I have a Nest E thermostat, and like many others have lost wifi and getting low battery because I have no common wire connected.
After looking at my old Lennnox 78ugf3-75-1 furnace, I noticed that my AC Y wire is connected to the Common terminal(called 'T') on my furnace circuit board. Oddly enough, nothing is connected to my Y terminal.
The electrical cable has a green unused wire that I plan to use for my C wire, but before I do that, should I move the Y wire to the Y terminal or leave it on the C terminal? It must have been installed this way years ago as I've never swapped the wires before.
Best Answer
Your AC compressor needs two wires - Y and C. Y comes from the thermostat and C comes from the air handler or the common side of the 24vac transformer.
The picture you have with the mid-line wire splice is where the three conductor wire (green white, red) branches off and goes outside to the compressor. Since Y is the standard for "compressor, someone decided to cut the Y wire in that place and use either end of it. One side of the Y wire goes back to the air handler to pick up the C wire (T terminal), and the other side goes to the thermostat to pick up the switched Y wire.
So, the Y wire and it's connections seem fine. What I would do is move the Green wire to the G terminal on the air handler and the thermostat since that is the standard color, and then use the Blue wire for C. It can connect to the T terminal on the air handler and then the C terminal of the nest.
I think the Nest has a voltage check function, but a simple multimeter could be used to verify that you do indeed have 24VAC between the R and T terminals of the air handler. Based on the diagrams, the T terminal is the C wire, and it was working before, so there shouldn't be a problem.