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Cleaning a McDonnell Miller 150 on a small commercial boiler

04090 Member Posts: 142
This boiler has a recent habit of flooding.  When that happens, water flows from the horizontal copper pipe above the condensate tank into a slop sink not seen in the photo.

I'd like to take apart the McDonnell Miller 150, which controls the condensate pump, and clean out expected muck.  We've been good about flushing the device, but I don't think it's ever been cleaned out.  I was taught to rapidly pulse the drain valve open and closed to flush the device, but see online that might not be the right thing to do.

To clean it out I assume it's (1) disconnect power and the wires leading to the device, (2) remove the series of bolts holding the float assembly to the housing (2a) pray (3) remove the float and (4) gently scrub away.  Reasembly in reverse with a new gasket and shellac.

Is that it,any caveats or other things I should be looking at as possible causes of boiler flooding?


update:  It's been 24 hours and the boiler continues to flood.  Yesterday I opened the feed valve into the condensate tank momentarily in an attempt to flush it, and ran many buckets of water from the McDonnell flush valve.  Everything looks good... what else to do to control flooding?  Watching the water drip from the pipe that comes from the condensate tank to the slop sink shows it's a very slow but steady flow.  I turned the water supply off to the boiler.  The manual bypass water feed ball valve is not leaking.


update 2:  Thinking of replacing the 1156F and 9DSM3 assemblies which feed water into the condensate tank, it appears they work in harmony.  Is that wise?


<img src="http://i50.tinypic.com/2zegnrb.jpg" width="1200" height="1600" alt="" />


  • 04090
    04090 Member Posts: 142
    edited October 2012
    Getting there....

    Replaced the S1156F STD pressure regulator and a 9DSM3 backflow device.  Both were full of encrusted muck and very simple to replace, and the boiler is no longer flooding.


    Cleaning out the McDonnell Miller (LWCO and condensate pump switch) next; it hasn't been opened at least since 1999.  Have one gasket in hand, any caveats before digging in?  Is it done as thought in the first post?
  • icesailor
    icesailor Member Posts: 7,265

    Why do you have a Watts 1156F hot water boiler feeder feeding a steam boiler?

    I thought steam boilers were fed through the low water cut-off or some device with a float. And not depending on pressure.

    Unless you have a hydronic loop off the steam boiler and the hydronic is a closed loop from the steam boiler. In which case, do you have a leak from the hydronic into the steam boiler?

    Inquiring minds may be wondering
  • 04090
    04090 Member Posts: 142
    For an inquiring mind, one to another

    The S1156 STD feeds water into the steam boilers condensate tank.  It maintains a water level about 2/3 full on the glass gauge


    From there, the boiler water level is maintained by a switch in the MM LWCO which controls the condensation pump.

    There's a single gate check valve between the boiler and the condensation pump.  I replaced that because it was leaking boiler water into the condensation tank a few years back and wracking havoc with the system. 

    I don't understand the dynamics of that myself, but it works. 

    Although I know it's not recommended, I let the system cool down and totally drain and flush the boiler (and the lines leading to the red Everhot tankless) of muck annually with a pump.  And after that I bring the water to a boil and let it run awhile.  I use the drain nut under the glass gauge and try repeatedly to fill and drain the tube so it's clear.  There's an enormous amount of mud and muck in that system.  I had the condensate pump rebuilt a few years ago after a bearing failed and while it was off I scraped and flushed a heap of mud out of the tank. 

    Can't wait to see what's around the float, and someday I'd like to see what's inside the Everhot.

    It's maintained annually by the oil company, but they won't touch anything plumbing related on their seasonal overhauls and wern't too swift once diagnosing a water related issue.  That's when I took it on myself.
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