I'm trying to install a new wireless thermostat but my old setup only uses 2 wires; one white and one red. On the side of my furnace there is this transformer mounted.
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There are four cables on the other side of the transformer; white, black, blue and yellow.
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The white cable going to the thermostat is tied in with the blue cable from the transformer. Would I able to use any of the other cables from my transformer as my connection for the C wire? Any help would or tips would be appreciated. Let me know if you need any additional information. Thank you
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1 Answer 1


Blue and yellow are 0v and 24v, since blue is fed to the stat, the yellow is 0v for the "C" connection.

  • I tried connecting C to the yellow on the transformer but the thermostat is not turning on. Also should the Blue 24v wire on my transformer be fed to the red cable to the thermostat instead of the white?
    – Rayluc
    Commented Dec 12, 2020 at 17:04
  • I was apparently confused by your description. The white (and black) from the transformer should NOT be connected to the stat. You need a minimum of 3 wires from the cable to work, they will connect to R,W, and C. The blue wire looks like it is only connected to the white wire going to the stat, it should connect to the R. The red wire in the cable coming back from the stat looks like wasn't connected to the transformer, it should be connected to the W terminal on the stat. An additional wire from the cable needs to be added to the yellow, and connected to the "c". Commented Dec 12, 2020 at 19:51
  • My old stat with batteries works with the current setup where the blue 24v wire from the transformer is tied in with a couple of blue wires and a white wire connect to W on the stat. The red wire going to R on the stat is only connected with one brown cable going to the furnace. So you are saying with the new stat I should swap the connections and connect the red wire to W and the white wire to R and run a wire from C to the yellow 24v on the transformer? Thank you
    – Rayluc
    Commented Dec 13, 2020 at 1:41
  • The blue wire in your third photo has a label that says Thermostat Connection and appears to be connected to a single white wire in the thermostat cable, it needs to attach to the R terminal. On a two wire stat they may have reversed the R and W and it would make no difference. Commented Dec 13, 2020 at 2:33
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    I just got it up and running. Thankyou for all your help. I really appreciate it.
    – Rayluc
    Commented Dec 13, 2020 at 3:33

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