I'm looking to install the Ecobee3 Lite thermostat. My current thermostat has a blue C wire running to it, so my understanding is the PEK doesn't help because that is used if you have no C wire.

On my furnace side, the C wire is unused, so I'm trying to figure out where I need to connect it because I do not have a typical control board that everything conveniently plugs into.


Wires not held in my hand are what is coming from the thermostat with the C wire wrapped around the other wires.

The wire being held (I'm assuming it goes out to the AC unit) connects a small white wire to the yellow wire of the thermostat. It then connects a small red wire from the AC to a larger brown wire that goes into the unit.

Wiring Schematic

Based on the schematic, the Brown wire is listed as connecting to the C wire going into the unit, so does that mean I can have the C wire from the thermostat, the R wire from the AC and the brown wire connected in the same cap? Would this give me the power I need to go to my thermostat?

enter image description here

Here's a picture of what everything goes into. I saw on another post that the C wire can also connect to the ground wire?

  • It's hard to tell what's going on in those pictures of the wires, but the brown wire coming off the 24v transformer is your "C" wire. Follow that. It's a little odd that there is a red thermostat wire connected to that thick brown wire in your first pic. Not sure where the red goes and where the brown come from.
    – JPhi1618
    Aug 7, 2019 at 18:20
  • In the third picture, it shows where the brown wire goes into the middle of the three components. Based on the schematic it would be the Fan relay, and splits off to both the pump relay and the transformer. The connection into the fan relay is in the top right corner of that black box in picture three, with the top brown wire being what goes out of the unit and connects to the red wire in the first picture.
    – Chris
    Aug 7, 2019 at 18:37

1 Answer 1


Based on the images and your comments, I would say that the wire splice you are holding in the first image with the thick brown and thin red wire would be where you want to connect your C wire. Blue is the typical color to use, and I believe you have an unused blue wire going to your thermostat that will work (the cable with the unused blue and brown should be the thermostat wiring).

The thin red wire in that bundle and the thin white wire from that yellow/white splice go to the "cooling unit relay" from the schematic. So the thin red would be the "common" side of the cooling relay and the white would get voltage from the thermostat.

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