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low water cut off

bradley Member Posts: 18
I want to install a low water cut off.  The low voltage terminal board in the slant fin boiler that I have has terminals for a flow switch, wall thermostat, and outdoor thermostat. How and where do I wire in the low water cut off? 


  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    It may depend on local code.

    I am not a pro, but my impression, from what I read at this site,  is that even if your code does not require a LWCO, you should have one.

    My (80,000 BTU/hr) boiler says its controls are such that no LWCO is required in some jurisdictions. My contractor said one is required here, so they installed a M&M probe-type unit. My boiler has two terminals specifically for wiring a LWCO, but the contractor wired it in series with the main power into the boiler instead. So if low water is detected, everything goes off. I do not know how much of this is required by code, and how much is at the discression of the contractor. Inspectors approved it, whatever that proves.
  • bradley
    bradley Member Posts: 18

    A lwco does seem like a good idea to me also.  And I've talked to others who say its a good idea and then go ahead and plumb in feed water.

    Anyway, my boiler has no terminals for a lwco. I have ordered a 24v lwco.  I've been told to wire the lwco in series with the wall thermostat.  But this makes no sense to me because the thermostat only tells the circulator to come on.  The boiler will fire up according to and internal aquastat.  Would I not be better off to wire the lwco in series with the flow switch?   
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666

    ..., so please bear that in mind.

    If your boiler control does not have a 24-volt connection for the LWCO, I suggest you exchange it for a 120 volt LWCO and use it to cut off all power to your boiler. Another possibility might be to put the 24-volt contact in series with all the other alarm contacts that lock out the burner; the ones that require manual reset.

    Personally, I favor the 120 volt LWCO that turns everything off. Anything else is dependant on whatever it is hooked to working, and in the case of something this important, the more of it you can turn off, the better. Adding cold water to a dry fired boiler would make a big mess.
  • meplumber
    meplumber Member Posts: 678
    Slow down a minute.

    Is this a hydronic or steam boiler?

    If it is hydronic and I assume that it is because you are in the Radiant Forum, then you are looking for a LWCO that would kill power to the boiler in the event that the boiler loses water to prevent an explosion from the boiler firing on an empty vessel.

    If so then the LWCO should be plumbed either in the supply or return piping at least 6" above the crown sheet of the boiler and wired on the line voltage that supplies power to the boiler.  Wiring on the low voltage side is not allowed in most of the jurisdictions that I am familiar with.

    Others on the board may know of AHJ's that allow this but none that I am aware of.

    If this is steam then we have another procedure.
  • bradley
    bradley Member Posts: 18

    This is a hydronic radiant system.  I live in Ontario, Canada.  My boiler is an electric fired monitron slant fin EH-8.  The manufacturer rep told me that I could wire the LWCO on the low voltage side in series with the wall thermostat.  I just wanted some confirmation of this.  Not sure it should be wired with the wall thermostat, which in my boiler only tells the circulator to come on.  The flow switch allows the boiler to fire, this seems a more sensible place to add the LWCO.  I will plumb the LWCO high on the supply line.  I want a LWCO for the reason Dan states in his book:  "Protect your hydronics systems with low water cut-offs, not feed valves".
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    wire the LWCO on the low voltage side in series with the wall thermostat

    I do not know the codes, and they are surely different in different jurisdictions.

    My W-M Ultra 3 has a connector on the control board for connecting a 24-volt LWCO. They recommend using it. My contractor did not do this, but connected a 120 volt LWCO in series with the entire power to the boiler. Perhaps the code here allows the LWCO to be connected as recommended by the manufacturer and the contractor was unaware of the low voltage connection to the controller. But perhaps the code does not permit that connection.

    In my non-professional opinion, I would like the LWCO to turn off as much stuff as possible, so as not to rely on things that might fail, and since according to Murphy's Law if anything can possibly go wrong, it will, I prefer turning everything off. Wiring the LWCO in series with the thermostat seems a terrible idea. What if the part of the controller where the thermostat is wired is defective and the boiler thinks the thermostat is calling for heat even with that circuit opened by the LWCO? How much better it would be if instead of that, the LWCO cut off all power to the boiler. But even if the controller did not fail like that, what if the part where the LWCO goes fails? Another of Murphy's laws goes, if several things can go wrong, they will all go wrong, all at once.

    My LWCO is an M&M RB-122-E that can switch either 120 volt or 24 volt circuits, but the contractor took the 120 volt option.
  • meplumber
    meplumber Member Posts: 678
    Electric boiler makes all the difference.

    We don't see too many electric boilers in New England.  Expensive Hydro rates.

    Not really my area of expertise.

    If it is oil or gas, then I am with you.  On this thing I would be guessing.

    Good Luck.
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,314
    That location would make sense if

    you are using a low voltage LWCO. If you use a line voltage LWCO it would need to be rated for the amperage draw of your heating elements, I do not think they make such and animal and if they did it would cost more than your boiler. Do you have a flow switch? I always wondered who could afford to run an electric boiler. It would not be cost effective here in Massachusetts. The questions the wall answers amaze me. So on that note listen to the manufacturer
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • JharrisSeattle
    JharrisSeattle Member Posts: 26
    edited December 2010

    "Would I not be better off to wire the lwco in series with the flow switch? "

    Power the lwco with a seperate source of 24v, pull the jumper from the power source side and the burner circuit contact, and wire in the burner circuit contact of the lwco series with the flow switch. This will insure innteruption of the elements on a low water condition. Some may argue that you do not need a lwco if using a flow switch since you may not have flow in a low water condition anyways. But this could be added protection if the flow switch gets stuck in the closed position.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Belts AND Suspenders.....

    THe code in the US reads, that if the manufacturer wants to use a flow switch in lieu of a low water cut off it is acceptable to do so, but one or the other must be used. If you do both, then you have double protection which is a good thing, because I have seen flow switches stick in the closed position.

    As for wiring, follow the manufacturers recommendations, and if they don't say, then putting it into the safety string (flow switch) versus the comfort string (room thermostat) makes more sense.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
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