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Low Water Cutoff switch

JohnA Member Posts: 7
Despite my changing the water each week, the switch inside my low water cutoff ends up getting stuck about once a year. My oil company's plumber usually just shoots something like a rust remover into the low water cutoff chamber. What kind of rust remover spray is safe to use? Also, what is the white sealant he puts around the screw that provides access to this cutoff chamber?


  • Rod
    Rod Posts: 2,067
    edited April 2011
    Low Water Cutoffs Save Lives!

    Hi-  Below are  the maintenance recommendations for the McDonnell & Miller Series 67 & 767 Lo Water Cutoff which  is commonly used on a lot residential steam boilers. 

    The Low Water Cutoff is an critically IMPORTANT SAFETY DEVICE so shouldn't be "mickey mouse" fixed with a spray can of  "rust remover"!!! 

    Find somebody competent to inspect and replace the defect parts or completely replace the whole unit!

    - Rod



    • Blow down weekly (at least once) when the

    boiler is in operation.

    • Disassemble and inspect annually. Replace

    the low water cut-off if it is worn, corroded,

    or if components no longer operate properly.

    • Inspect the float chamber and equalizing

    piping annually. Remove all sediment

    and debris.

    • Replace every 10 years. More frequent replacement

    may be required when severe conditions

    exist such as rapid switch cycling, surging water

    levels, and use of water treatment chemicals
  • JohnA
    JohnA Member Posts: 7
    edited April 2011
    More reliable lower water cutoff

    Is there a good alternative to the 67/767 Series? How about a system with some kind of probe that detects the presence of water at a certain level, instead of a float that seems prone to malfunction at a fairly regular interval and requires weekly attention by the consumer?
  • Rod
    Rod Posts: 2,067
    edited April 2011

    Hi -  There are basically two types of Low Water Cutoffs. One is the float type like the Mc Donald & Miller - Type  67 and the other is the probe type, like the series made by Hydrolevel.  Both types require regular yearly cleaning & maintenance. The float type also requires a weekly blow down to make sure the float area remains clear of dirt and debris.   Some owners prefer the probe type as it doesn’t require weekly blow downs though on the other hand , a benefit of having to do the blow downs makes sure you check your boiler at least weekly which you should do in any case. There is also a school of thought that from a safety stand point, it ‘s not a bad idea to have both types installed on your boiler. (Sort of like wearing a belt with suspenders.)

        To install a probe, there must be a dedicated  threaded  port in the boiler at the appropriate location.  On most late model boilers this is available but may not be on older models.

        If you already have an automatic water feeder on your boiler, you have to be sure it is compatible functionally and electronically with the low water cutoff. 

    You can get Low Water Cutoffs (both the M&M Type  67 and Hydrolevel ) at Pex Supply on the internet. (They also have Type 67 replacement parts)  Info  on  Hydrolevel:  http://www.hydrolevel.com/   You probably want to get  a good pro to handle the installation and setup and they can do the yearly maintenance/cleaning  on the boiler/ burner at the same time.

    - Rod
  • JohnA
    JohnA Member Posts: 7

    Thanks, Rod. The advice/information is much appreciated. I do have a professional doing yearly maintenance, but I believe they consider the Low Water Cutoff to be separate from that maintenance and therefore requiring a plumber's attention. It seems that if the manufacturer recommends a complete cleaning each year then that's what my boiler/oil company should be doing.
    RDSTEAM Member Posts: 134
    too much fresh water

    you shouldnt change your water every week. you will cause more harm then good. blow down the LWCO once a week, drain and flush once a year. always remember to bring your fresh water to steaming temperature after you refill....
  • JohnA
    JohnA Member Posts: 7
    Question about draining vs. blowing down LWCO

    What is the difference between draining and blowing down the LWCO? Either way, you are draining some of the water from the boiler, correct? Should the LWCO be drained until the water stops flowing, or simply long enough for the dirtiest water to leave the LWCO? Thanks.
  • Jamie Hall
    Jamie Hall Member Posts: 21,092
    I expect...

    that there are as many ways to blow down a LWCO as there are people doing it.  But keep in mind why you are doing what your are doing: all you need to do is to get the gunk which settles in the float chamber out of the float chamber.  You're not trying to clean the boiler!  Therefore... the approach I personally use is to open to blowdown (boiler and water feeder OFF) until either it starts to flow in a bit of a spurt or until it is vaguely clear -- say a quart or two.  Close.  Repeat until the water is vaguely clear when the blowdown is first opened.  If this is done weekly (less on a reasonably clean system) the total will be a gallon or two -- rarely more.  Then open your manual feeder to bring the water level back up to where you want it (you did record it when you started, didn't you?), close the manual feeder and open the automatic feeder again and turn the boiler back on.

    (Why do I bother to turn the auto feeder off and restore the water level manually?  Because my autofeeder has a nice counter on it which tells me how much water the autofeeder has added because of losses -- and I very much want to know if I'm losing water I don't know about.)
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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