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)