I have a new Insteon Thermostat that I'm using to replace my prior Honeywell t-stat. This is installed to a relatively new HVAC system (both AC and Heat) that DOES have a "c" wire. I've attached the wires correctly in that the terminals on the furnace control side are matched to the terminals on the t-stat by "code" (meaning G goes to G, Y goes to Y, W goes to W, C goes to C and R goes to R).

I'm not quite sure that the secondary 24VAC is wired into all of this correctly and perhaps that's why the compressor comes on as soon as the system is energized.

Below is a photo of the wiring diagram on the furnace. NOTE: On the furnace terminals, the wire terminating into "R" is red, the wire into "Y" is yellow, the wire into "G" is green, the wire into "C" is blue, and the wire in "W" is white; you'll note that this is maintained on the t-stat side as well.

Wiring Diagram on Furnace

And here is the photo of the inside of the thermostat:

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Any help anyone can offer on how or what might be wrong would be most appreciated.

TRANSFORMER NOTE: The red wire off of the 24VAC transformer is "spliced" into the red wire going between the furnace and the t-stat. Similarly, the white wire off of the transformer is spliced into the blue (common) wire between the furnace and the t-stat. I have a feeling this is the problem, but I'm not sure how to "fix" it.

  • Have to ask... Did you note how the old thermostat was hooked up? Were the wires connected to the Honeywell by their "code" ?
    – JPhi1618
    Oct 14, 2015 at 19:26
  • The old t-stat only made use of 4 wires. The "c" wire (blue) was in the wall and not connected. It used batteries instead to power the LCD. The remaining wires were all connected by their code with the specific note that red was wired into a terminal labeled "Rc" and there was a jumper in place to another terminal labeled just "R." The old t-stat DID have a "c" terminal but inside the back of the unit, it said that connecting "c" was optional if you chose to use batteries, so I guess my installer took that option for some reason as it did have batteries in it.
    – Greg
    Oct 14, 2015 at 19:45

1 Answer 1


Ultimately I figured out what I was doing wrong with help of this question: Adding a C Wire To A New Honeywell Wifi Thermostat

I was mistaking the 24VAC transformer for the relay to the outdoor unit. The only thing I really needed to do to the original wiring was connect the blue wire ("C") going up to the t-stat to the splice bundle near the furnace.

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