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WM Boiler Control Wiring Question

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pupmans
pupmans Member Posts: 5
Helpful Folks:

I had started a thread on the Steam section but it turns out my problem is quite a bit different (and less steam related) than I thought so I'm looking for a broader range of expertise.

In an effort to add a water feeder to my Weil-McClain EG35 of old vintage, I've discovered that my wiring does not match the factory schematic and is a little inexplicable - and fried a couple of thermostats now to boot.

My two questions
1) Am I safe?
2) How do I get my water feeder running with as little hassle as possible.

In brief, (with pictures attached) the way I am currently wired is that, from my transformer, I have hot and neutral running to the M&M PS801 Low water cutoff, going to terminals 1&2, which makes sense. I then have two feeds coming out of the LWCO- from the "Burner" circuit number 5, I have a line heading back to the front of the furnace that has continuity with the gas valve, and then from the "common" circuit number 4, I have a line that goes to the high temp limit, then to the Pressuretrol (essentially the switch leg type safety controls) and from there to the thermostat, where it "presents" as Rh. Coming back from the thermostat I then have W going to the burner circuit. As I can tell, it's been like this for many years. To me this is a little academic but in a line voltage context we've always been told to break hot, not neutral, because open neutrals are bad. At a common line essentially being a neutral...

So into this setting I tried to introduce a VXT24 water feeder, which needs a hot, neutral and feed. I hooked them to the appropriate terminals per the wiring diagram on the LWCO (terminals 1, 2 and 4) and the water feeder powers up and runs (with the manual switch), but a low water signal doesn't turn on the feeder.

In reviewing both the VXT instructions and the factory WM schematic, there is a jumper from hot (terminal 1) to common (terminal 3) on the boiler. So I tried implementing that (not knowing the lines from the common ran to the safety controls) and got the following result:

1) Water feeder works perfectly (feeds when LWCO clicks on)
2) Burner does not start
3) Fried a nest thermostat

In investigating all this I determined the thermostat and safety controls are in the common leg coming off the LWCO, not the burner leg...and now I'm stuck. The very strange thing is I checked my fried thermostat and even with the jumper in place in the LWCO, I just see 24v drop between RH and W on the stat, so I'm thinking perhaps a phase issue? So I'm trying to figure out why the system was wired this way, if it's safe, and how to actually get a common to my feeder. I'm concerned the common isn't functioning as common, given the LWCO is essentially a SPDT switch that sends current to the burner or the feeder. So if that's the case why are all my safety controls there rather than downstream of the burner lead on the LWCO.

Pictures of the factory schematic and LWCO wiring are attached. I will try and diagram the extant wiring later today but I haven't confirmed everything yet...physically getting meter probes to all the places I want to check is a two person job. As an aside the factory transfomer in the boiler is a 5 terminal job as per the spec (and my review) and I really don't understand what the 5 terminals exactly do. If I understood that I would have some hope of figuring this out.






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  • ratio
    ratio Member Posts: 3,702
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    We break hot for a number of reasons, one being that if you break the neutral, the load is still hot & presents a electrocution risk; also, if the neutral is inadvertently grounded the load will be energized. That said, many motor starters break the neutral for overload protection, so it's not unheard of.

    How many control & safety devices are in the circuit? I still think you should remove all the old wiring & redo it either to the factory schematic, or something better (I like to break R for safeties [i.e. manual reset LWCO] & W for automatic/operational controls [i.e pressuretrol])

    The transformer sounds like it might be a 208/240 volt multi-tap model, there should be a schematic or drawling that shows the wiring.

  • pupmans
    pupmans Member Posts: 5
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    Hi Ratio:

    WIth 24v the safety impact is a lot less, but agreed it's better not to break neutral without good reason. The issue is, I can't figure out the original implementation's reason ;)

    As far as control and safety, we have automatic LWCO, then a temp limit switch, then a Vapostat. I will look tonight as I'm sure there is more inline in the boiler itself, but that's all the outside stuff. The LWCO is the only one that is more than a simple switch. Why do you prefer true safeties in the Hot and ops controls int he common?

    The transformer is no more than 24v nominal across any two points- I will examine tonight and see what else I can discern. The schematic from factory shows a 4 point transformer but all 24v. C and R are obvious but admittedly I don't get why things are coming back to transformer for W and Y.

    I am going to try to figure out the transformer and draw a diagram of what's in place tonight...the act of doing that might make some sense out of this in my head.

    Thanks for the help.
  • ratio
    ratio Member Posts: 3,702
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    I was unclear. I like manual reset –or Oh Crap– safeties breaking the R, as in killing the thermostat power. That's a lot more likely to get addressed. Things that are automatic reset & can be expected to cycle as part of normal operation, like a pressuretrol or vaporstat, can break the W, turning the burner off without effecting the heat call. Fewer callbacks for working-as-intended and still more or less a Big Deal if something else goes wrong.

    Post a pic of the xfrmr, & the schematic if you can.

