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Low Water Cutoff Failed?

Hey All!

Just fired up the boiler for a quick test prior to the actual heating season. I opened the valve on the low water cutoff while under steam pressure and it appears that the burner didn't shut off. Last year, I could have sworn it did. Is this a sign that the LWOC has failed? [per Fred, it's a McDonnell Miller #67 LWCO, thanks Fred :)].



  • JUGHNEJUGHNE Posts: 5,787Member
    Float may be sticking.
    How old is it?
    Maybe a picture?
  • pecmsgpecmsg Posts: 837Member
    Did water come out.
  • FredFred Posts: 7,920Member
    When was the LWCO last taken apart and cleaned? Shutdown may take 5 to 10 seconds after Opening the valve. The water level in the float chamber has to fall low enough to trip the switch.
  • pkwastavichpkwastavich Posts: 12Member
    @JUGHNE - Not sure how old it is. Here's a pic of the boiler from last year. Its not the focus, but you can still see it.

    @pecmsg - Yup, water did come out (and steam)

    @Fred - It was probably never taken apart or cleaned. Also last year I didn't realize that I was supposed to open it under steam pressure and had only been draining it while cold. I did just fill the boiler prior to opening it, so maybe the water level just isn't low enough?

  • pecmsgpecmsg Posts: 837Member
    > @pkwastavich said:
    > @JUGHNE - Not sure how old it is. Here's a pic of the boiler from last year. Its not the focus, but you can still see it.
    > @pecmsg - Yup, water did come out (and steam)
    > @Fred - It was probably never taken apart or cleaned. Also last year I didn't realize that I was supposed to open it under steam pressure and had only been draining it while cold. I did just fill the boiler prior to opening it, so maybe the water level just isn't low enough?
    > Thanks,
    > Pete

    After 5 or 10 secs it should have shut down.
    Time to get it rebuilt or replaced!
  • FredFred Posts: 7,920Member
    It probably needs to be taken apart and the float chamber scraped out. If it has a lot of crud in it, it can build up enough that it won't let the float drop down enough to trip the cut-off switch or the float can actually hang up on the side walls of the chamber. It is not a hard job to clean them out but you do need to buy a set of gaskets to put it back together.
  • pkwastavichpkwastavich Posts: 12Member
    I'll let it open for a little while longer and see how that works. I had only left it open for ~3 sec before steam came out, then shut it. Last year it shut off pretty much immediately though. If that fails Ill see if I can clean it.

    Thanks All! :)
  • nicholas bonham-carternicholas bonham-carter Posts: 7,865Member
    Check the LWCO outlet pipe for any restriction which might prevent the float chamber from draining quickly enough to drop the float.
    I was replacing the 4 feet of iron pipe over to the bucket every season, until I tried Steamaster tablets. Puting a ball valve on the end drop of pipe and leaving the #67 valve open prevents the rust particles from drying up and sticking together clogging the pipe.—NBC
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,921Member

    The control has to work and you don't have to have steam up. It will work with cold water and hot water . It has to.

    Keep in mind that McDonnell Miller recommends replacement after 10 years.

    Keep in mind that the low water cutoff is the second most important control on the boiler after the safety valve.

    You don't want to take a chance and loose the boiler so replace or clean's you choice but the control has to be 100%
  • pkwastavichpkwastavich Posts: 12Member
    Just left it open for 10 sec w/ the burner on. Still didn't shut off. Since I don't know how old it is I'm going to assume it's > 10 years and replace it.

    Thanks all for your help!
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,921Member
    You can always rebuild the old one for a spare. I know the controls are$$$ but cheaper than a boiler.

    Make sure the interconnected piping is clear
  • pkwastavichpkwastavich Posts: 12Member
    Figured I would keep it around as a spare.

