As a relatively new home-owner, I recently came across the whole C-wire issue for the first time, having had to recently replace an old thermostat, and there is no C-wire connected inside the furnace.

The furnace we have is the Goodman GMP100-4 and is 25 years old, but the last homeowners replaced the circuit board in 2015. There are two unused (black and blue) wires in the 18/6 wire that goes from the thermostat to the furnace, and I wish to use one as a C-wire.

Please could you help me out with where this would go in the furnace? The circuit board is visible in the attached pictures along with the whole set up. I'm fairly handy in general but new to this particular situation/electronics.

I really appreciate any time you can give. Thanks!

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  • Hello, and welcome to Home Improvement. Is this the manual for your furnace? – Daniel Griscom Feb 25 '19 at 20:56
  • It does apply to this model, although the older paper one I have is slightly different – Andrew L Feb 25 '19 at 21:19

That black box in the top left is the 24vac transformer that powers the circuits of the furnace. The blue wire that appears to have a brown ground wire piggy-backing onto it is what the C wire needs to connect to. Sometimes there is a connector exposed on the circuit board for a C-wire connection, but not always.

If there's no other place to connect the C-wire, under that ground wire screw would be once place to connect it.

Use the blue wire in your thermostat wiring for the C-wire - that's the common color for it. Don't use the black.

On second look at the pictures... The leftmost wire nut in the bottom of the pictures has a thick bluish wire that goes up to a ground screw and a small red wire that goes somewhere else. You could tie the C-wire in that connection rather than trying to put it under a ground screw yourself.

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  • Yeah -- the wire nut between the thick blue and the thin red wires is definitely the right place to tap C at. – ThreePhaseEel Feb 26 '19 at 5:04
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    Did as suggested by both above- tied in with the thin red and thick blue in the leftmost connector. Works like a charm, thanks – Andrew L Mar 4 '19 at 21:50

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