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flooding air vent

so, i have finished a boiler change out and the system works great except for one thing, and I am not even sure that this was even part of the orginal system, I have a dry return coming from the unit above the boiler room and then it drops into a steam trap, a Hoffman 8c, why I can't figure out, and then into a tee with a Hoffman 75 air vent before dropping into the wet return lines. This air vent floods as the boiler approaches pressure cut out. All wet return lines have swing checks on them between the air vents and and the hartford loop. This air vent is at least 5 feet above the water line on the boiler, none of the other air vents are flooding, and I have the pressure as low as the pressuretrol will allow, basically, cut in at 1/2 psi, differential at 1 psi. Is this all about pressure and height? I have this vent as high as I can possibly go without cutting the ceiling and going into the joist space, which I will do if that would get me high enough, but I am not sure it will. Will a vaporstat fix this problem. I have to fix this problem. The boiler height was determined by the wet return and the need for the Hartford loop tie in to be at least 2 inches below the water line. Any advice or suggestions would be greatly appreciated. Thanks Mike Miller


  • Jamie Hall
    Jamie Hall Member Posts: 22,924
    Five feet

    is only 2 psi.  Couldn't tell you why the steam trap is on that line -- unless it's the only one, in which case you probably need it but it's just in a slightly odd position.  It's a guess, but I would be inclined to guess that your pressuretrol is cutting out around 2 psi -- they are slightly inaccurate at low pressures -- and that that may be much of the problem.

    Somebody probably put the checks on the other lines for the same reason -- too high a pressure flooding the vents and the returns.

    When in doubt... crank it down!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • lutorm
    lutorm Member Posts: 78

    Is it possible the return line is clogged? That way the condensate would back up.

    The absolute pressure really shouldn't affect the water column, since both sides of the wet return are pressurized. It's just the pressure drop through the mains that has to be counteracted by the water column. Or have I misunderstood completely?

  • Jamie Hall
    Jamie Hall Member Posts: 22,924
    edited December 2009

    you should have boiler pressure on the boiler end of a wet return.  However, the drips and all coming into the other places of the wet return from the dry returns or radiators should, if the system is vented properly, be at or close to atmospheric.  Hence the need for a minimum distance between the water line and the dry returns!  Note, though, that if the steam mains drip into the wet return, they will have steam pressure on them -- and the water level in those drips will be about the same as the boiler.  Which is why the wet return must be significantly below the boiler water level.  And why the dry returns must be significantly above it!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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