I'm looking to wire a Honeywell Lyric T5 to my furnace (I do not have an air conditioner, I do have a humidistat). My current wiring at the thermostat is G,W,Y,R. At the furnace it's G,R,C, and 2 at W, there is a spare wire capped off (same colour as the Y wire at the thermostat).

The humidistat is connected to W and C.

Since I don't have air conditioning can I sub out the Y wire for the C wire I need for the new Lyric? My new set up at the Lyric would be G,W,R,C. The furnace would be G,R,2C,2W. The humidistat would not change.

Thanks for your help.

  • Does the "spare wire capped off" happen to be blue? Commented Dec 21, 2017 at 17:52
  • No, it's orange. The blue wire is connected to the C furnace terminal and the humidistat. Commented Dec 21, 2017 at 18:41

2 Answers 2


Yes, you can swap out and reuse the Y wire as a C wire, since you don't have an air conditioner. Or use the capped off wire (that's probably better).

hey're just copper wires, and there's no firm color code anyway. Take photos before you disconnect anything, and consider leaving a note tucked inside the thermostat documenting whatever you came up with.

Check this excellent answer for diagrams: https://diy.stackexchange.com/a/10527/5960

Consider posting photos to SE when asking questions.


I would need to see a diagram but basically the terminals are labeled with letters that should but do not have to correspond to different colors. This was a convention made popular by Honeywell. R=power from one side of the transformer C= power from the other side of the transformer W= switch from the R side through the gas valve or heating circuit Y= switch from R through contractor control circuit and G operates the fan relay. So as long as the same wire is connected to C at the furnace and C at the thermostat it doesn’t matter what color the wire is although it may be a little confusing for anyone trying to troubleshoot later. The first thing I do however is note if the wire colors conform to convention or not and make a mental note.

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