Trying to install my new ecobee SmartThermostat. Furnace is a Trane heat pump, 20 years old from when the house was built.

When I looked at the existing thermostat, it has RGWY. Existing

So I thought I needed to install the PEK. I went up into the attic and found a wire nutted snarl. Someone probably assumed no one would ever have to look at their work 🤦‍♂️

Anyway, it looks to me like the blue comes from the control box and is tied together with the white wire going to the thermostat, two white wires going back to the control box, and the wire running to the actual furnace.

furnace wiring

I added the labels. If I’m correct here, then what I actually have at the thermostat is GRCY. Does this make sense? Will this work if I use that white wire as C and have no W?

Oh, I also measured the voltage between R and W/C, and it read 28 VAC.

Thanks for the help 😊


Here is the wiring diagram and control box wiring:

wiring diagram spliced bottom half top half control

Plus the heater wiring and diagram: heater

I pulled the rest of the wire out of the wall and the 4 wires tied back in the furnace are clipped off here, so I could theoretically do some rewriting if necessary.

Update 2: Unwrapping the tape

enter image description here enter image description here

It looks like they have the blue/C jumpered to the white/W using a capacitor.. Weird. I can’t think of any reason they would do that.. Maybe using it as a fuse? I’m so confused, lol

So I think I’m going to take the capacitor out, connect my thermostat C wire to the blue/C going to the AC, leave the white/W without the jumper. Thoughts? Should I leave it in?

Thanks again!

  • 1
    Can you post the wiring diagram for your furnace/air-handler please? Jan 30, 2023 at 2:14
  • 1
    Photos of the control box wiring could be helpful too. The photos so far show a cable that has 4C visible (green, red, white, yellow -- check for more hidden behind the mounting plate!), while in the blower compartment one cable has 4C (red, white, blue, green) and one seems to be 7C or maybe 8C (red, white, yellow, green, with blue, orange, and one or two others twisted around it). Also look around the furnace for splices; it's possible one of these in the blower is spliced to something else before going to the thermostat.
    – Greg Hill
    Jan 30, 2023 at 3:18
  • Congrats on taking some "before" pics! Far too many forget that important step. Also, that looks like some pretty standard (if dusty) furnace side wiring. At the low voltages used to control HVAC equipment, it's not that big a deal.
    – FreeMan
    Jan 30, 2023 at 12:32
  • Thanks for the assistance.. I’ve added some more pictures
    – Steve W
    Jan 31, 2023 at 0:50
  • That "low voltage field connection" has three white wires and a blue wire -- are all three of those connected with the two small white wires? It's wrapped in a ball of electrical tape and I can't tell from here. :-(
    – Greg Hill
    Jan 31, 2023 at 3:51

1 Answer 1


I ended up removing the resistor (or whatever), connected my blue wire from the thermostat to the C/blue bundle and left the white from the thermostat hooked into the white/W.

At the thermostat I hooked in the R, G, Y, C, and W. During initialization it didn’t detect the W, so I manually entered it. I tested both heat and cool, and everything worked fine.

The weird resistor jumper is still a mystery, but now that it’s working and installed, I’m happy to let it lie. Thanks for the suggestions and support!

  • 1
    Was going to answer this myself, but you beat me to it :) I'm fairly sure the mystery device was indeed a resistor that was added to provide a bypass path around the heat sequencer coils to power a "power stealing" thermostat without the heat turning on unexpectedly Feb 2, 2023 at 3:59

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