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  • 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?) enter image description here this is the transformer? enter image description here

this is behind the panel enter image description here

enter image description here enter image description here enter image description here

enter image description here enter image description here enter image description here enter image description here

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.

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3 Answers 3

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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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  • Sidenote: if you can't find a steam boiler expert in your area, (and even perhaps if you can), you'll probably want to read Dan Holohan's We Got Steam Heat for more advice on your system overall -- steam systems behave very differently from forced-air, and applying forced-air logic to steam systems can be quite counterproductive as a result Mar 9, 2021 at 3:03
  • Given the way the low voltage wires fold back into the junction box, I don't think we know for sure how the pressuretrol is wired but I agree with the advice, this boiler deserves a look by someone knowledgeable.
    – jay613
    Mar 9, 2021 at 3:27
  • Thanks for your reply. Ive added a couple of photos of the thermostat wires. Based on your input I suspect that this is something more than I can handle. I was hoping to avoid plugging in a 24VAC transformer Mar 9, 2021 at 3:35
  • @user2285215 -- we thank people around here by upvoting and/or accepting their answers :) Mar 9, 2021 at 3:55
  • Just to reopen this can of copper worms, I think the thermostat wire enters through the EMT on the top right side of the cutoff box, the same EMT as the power source. I think the green is already connected to neutral! Ironic. But this doesn't change the outcome ... It should be redone by an electrician as there are serious safety issues here.
    – jay613
    Mar 9, 2021 at 10:28
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What are the safety issues here?

You have several issues that worry me:

  1. your lack of experience with both electrical and steam boilers
  2. the lack of a circuit diagram or a reasonable ability to create one from the spaghetti in that junction box. A link was provided in one of the comments but there is no certainty your system followed the diagram in that document.
  3. your boiler is wired terribly (see below) so the chances of your making a mistake are higher than they could or should be.

@ThreePhaseEel highlighted one big concern. Briefly: your thermostat is wired along with several safety devices any one of which can turn off the boiler. It is possible to wire the boiler so the thermostat alone can turn it on, bypassing the safety devices. That would be very bad.

I’m highlighting a different safety issue: The purpose of that metal pipe (top right) and of the two metal boxes is to isolate and contain the high voltage wiring. The entire point of that metal plate that the transformer is mounted on is to provide and ensure that the high voltage wiring is on one side of the plate, inside the box, and the low voltage wiring is on the outside of the box. The thermostat wiring should be outside that conduit. The thermostat wire can be cable-tied to the outside of the conduit if it happens to be heading in the same direction.

What you have is the low voltage wiring from the transformer wrapped around the edge of the metal plate, wedged between the plate and the sharp edge of the box and re-entering the very box that is literally designed keep it out. You have the thermostat running through the conduit with high voltage wiring, then you have all the high and low voltage wires mixed up in a spaghetti junction inside the box.

The main dangers are:

  • Difficult to inspect and maintain correct wiring and function of the safety devices
  • Faults or errors in wiring can cause high voltage to be applied to your low-voltage devices including the thermostat. That could start a fire and/or shock people who come in contact.
  • Unintentional wiring errors such as installing a new thermostat could bypass safety controls. It's almost impossible with this mess to see, understand, or attempt to do things correctly.

I strongly suggest you have all this corrected by someone who is knowledgeable and who has some pride in their work. I'm pretty sure you can easily provide a C wire to your new thermostat, in fact I think it may already have on in the green wire but don't make any changes without cleaning this up.

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  • Thanks for your in depth answer! Actually I have never touched this boiler, it's in my parents home. Evidently whoever wired/installed it didn't know what they were doing or just didn't care. I will try to get an electrician to fix these safety issues Mar 9, 2021 at 15:55
  • jay613 or @ThreePhaseEel would either of you happen to know someone you recommend in the New York City area? Mar 9, 2021 at 19:40
  • @user2285215 -- not offhand, although you're fortunate in a sense in that NYC is a place where your chances of finding someone who actually knows their stuff is non-nil :) Mar 9, 2021 at 23:33
  • Ask the question on hyper local chat groups like a local moms group on fb or Nextdoor or whatever is popular in your town or neighborhood. Best bet for connection to a tradesman with a reputation to protect.
    – jay613
    Mar 10, 2021 at 1:00
  • And you want an electrician with reasonable steam plumbing knowledge, not a plumber with hazy electrical skill. Any electrician ought to be able wire the boiler correctly by following the schematic provided in one of the comments here, while also doing everything safely.
    – jay613
    Mar 10, 2021 at 2:08
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There's something concerning in your third picture: the low voltage black and white wires from your transformer appear to disappear into the high voltage box behind the transformer. It's not totally clear from the picture. Can you confirm that? And if so, where do they go once inside?

None of your photos show the basement end of your thermostat wire. Hopefully you can find that nearby with the unused green and blue wires visible.

This white wire is your neutral enter image description here

Just to the right it joins the controller with a push-on connector so that's not a convenient place to connect your thermostat C wire. I can't tell from your photos where that white wire goes off to on the left. Presumably it ends up eventually as the white wire on the transformer.

Perhaps you could draw a partial schematic ... don't need all the wires from the controller to the ignitor and gas valve. Just need to know where the transformer neutral side is available to be connected to, and where your thermostat wire is. You should hopefully confirm also that one of the in-use thermostat wires connects, somewhere, to the + side of the transformer.

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  • thanks for replying. I will try and take better pictures and add them when I can. Mar 9, 2021 at 2:12
  • tried to edit comment but couldnt. Im not 100% sure where the wires in picture 3 are going atm. Yes I am having trouble locating the thermostat wires as they are in a shielded cable before they get to the basement. they also are running above the basement ceiling. its difficult to trace where they are on the actual boiler. As to your partial schematic request i don't fully understand. can you explain like im a 3 year old? (transformer neutral side, + side of transformer (i assume this is the black terminal?)) Mar 9, 2021 at 2:20
  • Note that the OP here has to be extra careful as to where they connect their thermostat heating terminals (Rh & W) so that they don't bypass their low-water cutoff, as this is a steam boiler (!!!!) Mar 9, 2021 at 2:43
  • @ThreePhaseEel agreed, though without a full schematic (which I don't think we'll be getting) I think that's beyond the scope. Hopefully they are already correctly connect in that manner and all we're doing is finding a good connection point for C.
    – jay613
    Mar 9, 2021 at 2:54
  • @jay613 -- it's worse than that, their pressuretrol's in line with the 120VAC side of things from all appearances, so there's no way they'll just be able to pull C off the boiler and call it a day Mar 9, 2021 at 2:58

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