My thermostat failed and I am replacing it with a Honeywell Lyric T6 pro.

The previous configuration had G,W,R connected with Rc jumpered. The thermostat wire was 5-wire, 22ga I think. I used an extra wire to run power from the transformer (28vac) up to the common. However I still cannot get it to turn on. I tried checking voltage at the thermostat but I'm actually not sure how to check without access to the neutral. Using any ground I could find I was getting 6-7 volts. I'm sure this is not valid but regardless, its not turning on.

Also, There is a C terminal on the furnace board. The instructions make it sound like the wire should go straight from the transformer to the thermostat. Should anything be connected to the C terminal on the board?

  • Pictures of the wiring at the 'stat and furnace. Brand/model of the stat & furnace. These things are always helpful.
    – FreeMan
    Oct 16, 2023 at 12:11

2 Answers 2


The solution in this case was simple since the furnace board had an unused C terminal, I just connected a spare wire to it and ran it to the C terminal on the thermostat.


You need to run both, the R and C from the transformer to thermostat to have 24 volt at thermostat that it needs for its operation.

The G is the Fan (indoor blower)

The Y is cooling (Compressor ON)

The W is heating (Furnace on Gas or electric)

  • I thought R was the signal wire for the heat. Doesn't it need to come from the board?
    – mreff555
    Oct 16, 2023 at 0:23
  • R comes from the board (transformer) so does the C. The W is for heat command
    – Traveler
    Oct 16, 2023 at 1:12

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