  • pupmans
    pupmans Member Posts: 5
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    OK, finished my tracing. I have attached a schematic and photo. Arrows on schematic aren't meaningful given this is AC...Key differences from factory schematic:

    1) My transformer has 5 terminals, not 4- but only 3 are in use. And the W terminal is broken off with nothing attached, whereas in factory schematic it seems to represent a control loop.

    2) My control devices are downstream of the LWCO Common terminal, not the burn terminal...burn goes back to the Y terminal on the transformer and from there to the gas valve. In factory schmatic they are in a loop off the W terminal to the Y terminal of the transformer.

    3) My LWCO is not jumped from Hot to Common (instead, the common is getting the hot power after it goes thru the safety controls). I think in this instance the LWCO Common isn't really common...it's just the hot side of a SPDT switch that goes to burn or feed.

    I cannot imagine why it was done this way, except maybe someone broke the W terminal on the transformer and hacked together another way.

    I can imagine why my water feeder wasn't working; I'd think you'd have to have a call for heat to get power to that common.

    But I can't figure out how I below up 2 thermostats by jumping from H to C on the LWCO....C was getting current from elsewhere but it seems valid.

    Schematic as wired:


    Transformer Pic:




  • ratio
    ratio Member Posts: 3,702
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    Ok, the extra terminals on the transformer are just places to land wires, nothing special. According to the schematic you drew, the power is going from the transformer, through the thermostat & safeties, then to the LWCO 'COM' terminal. With that configuration, the LWCO 'COM' terminal will only see power when all the safeties and the thermostat are closed. Currently, the feeder will only feed when there's a call for the burner.

    To correct, move the wire from the xfrmr R terminal from the thermostat to the pressuretrol, where the wire from the stat is landed now. That will leave the thermostat unconnected. Use the thermostat red & white to replace the jumper from the LWCO terminal 5 to the xfrmr terminal Y (red to 5, white to Y). That will supply continuous power to the LWCO contacts so the feed valve will operate.

  • pupmans
    pupmans Member Posts: 5
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    @ratio I implemented as you described and things worked, mostly..burner fires, safeties work, feeder works when LWCO strikes. Thank you!!!

    Now I think I need to make an adjustment to get my thermostat working perfectly.

    So I removed my light switch and put back a Ecobee and on a call for heat, I got fire eventually but the gas valve was clicking. On call to Ecobee support they stated that if I use an external transformer (which I was) I might need to put in a relay for isolation. Not having a relay handy but now being VERY familiar with my boiler wiring I took a spare line and made a common to the stat. So far, so good...gas valve acts normal.

    Then I go to test the LWCO and feeder again and I realize that, since I no longer have an external transformer, any LWCO or pressuretrol activation will shut down the thermostat. Given it’s electronic and has to boot and so forth, I’m thinking this is problematic, since in my oversized boiler life, cycling on pressure is a thing.

    So now I’m trying to decide, should I

    1) put back the external transformer and a relay or
    2) Wire the stat behind the LWCO but in front of the pressuretrol
    Or
    3) Isolate the stat with a relay and keep the boiler transformer as the power source OR
    4) Put the stat in front of the LWCO?

    I’m not entirely sure if powering the LWCO on and off on calls for heat is problematic.
  • ratio
    ratio Member Posts: 3,702
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    If you implemented the changes I suggested exactly (and I didn't screw anything up!), you're good to run on the boiler xfrmr—no extra xfrmr needed. I'll describe it connection to connection: tie all the commons together (LWCO, feed valve, gas valve, ignition controller, thermostat) on the C terminal of the xfrmr. Take the R from the xfrmr to the hot on the LWCO, feed valve, & first safety. (The order of the safeties doesn't matter.) Go from the first safety to the second safety (then third, etc.), then from the last safety to the COM terminal of the feed valve. This way, any safety opening will kill power to the thermostat, & perhaps generate an alert from a smart thermostat. From the 'feed' terminal on the LWCO to the feed valve, from the 'burn' terminal on the LWCO to the R terminal on the thermostat. From the W terminal on the thermostat to the Y terminal on the xfrmr. Looks like the gas valve might be connected to the Y terminal already?
  • pupmans
    pupmans Member Posts: 5
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    @ratio I did implement as you described. AndI’m just running on the boiler transformer now. The issue I have is that in my system, cycling on pressure is A thing and is SOP. The boiler is for 2x EDR compared to my actual radiation. I would like to avoid cutting power to the stat when cycling on pressure as that happens all the time.

    Cutting power to the stat for LWCO is ok, and I can set an alert (that shouldn’t happen often- my reason for adding the feeder is this winter I won’t be in to check weekly...but when checking weekly I never hit the LWCO. I could also set an alert from the feeder.

    So my question is, is it better to put the stat ahead of the LWCO (and have no power to the LWCO when not calling for heat) or behind the LWCO but in front of the pressuretrol? Or, I could put back an external transformer and a relay, but that seems complex relative to the benefit.