I'm trying to install a Nest learning WIFI thermostat to my boiler. However, I'm not sure where to connect the common wire. I've went through multiple posts on this forum but unfortunately wasn't able to find a resolution.

Below is my boiler's schematic wiring diagram. Diagram Link Schematic wiring diagram

I'm assuming that TSTAT BK(C) is the common wire? However, my boiler is connected to low water cut off and high water limit. In attached picture, blue pen drawing shows existing wirings. The Low Water Cut Off has 4 wires, and one is already connected to the C Common terminal. Can I just connect my thermostat’s C wire to the same terminal? (Green pen in drawing) Thanks!

Also, please let me know if the wiring is correct because the previous owner installed the boiler himself so the wiring might be off.

Diagram Drawing

Updated Diagram

Second Boiler

Low Water Cut Off



boiler picture

  • Is your boiler providing hot water to your heating system, or steam? Commented Nov 13, 2021 at 4:55
  • It's providing hot water to the heating system. Thanks for the question
    – Kevin2344
    Commented Nov 13, 2021 at 14:42
  • Can you post photos of your boiler's nameplate please? Commented Nov 13, 2021 at 14:47
  • Just added a few more pictures to the thread. Let me know if you need additional information!
    – Kevin2344
    Commented Nov 13, 2021 at 23:20
  • the manual for your boiler can be found here (it's a CGa Series 3, not the EG/PEG that other posters here had) Commented Nov 14, 2021 at 1:51

1 Answer 1


You'll need to work around what the installer did

Your boiler's installer put the safeties (high limit aquastat, LWCO) in line with the thermostat instead of wiring them in with the spill switch as the CGa manual calls for. As a result of this, you'll need to tie the green wire into the R wire from the transformer and control connector, then connect it to Rc on your Nest, in addition to connecting the blue wire to C as you indicate in your diagram. The red wire then lands on Rh, with the white on W, as you'd expect; this means that your boiler won't confuse your thermostat if it ever malfunctions and cuts out on low water, overtemperature, or such.

  • I've added a new digram with dash lines following what you recommended. Hopefully this is what you meant. where is R wire from the control board? Just curious, why do we need to connect green wire to red transformer and also connect it to Rc?
    – Kevin2344
    Commented Nov 14, 2021 at 3:48
  • @Kevin2344 -- to make sure the thermostat doesn't get confused by a power cut if the low-water cutoff or high-limit aquastat kicks in to shut the boiler down Commented Nov 14, 2021 at 4:00
  • @Kevin2344 -- yeah, your diagram looks correct Commented Nov 14, 2021 at 5:11
  • I added a new diagram for my second boiler. However, I noticed the wiring is very different than the first one. Can I still use the suggestion above with the dash lines?
    – Kevin2344
    Commented Nov 14, 2021 at 16:31
  • @Kevin2344 -- swap the wires going to RH and W on the thermostat, and you should be good for Boiler #2 Commented Nov 14, 2021 at 16:32

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