I'm replacing an old thermostat from the early 90's with a new wifi thermostat.
When removing the old one, it only used R,G,Y,W wires, and the new wifi thermostat requires a C.
After fully removing the old thermostat, I saw there were wires tucked away in the wall. One was orange and the other blue. I'm assuming (just based off pictures) the orange is the C, but what could the blue one be?
Also, is there a way I can test the wires to determine which is a C? As of right now, I have the power turned off at the breaker for both the furnace and AC.