I have a problem similar to the one in this question. My R wire from the thermostat to the G terminal on boiler and the W wire from the thermostat to the Y terminal on the boiler. The picture is a bit confusing since the red and white from thermostat were connected to 2 black wires.


1 Answer 1


You'll need to run a 4 wire thermostat cable

The issue with your proposals so far is that unlike many setups, your boiler (and some other Weil-McLain boilers as well) puts various safety controls ahead of the intended thermostat connection point in a steam configuration (see page 8 of the control supplement), so the wire that's typically connected to R isn't actually constant power. To avoid thermostat derangement in the case of a safety cutout or worse yet, bypassing a critical safety control (such as your all-important low-water cutoff!!!), you'll need to pull a 4-wire cable instead of your existing 3-wire cable, then wire it as follows:

  • Nest Rc to boiler R
  • Nest C to boiler C
  • Nest Rh to boiler G
  • and Nest W to boiler Y

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