I am attempting to run a new 18/5 line in order to hook up an Ecobee 4 to my Weil Mclain boiler, as my current thermostat only has a R & W wire. I am having trouble figuring out where exactly to hook up the new C wire. Any insight would be helpful.

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  • Hi and welcome to this site. Two things to ask here. Can you provide a clearer more detailed picture of the circuit board that has the transformer with the CAL label on the top? Secondly can you find if there is a boiler wiring diagram on a sheet of paper pasted onto the inside of a cover or panel of the boiler? – Michael Karas Oct 28 '19 at 13:57
  • Hi Michael, thank you for the welcome to the site. I have added a better picture of the transformer and of the wiring diagram for my boiler. – Jeff Lombardi Oct 28 '19 at 23:40
  • Hi. It appears that the wiring diagram is going to provide the information need to locate where a C wire would connect. Unfortunately the wiring diagram is not a straight on shot and gets too much out of focus when trying to zoom into it. Could you try getting a better photo of the wiring diagram? – Michael Karas Oct 29 '19 at 0:21
  • Michael, I was finally able to get the wiring diagram in better focus. – Jeff Lombardi Oct 29 '19 at 1:01
  • I am having a problem understanding how the "LOW WATER CUT-OFF CONTROL" has any direct relationship to controlling room temperature from an Ecobee thermostat. Are you sure there is not a separate heating control unit plus a wiring diagram for that. It appears that the small gauge red and white wires in the first photo connect into the old 2-wire thermostat cable. What is not at all clear is what the black wires on that blue three terminal block connect to. Do they some how interconnect some of the wires attached to the black 6-position terminal block in the second photo? – Michael Karas Oct 29 '19 at 1:13

Looks like it should be terminal 2 based on the left hand wiring diagram. Verify it using a multimeter (should have 24v between 1 and 2, and 0 between 2 and chassis ground)

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