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Heating & Air Conditioning

use of 'C' (common) wire in digital thermostat

15 years ago

I bought a new programmable thermostat yesterday and hooked it up without any problem, works great. In reading the documentation, it says that if I use the 'C' wire, that the thermostat can utilize a remote control that came with the unit. The 'C' wire also powers the display panel and only utilizes the batteries if the power is off (power outage).

I'd like to utilize the 'C' wire; however, I'm not sure how to do it. Calling a repair company will cost at least $75-100. I am a computer network specialist so I figure if someone helps me I can certainly do this.

The wires I connected to the thermostat are R G Y W RC. There are other wires in the bundle that aren't used (ie: blue, orange, black, etc).

They terminate on a circuit board on my Lennox C26-31FC-1 system on their respective color screw-downs. There is a 'C' screw-down with a wire attached but it's not a color that is in the bundle terminating at the thermostat.

If someone can possibly help me with this, I could easily send a digital picture of the circuit board where the wires terminate.

Would it be as easy as using any one of the unused wires in that bundle (say the orange one) and connecting it the the 'C' connector on the thermostat and then the 'C' screw-down on the circuit board?

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Comments (2)

tlewis_hvac
15 years ago
If the color at the unit on "c" is not in the bundle at the thermostat, there is a junction in that thermostat wire. You need to make sure whatever is hooked to "c" is followed all the way back to the thermostat, whether is is blue, brown, etc.
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energywarrior
15 years ago
Tlewis is correct in that you need to be sure that it goes back to the C terminal. The C terminal is the Comon or Neutral side of the equation. The Rh/Rc terminals are made the same terminal by a jumper wire that goes from one terminal to the other, the wire that goes to that termial is the hot lead coming from your transformer.

The C terminal is your neutral side, or ground side-depending on the configuration of the transformer. The wire that goes to this terminal must go to the neutral side or ground side of the transformer. Ususualy you can ohm out the C terminal to Ground and ensure its the proper terminal. There will be continuity between C & ground but virtually no resistence if the Neutral and ground are the same.

E.W.
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