I'm attempting to install a new ecobee3 on an older Coleman Evcon gas furnace and AC HVAC system. The system is what I would consider typical residential as far as I can see. There is currently a Honeywell RTH2410B1001 programmable stat installed and (seemingly) functioning as expected. The wiring at the thermostat is the typical Y R G W wiring (actual wire colors also match the terminal label designation).

When I went to install the ecobee3 PEK module, I noticed that the yellow wire (the thermostat Y terminal) is connected directly to the HVAC control board C terminal, and the control board Y terminal is empty. The other wires (R, G, W) are as expected.

I'm not sure what to do at this point. I'm reluctant to simply follow ecobee's instructions (waiting for their support to call me back with with more information) for connecting the PEK module wires to the corresponding control board terminals, and then plugging in the stat wires to the terminal board on the PEK module based on the thermostat wiring (essentially ignoring the fact that Y -> C). Also, I just don't understand how the AC compressor relay is energized and functions if the Y stat terminal is sent straight to common. (I found the splice up in the drop ceiling between Y and the AC compressor relay).

Any help anyone could provide would be much appreciated.

Furnace Model Furnace Control Board t-stat wiring Honeywell t-stat wiring Furnace Control Board wiring schematic Splice between Y t-stat wire and what looks like the ac compressor relay, then to the C terminal on the Furnace Control Board

  • Make and model of the air handler/furnace, and some photos of the wiring would be helpful.
    – Tester101
    Jun 22, 2015 at 1:15
  • Never mind regarding the direct wire from Y on the stat to the C on the control board. I found the wire splice the previous homeowners did up inside a drop ceiling to the outside AC unit, and see how the whole system is working. Now I'm just at a loss on what to do with the PEK module to get the ecobee3 a C wire equivalent back at the mounting spot on the upstairs wall. I'm going to make the assumption that the PEK module isn't designed to handle this implementation, and it could cause damage to the equipment if I follow their instructions. I guess I might have to just run an actual C wire... Jun 22, 2015 at 1:30

1 Answer 1


It looks like there's an unused blue wire in the cable. Just hook that up at both ends to the C terminals, and make sure you connect them together at the splice.

You might have to use a twist-on wire connector and a bit of scrap wire, to extend the cut off blue wire to reach the terminals.

  • That's what I'll do. There's multiple splices along the way, but I think there's enough slack to connect the blue wire back to the furnace. I pulled off the existing thermostat mount and pulled the blue out of the wall upstairs - plenty there. Thanks! Jun 22, 2015 at 2:17
  • Just as a follow up, I left the existing wiring alone on the control board, connected the unused blue wire in the five wire thermostat cable to common on the control board, and connected the blue wire at the thermostat to the C terminal. The yellow Y at the ecobee thermostat is still connected to C on the control board (the AC compressor relay is wired inline with the Y terminal back to the common on the control board). The blue wire is now basically a common, and the ecobee3 powered right up and has been working as expected since. Thanks Tester101 for pointing this out! Jun 22, 2015 at 4:57

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