I have an electric furnace that has power, fan, electric heat, but I do not have a "C" wire. The thermostat wiring has several unused wires that are routed to furnace. It is a matter of applying 24VAC to the existing Blue wire that would be "C" wire. Why can't I jumper the 24VAC Power (red) to the C wire? I know there is a 24VAC transformer, but not sure if I should mess with it.


You have things the wrong way around

The way conventional mechanical thermostats work is similar to a wall switch -- you only need to provide constant 24VAC power (R) and a switched-hot to the load (W/Y/G) in order for it to be happy. However, modern electronic thermostats either recommend or require a return path for their own internal power -- the C wire you are talking about is that return path to the other side of the 24VAC transformer, same as how a neutral in a mains circuit returns electricity back to the utility's service. So, it needs to be connected to the C terminal on the control board, or to the same point the C wire from the air conditioner outdoor unit/compressor connects to (find the wire from the air conditioner that connects to the Y wire from the thermostat -- the other wire in that cable connects to C, considering that you have a conventional air conditioner).

(If you tried jumpering the 24VAC power (R) to the C and R terminals on your thermostat, it simply would refuse to turn on as there would be 0V across its internal power circuitry, provided another power source such as batteries or a power-stealing circuit is not present of course.)

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