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.
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.