    Anyone happen to have a video on removing one of these things? Having a heck of a time finding one. Can't seem to figure out what comes off first.
  • nicholas bonham-carternicholas bonham-carter Posts: 7,865Member
    How about posting a picture of your LWCO, and maybe we can mark it for you.--NBC
  • pkwastavichpkwastavich Posts: 12Member
    Of course! See below!

    I've made a bit of progress since the last post. Roughly in order I've:

    1. Turned off the gas to the boiler
    2. Set switch on pilot light to off
    3. Flipped main power on boiler off
    4. Drained boiler via old LWCO
    5. Removed sight glass [which this video helped with
    6. Removed wiring from old LWCO
    7. Loosened the compression nut on top of the LWCO

    From here I'm stuck. I think I need to twist the top pipe clockwise to remove the tube from the top of the LWCO, but I don't see where to stick a wrench. The top and bottom of the T are flat, so maybe there?

    No idea how to get the bottom one out though. There is a large nut there, and I can get a wrench on it & turn it, but it turns in the T instead of the LWCO side.


    Bonus Question:
    The new LWCO is reversed compared to the old one. Water inlet is on the left, electrical on the right. Think it's safe to remove the valve on the bottom and flip it around? I'd hate to kill the new one. If not, I think I have enough clearance to just mount it backwards.

    Thank you,
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,921Member
    loosen the nut holding the brass tubing to the upper tee, take it all the way out. Loosen the compression nut on the bottom low water connection on the tubing all the way and slide it up the tubing. Grab the brass tubing and wiggle it out. It will be a little difficult.

    Once that is done you can spin the low water cutoff off the nipple at the lower connection. If it hits the top connection you will have to take the upper tee off.

    yes, take the drain valve off and spin it around
  • FredFred Posts: 7,920Member
    All you need to do is loosen the nut on the top of the LWCO vent tube. Put a wrench on the coupling nut on the bottom Tee and another smaller wrench on the smaller hex of that fitting (on the Tee side) to keep it from turning and loosen that large nut. Then you can lift the LWCO away. Yes, you can reverse the blow-off valve so that it faces out. Clean out the upper and lower Tee before mounting your new LWCO.
  • pkwastavichpkwastavich Posts: 12Member
    edited September 7
    Hey All,

    Good news - I got it off! Turned out to be fairly easy. Just needed 2 wrenches as Fred said.

    I also got the blow off valve flipped around. Thankfully that was pretty easy too.

    One more question. The coupling nut seemed to have some sort of gasket inside [edit, or maybe this is a union? idk my pipe terms]. Hope that's the right word. It has some grey grease on it. I imagine I want to clean off the old grease and put on new. What sort of grease do I use? I have some rtv silicon, but idk if that will work.

    Some more pics

    Thanks again all. You've all been super helpful!
  • FredFred Posts: 7,920Member
    @pkwastavich , That coupling doesn't need any grease on it. Someone probably lubricated the treads just to make it easier to turn. It may even be "Never Seize" to make for easier removal.
  • pkwastavichpkwastavich Posts: 12Member
    Ahh, glad I didn't put anything on it then :)

    Was able to get the whole thing back together and started up. No leaks as far as I can tell.

    Started up the boiler, opened the LWCO valve & sure enough the boiler shuts off! Yay!

    Worst part of this whole thing was wiring the new LWCO. Getting the wires back on was a total PITA. Wife heard me yelling at one point and had to come down and make sure I was ok, lol.

    Thank you all for your help! Much appreciated!

  • SteamheadSteamhead Posts: 13,132Member
    That looks like an old "snowman" boiler.................
    All Steamed Up, Inc.
    "Reducing our country's energy consumption, one system at a time"
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Baltimore, MD (USA) and consulting anywhere.
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,921Member
    @Steamhead , and the "snow" is bad stuff, we all wish it would melt and go away
  • JUGHNEJUGHNE Posts: 5,787Member
    The "Snowman" was in a U tube video the OP posted.
    There are pictures of his (modern) boiler posted.
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