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I am trying to figure out the proper wiring for a C wire for my thermostat.

My 24v transformer and the current thermostat wiring at the furnace are shown in the pictures below.

Can I wire a thermostat with Red White and Common to this Furnace?

enter image description here

enter image description here

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  • Can I wire a red white and common with this? ... what does this mean?
    – jsotola
    Jan 16, 2023 at 5:23
  • Can you post the wiring diagram for your furnace please, or the make/model of the control valve in the picture for that matter? Jan 16, 2023 at 5:53
  • that does not look like your 24 transformer, better use multimeter and find out
    – Traveler
    Jan 16, 2023 at 6:28
  • @ruskes it's a gas valve. The transformer is not shown. The induction winding symbol imprinted represents the gas valve solenoid. You bring up a good point that the OP might be confusing this for the transformer.
    – P2000
    Jan 16, 2023 at 6:37
  • 24v transformer and current wiring in pic below
    – Traveler
    Jan 16, 2023 at 7:52

3 Answers 3

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The first picture is of the gas valve of your furnace. The white induction winding/coil symbol imprinted represents the gas valve solenoid.

The second picture is of the transformer.

(If you are reading this and wondering why I am stating this, that's because the question was edited and has evolved, and we are concerned the OP or other reader might have thought that the gas valve was the transformer)

It seems the thermostat/transformer cables at the valve are swapped in terms of the labels at the screw terminals. This is odd but not a problem (obviously it worked over the past years).

The black cable is for the transformer. The brown cable is for the current thermostat.

Your new thermostat with a "C" common will require at least 3 wires.

This is will be the new thermostat wiring for a gas standing-pilot furnace:

enter image description here

I suggest you correct the transformer/thermostat swap so that the labeling is correct and clear for now and the future.

To use a new thermostat that requires "R" and "C" for 24VAC power, you need to run at least a 3-wire cable to the thermostat. (I suggest upgrading to a 5 wire, see below)

Your new thermostat wire will have the "C", "R" and "W" terminals in use.

Your old brown thermostat wire is for a "mercury" style thermostat. Colour coding (red/white) did not matter, but it will now matter for the new thermostat.

The furnace TH terminal at screw #1 is the call to heat, and is wired to the new thermostat's "W" using the white wire of the brown thermostat cable.

The furnace TR terminal at screw #4 is wired to one of the black transformer's wires and to the "C" of the new thermostat. It does not matter which of the two transformer wires you use for this.

The other wire of the transformer goes to the new thermostat's "R" using the brown cable's red wire. You can use screw #2 for that, as before, or a wire nut.

If you are sticking to the swapped wiring, then the C & transformer go to #1, and the white to #4.

But I would switch it around now and fix the swapping. It will be more readily understood by someone else looking at a future picture.

This wiring diagram shows the colour coding for a thermostat with C-wire:

enter image description here

Note that your furnace does not have a separate fan control or a cooling signal (air conditioning). These terminals on the thermostat are left unconnected.

If you are running a new cable to the new thermostat, so that you have a wire for the common, it is a good idea upgrade to a 5 wire cable to provision for a future furnace upgrade with fan and cooling.

It is best if you include the furnace model number and a picture of the wiring diagram in the manual into your question. One thing to double check is the wiring for your NC high temp relay.

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  • Brown wire is thermostat wire. Black is transformer (Forgot to upload a pic of that). Am i right in my thinking that since its just a 2 wire mercury style thermo that it didn't matter if the red or the white was to the TH/TR. If rewiring, i would swap those so white is in #4, red #2, transformer #2 and transformer #1. The C wire would go to #1 as well. right? Jan 16, 2023 at 13:34
  • @ChrisPeterson ok so it seems the thermostat/transformer cables at the valve are swapped, which is odd but not a problem (obviously it worked over the past years). You should swap them back for future clarity. I will edit my answer. Look at the last diagram, showing a new blue C wire. If you are sticking to the swapped wiring, then the C & transformer go to #1, and the white to #4
    – P2000
    Jan 16, 2023 at 14:39
  • Thank you so much! Jan 16, 2023 at 15:08
  • @ChrisPeterson ok great! I edited your question so others can more easily find it if they have the same situation.
    – P2000
    Jan 16, 2023 at 16:07
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24v transformer and current wiring in pic below

That is not the 24V transformer, that is the Gas valve.

Keep looking for the 24 V transformer that will give you 24 Volt and the C wire for the thermostat.

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If you don't want to run new cable there is a product called the "Common maker" sold on amazon that is made exactly for this purpose

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