I have a new smart thermostat and looking to connect to my boiler. As you can see in the picture there are (2) yellow wires which are labeled 24v control. One yellow wire is connected to the Y terminal and the other yellow wire is connected to the G terminal. The wire from the current thermostat is connected to the Y terminal and the G terminal with the common wire wrapped up and not connected to anything. Do I connect the control wire to both yellow control wires? Or just one and if only one, which one does it connect to? Or does it connect to a different terminal? enter image description here

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  • Hello, and welcome to Home Improvement. There's (probably) a circuit diagram for your furnace on one of the side panels; would you add a picture of it? And, you should probably take our tour so you'll know the details of contributing here. Nov 25, 2020 at 13:41
  • Can you post a photo showing the other half of the wiring diagram label please? Also, what make and model is the new thermostat you're trying to install? Nov 26, 2020 at 1:12
  • I have posted the full wiring drawing along with the thermostat I plan on installing. Nov 27, 2020 at 2:23
  • @inneedofhelp -- do you have more than one spare wire at hand in your existing thermostat cable? Nov 27, 2020 at 2:39
  • yes there is a spare wire that is in the wall and at the boiler. Nov 27, 2020 at 3:16

1 Answer 1


Your boiler wiring is a bit tricky...

Your boiler, along with some other Weil-McLain boilers, is a rather tricky beast in that it does not supply 24VAC to the thermostat connections at all times. Instead, it only supplies 24VAC to the 'stat if the low-water cutoff has not opened its contacts to shut the boiler off, which is a problem for a smart thermostat because without that constant supply, it might "lose its mind" if the low-water cutoff on your boiler ever kicks in.

...however, we can overcome this, but we'll need to pull a fatter cable to do so

The good news, though, is that like many modern thermostats, your Lyric T5 supports what are called two transformer systems, with separate power sources for heating and for cooling. Furthermore, in these two transformer systems, the Lyric T5 pulls power from the cooling transformer (Rc/C). This is good because it means that once you pull a fatter thermostat cable through to have enough spare wires, it's possible to wire your thermostat so it gets constant power, yet doesn't inadvertently bypass the low-water cutoff. (Consider that bypassing the cutoff would a) void any insurance you have on your house and b) render the boiler vulnerable to blowing up due to trying to run with insufficient water.)

You'll want to pull a fat cable for this, by the way; I'd recommend an 18/7 or 18/8 cable at a minimum so that you leave spare wires for the next installer to use. Once you have the new cable in:

  • Red goes to R on the boiler and Rc on the thermostat base
  • Brown goes to G on the boiler and Rh on the thermostat base
  • White goes to Y on the boiler and W/W1 on the thermostat base
  • and Blue goes to C on the boiler and C on the thermostat base

This way, the thermostat gets constant power from Rc and C while connecting Rh to W/W1 to call for heat. Once everything's in place and wired, I would run a simple test after turning things back on, though:

  • Crank the heat up to maximum to cause a call for heating
  • Head down to the boiler and activate the TEST function on the low-water cutoff, or simply unplug it from the boiler if your LWCO lacks a TEST function
  • Make sure that the boiler shuts off (stops heating) while the thermostat stays on when the LWCO is in TEST mode (or unplugged)
  • Once the test is concluded, make sure you reset the LWCO to the operating mode (or plug it back in if you had to unplug it) and set the thermostat to your desired temperature
  • Thank you. The only issue I have is all the connections are soldered. I guess I can wrap the t stat wire around the soldered connection. I also ordered a 24 volt adapter just in case. The t stat was almost free from the state and the adapter was 15$ But hoping for better efficiency and the WiFi connection will be a great help. Nov 28, 2020 at 2:51

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