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I have an older furnace and am looking to add a c wire for a smart thermostat. Currently behind my thermostat I have red/black wire, however, there are 3 additional wires that are not connected.

At the furnace the red/black are connected (as shown below) and the 3 additional wires are free.

From what I understand the C wire is 24 volt common. I have metered the furnace running/not running and found a post transformer wire that is 24 volt constant. Can I simply connect one of the spare wires to this terminal or is there potential for too much of a current draw blowing the transformer?

enter image description here

Red coil wire is from thermostat. 24v constant is the bottom right black wire (this is where I want to connect the C wire to)

  • What's the model number on that gas valve? – Tester101 Jan 13 '17 at 11:36
  • VR800C 1190 is the model number – user65052 Jan 14 '17 at 20:41
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Yes, the terminal at the lower right in the photo is C.

Here's the wiring diagram from the documentation, which I highlighted in different colors.

enter image description here

Here's your wiring, which I highlighted using the same colors.

enter image description here

  • Thanks! I will connect one of the spare wires to the TR and then to the C slot on thermostat. – user65052 Jan 15 '17 at 3:58

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