Wiring – install a C-Wire for Weil-McLain gas boiler? (EGH 115, 4 Series, Natural gas)

basementboilerthermostat-c-wirewifiwiring

  • I want to install a C-wire so that I can use a WiFi Thermostat.

  • Behind my current thermostat there are only 2 wires (red and white)

  • There are 2 wires tucked and unused, (green and blue)

  • I had tried using a Nest before and it eventually errored out because I don't have a C wire.

  • My boiler is a Weil-McLain EGH-115-4 series. My building is nearly
    100 years old.

  • I don't see any pictures of a wiring schematic anywhere on or in the
    boiler

  • I don't see the control board that has letters like in
    a general thermostat (C, G, Y, W1, W2, etc)

Where/Can I install a C-wire? Is it even possible here? I am kind of a newbie when it comes to this.
I've attached some pictures of my boiler. Thanks

This is behind the boiler panel
image behind boiler panel
This is behind the previous images Honeywell box (Aquastat?)
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this is the transformer?
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this is behind the panel
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I assume the shredded wire is the thermostat wire, the red, blue, green, etc wires are coming from there. It's really a rats nest of spliced wires.

Best Answer

This isn't just a matter of installing a C wire

Unlike most things that get called "boilers", which produce hot water, you have a live, honest-to-goodness, steam boiler on your hands here. As a result of that, and the way Weil-McLain wires the EGH series boilers when used in steam apps, you will need to use both spare wires in your thermostat cable, and be supremely careful what connects where to avoid the epic disaster that can result from a bypassed low-water cutoff. (Spoiler: on a good day, the safety valve pops and dumps scalding steam all over your basement, turning it into the worst sauna ever.)

Furthermore, it seems that the steam operating control (pressuretrol) was wired into the 120V side of the boiler's wiring, which is a recipe for baffling misbehavior when you try to hook up a smart thermostat as power to the boiler will be getting switched on and off by the pressuretrol during normal boiler operation. I'd strongly recommend getting a steam-boiler veteran out there to rearrange the controls on your boiler so that you can provide constant 24VAC to your thermostat.

If that's not an option, you could fit a 24VAC transformer near the thermostat and connect its terminals to Rc and C on the thermostat, while wiring the existing boiler thermostat wires to Rh and W. This way, the Nest gets a constant 24VAC supply, yet breaks the thermostat circuit on the boiler in the place the boiler expects it to, to avoid interactions with the low-water cutoff and that aquastat you mentioned, as it appears your boiler also heats up your hot water using an old gadget called a tankless coil that sits submerged in your boiler's water bath.

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