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Problem with Mercury Switch/Pressuretrol.. HELP!

First an admission. I'm far from an expert... just a property manager with a problem. In one of the buildings I manage, the steam boiler has the typical set up of a Pressuretrol (with a mercury) switch, a grey box that says "Cut in" and a high pressure emergency pressuretrol.

The problem I'm having is that if I set the mercury switch to cut out at under 5 lbs, the mercury tube doesn't come back far enough after the boiler turns off to allow the system to come back on again. That is, the mercury switch stays in the "left-side down" position instead of flopping back to the "right-side down".

The system works fine on just 2 lbs of pressure, so having it stay on until it makes 5 lbs is just a waste of fuel. After all, once the pipe gets to 200 degrees or so, it's not going to get much hotter.

I know I'm overlooking something, but I can't figure out how to solve this. BTW, the Pressuretrols are not quite level so at first I though that might have something to do with it, but it seems to me that leveling it perfectly will make it even harder for the mercury switch to flop back over (as it's tilted a bit toward the right looking at it face on.


  • Jamie Hall
    Jamie Hall Member Posts: 21,094
    A Pressuretrol

    also has a differential setting -- different ones have it set up slightly differently.  Some subtract from the cut out pressure to get the cut in pressure, but some -- and if yours is labelled "cut in" it is probably of this clan -- add the differential to get the cut out.  So... check the differential setting (it's a different scale) and see what it says.  Set the cut in as low as it will go, and set the differential for no more than 1 pound.  Then... and here's the fun part... when the boiler turns off (something for you to do tomorrow morning) you should be able to dial the cut in back up just enough to tip that mercury switch back over and you should be good to go.  Maybe tweak it up just a bit more, just to be sure.  And watch it for a cycle or two...

    The end result will be the thing will be set up to run at as low a pressure as you can get it to go -- and the pressuretrol being slightly off level won't make a bit of difference.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Lance
    Lance Member Posts: 243
    edited January 2010
    Other thoughts

    Mercury is no longer allowed, if you replace it you will also eliminate problems caused by mercury by incorrect placement on pigtails which move when heated and cooled. Always make sure the pigtail pipe will not rock the mercury back and forth. Good luck

    Ps. If the Mercury gets gray and contaminated, it will be sluggish and must be replaced.
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