuous wire has voltage on one end (furnace), it will have voltage on the other end (thermostat), so I don't understand how a wire that was working fine just a few minutes prior is suddenly not working. My only thoughts are that whatever was used to splice the wire so that it goes to both the upstairs and downstairs thermostat has tripped / blown, and the connection is no longer continuous. If this is likely the issue, how can I verify it / locate where the splice is? Or are there other potential problems that could cause these symptoms?
