Take the 2-minute tour ×
Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. It's 100% free, no registration required.

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

share|improve this question
A picture of the current thermostat wiring would be useful. –  Tester101 Jan 12 at 13:51

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Browse other questions tagged or ask your own question.