thermostatthermostat-c-wirewire
I am trying to install an Ecobee 3 thermostat to replace the upstairs one I have now but the 3rd wire isn't connected to anything on either end. I added the basement thermostat just to complete the wiring but I won't be changing that one. Can I connect the 3rd wire to the C terminal on the external transformer (top left)?
The YELLOW or Y terminal at the furncae/ air handler is a dummy terminal for convenience, it is not required to be used, you could send Yellow directly to the AC unit out doors. Common will be used since the Common leg of power comes from the transformer which is in the furnace. The Yellow going to common likely if you trace it back it in fact is Common leg of the 24 v power supply.
Don't use an external transformer. Find a way to run an extra wire from the furnace/air handler, and wire it as a proper C wire.
Best Answer
Yes, the terminal labeled
Con the transformer is where you want to connect theCwire to the thermostat. However, you'll want to make sure that the transformer is large enough to also supply power to the thermostat. If the thermostat draws too much power, you could end up burning out your transformer. It might also be a good idea to put an appropriately sized fuse in line with theRwire.