How can I implement an unused wire as a C wire for my thermostat?
Installation of a WiFi thermostat to control a forced hot air furnace, and you have a spare wire available to use for (C). Rather than get into the wiring of the furnace, I install a Taco SR501 single zone relay outside the furnace, near the service switch. The 120 volt AC for the SR501 can be connected to the furnace (usually at the service switch). The SR501 then can be wired using the T T terminals (red and white from the T-stat) and the green (or spare wire) connect to the C terminal in the SR501. Connect two wires from the burner head (T T) to terminals 5 and 6 on the SR501 (normally open). This works well, and the SR501 relay is about 50 bucks...