I just installed a nest thermostat. The previous thermostat did not have a C wire attached. It G, RC, (RH Jumper), Y and W. When I hooked up the nest thermostat it was short cycling the blower fan until I pulled the AC (Y) wire.
I've been reading this happens if there is no C wire then the nest is trying to draw power from the furnace.
I then tried to attach the blue wire which was unused in the previous thermostat to the nest C terminal. The nest threw an error saying it was not getting power from this wire.
This isn't an ideal situation because once summer comes i cannot use my AC. I'm looking at my furnace and looks like the C terminal is connected but that wire is also joined on the Y terminal? I attached a picture of the furnace wire configuration.
Wondering if there's something I can do here to give the blue C wire power at the thermostat or if I should go with the venstar add-a-wire?
Furnace:
Old thermostat wiring:
Best Answer
What you need to do is to move the Blue wire at the furnace over to the "C" terminal at the furnace. Then at the new thermostat connect the Blue wire to the thermostat's "C" terminal.
Note that at the furnace the White wire currently on the "C" terminal goes into a separate cable bundle that is paired with the Red wire in that same bundle. That cable most likely runs off to the air conditioning compressor unit.
It is not clear what the Yellow wire in the cable bundle from the thermostat connects to at the furnace end. There is a good chance that this is connected incorrectly or not at all. My understanding is that this Yellow wire should be connected into the "Y" terminal at the furnace where the miss-wired blue wire used to be.