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.

boiler boiler boiler


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.

  • Thanks. I own a multimeter, and I know how to set it to AC mode, and even know how to adjust the range. However, I'm not sure where and why to place the contacts in order to figure out out to identify the C vs. Load side. If I had to guess, I'd test the upper wire (supposedly a C wire) against a red thermostat wire, which I assume is attached to Load. But I'm not very bright. Any advice? – John Stamos Oct 22 '19 at 0:27
  • Test them both against the chassis of the furnace. I've seen a few furnaces where the R wire is actually the return from the thermostat, and there's another switch before hand. – gbronner Oct 22 '19 at 4:04

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