electricalthermostatthermostat-c-wirewiring
I'm adding a smart thermostat to one zone of a two wire, oil fired boiler system. I've roughed in a new wire and could use some help determining where to terminate it at the boiler. The wiring is far too neat and I can't figure it out. The wires terminated on "Main End Switch" go back to the relay on the boiler and connect to terminals labeled TT. Pics below. Thanks in advance.
After glancing through the White-Rodgers 1361 zone valve Installation Instructions, I'd say the C wire should be attached to terminal 1. While R should attach to 5, and W should attach to 4.
From the pic, I'm guessing the transformer is the black device under the junction box. Looks yellow and purple coming off of that, you're connecting to the yellow, so the purple would be the common. To be sure, you'd want to connect a multimeter and verify the voltages to ground. It would also be helpful to find the wiring diagram for your boiler to be sure you're interpreting these wires correctly.
Assuming purple is the common, you'd run your wire to that, preferably through that wiring block where the other thermostat wires are connected on the left. There's no issue connecting the common to multiple places.
Best Answer
You've got two yellow wires on the lower left.
One of those two is the C wire -- according to the wiring diagram it is the upper one, but check with a multimeter. Remember to set it to AC mode.
Once you figure that out, the easiest way to do it would be to connect it to a new terminal block. Then connect the C wire from the thermostat to that terminal block -- this will support additional new smart thermostats as you acquire them.