I know this question has been asked multiple times and I'm going to try some of the methods I have read in other threads but wanted to ask this question since I didn't see anyone else's wired the way I have mine wired.

Systems: AC and furnace
Old digital thermostat -> Ecobee 3

Old thermostat wiring:

  • First 2 wire:

    • Red -> Rh (jumper to Rc)
    • White -> W
  • Second 2 wire:

    • Red -> G
    • White -> Y

These wires run down into the basement to wire nuts where they split into wires leading to the AC unit and to the furnace. The wires leading to the furnace and AC are different colors so I have to undo the spaghetti to determine which colors are connected to the thermostat wiring.

Inside the furnace the wiring is:

  • Green -> G
  • Blue - > C (Com 24v)
  • White -> W
  • Yellow -> Y
  • Black -> R

What's strange is that I can follow that black wire out to the condensate line attached to the furnace and it's connected to an EZ Trap.

The thermostat wiring is not connected to R at all. I thought the EZ Trap would be connected to C if anything and not sure how the thermostat is working properly if not connected to R.



Is this wiring normal?

I was thinking of connecting my Ecobee with an external transformer. So connecting the wires from the old thermostat as is and connecting the wires from the transformer to C and to Rc.

Is this correct and the proper way to wire my system given my wiring at the furnace?

Here is the image of the wires.

spaghetti wires

I have to cut the tie holding the wires together and trace all the wiring back to see exactly what is connected to what. I do know the blue common wire is connected to a red and white wire.


Ok I traced back all the wires in that spaghetti mess and have found where they all go.

Thermostat -> Air Handler -> AC
White (W) -> White (W)
Red (Rh) -> Red (R)...goes to wire nut with black wire. Black wire leads to condensate pump and ez trap and R on the air handler
White (Y) -> Yellow (Y) -> White
Red (G) -> Green (G)
...-> Blue (C or Common) -> Red (also goes to white humidifier wire)
...-> Brown (HUM) to red humidifier wire

I hope that explains it all. Is it normal for the C wire to go to the AC? So how would I wire up the Ecobee to make it work?

  • Can you run another thermostat cable from the thermostat down to the junction-mess? Commented Jul 15, 2016 at 2:56
  • I was going to try to do that but it will be very hard because it's not a straight run and the hole in the basement has fire block insulation so I was looking for another solution. Would the wire just connect C (thermostat) -> C (air handler)? Right now the C wire from the air handler (blue wire) is connected to Red AC wire and White Humidifier wire. Can another wire be connected to it? It won't be too much load on the transformer board in the air handler?
    – robk27
    Commented Jul 15, 2016 at 3:50
  • You have the right idea with C (stat) -> C (air handler) -- unless the Ecobee's an absolute pig, it shouldn't overload the transformer Commented Jul 15, 2016 at 11:43
  • Thanks ThreePhase. Do you know about using external transformers? Could I use one and just connect C on the thermostat to one of the screws on the transformer?
    – robk27
    Commented Jul 15, 2016 at 11:53


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