750mv vs 12v Thermostat Question UPDATE 2

BFL13 wrote:
Some mentioned about the adjustable swing feature on the better ones. I got the lower cost digital without that. I can see how that could be a useful feature in some scenarios.

I have no clue why those two blue wires have a voltage across them. I started out thinking it was just one of them that was being switched and the other was a ground. Didn't look closely enough at the old one before starting all this. Anyway it turns the voltage on and off between the two wires.

The blue wire shows up in the schematic here and there to the various furnace switches, so it must all have something to do with that instead of just switching one of them. Whatever. "Not my problem!" :)

If there were no voltage across the wires (when not connected together), the furnace would have no way of detecting when the thermostat switch closed. The wires would basically be virtually shorted together already--which basically means that their voltage is identical--and without any voltage difference, there would be no current that flows when the switch closes and no voltage change to boot.

These are not power supply wires. They are signaling wires for the furnace to turn on and off. It's pointless in this case to try to determine if one is positive and one is negative; they just need to be connected to turn the furnace on and disconnected to turn it off. Similarly, it doesn't matter for a light switch which wire to the switch is positive and which is negative, and also there those descriptors are not entirely accurate.

(There's never any voltage between wires that are shorted together by a switch or anything else. That's basically what shorting them means; they are, from a circuit analysis standpoint, part of the same node.)