I have a galaxy cast iron gas boiler and i'm trying to determine where to attach the 'c' wire on the furnace for my new wifi thermostat. The cable is an 18/5 but is only using 2 of 5 wires currently. Based on other threads i've read it sounds like i can use one of the available wires for a 'c'.

I have attached a photo of where the thermostat wire connects to the furnace (A).

I'm not that familiar with thermostat wiring and i've tried to label the picture based on what is see on the furnace. (TH or TR)

thermostat Rh = TH2 thermostat W1 = TH1 (White Wire)

enter image description here

enter image description here

  • A picture of the current thermostat wiring would be useful.
    – Tester101
    Jan 12, 2014 at 13:51

1 Answer 1


Based on the second image, I'd say TR2 should be connected to the C terminal. However, for this to work, you'll likely have to swap TH1 and TH2. Since TR1 is associated with TH1, TH1 should be the R wire while TH2 should be the W wire.

The complete circuit should be:
TR1 -> TH1 -> R on thermostat -> W on thermostat -> TH2 -> TR2.

Then you can connect a wire from TR2 to the C terminal on the thermostat.

NOTE: If the transformer is not sized to support the additional load of the thermostat, you could end up blowing a fuse or frying the transformer.

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