Trying to install an Ecobee 3 Lite thermostat. The old thermostat had a W, G, R, and Y wire connected, but no C cable which is needed.
However, I saw an unused red and blue wire in the wall and pulled them out.
I realized that my control board only seems to have W, G, and R labeled, and instead of using screws it uses quick disconnects and prongs.
Those cables are wire-nut to the cables coming from the actual thermostat. You can see that there is also the unconnected blue cable and the red cable wire-nut with another red cable coming from the outdoor fan unit I believe, and then runs into the board. There is also the yellow cable from the thermostat wire-nut with a white cable also coming from the outdoor unit (again, just a guess).
The two red wires that are wire-nut together with the thicker blue-ish cable seems to connect to behind the transformer?
Could I somehow use the un-used red wire at the thermostat as a C wire? Do I connect a cable to the unused blue wire from the thermostat into my transformer somehow? Any tips or suggestions would be appreciated.
I have the PEK extender (uses C and R from the board as one R cable to the thermostat) that came with the new thermostat and initially thought I would need it but finding the unused cables made me think otherwise.
ALSO: there is an unused 24V labeled screw slot above the W, G, and R prongs that you can see in the pictures. Could this be used as a place for the C cable?