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why does the water level drop when burner shuts off

smcsmc Member Posts: 11
looking at a steam boiler approx 5 yr old 500,000 input. .5 cut in plus 1# takes 50 minutes at least for condensate to return (may need feeder tends to overfill). not sure why when burner cycles off on psi water level drops in glass. any ideas? system seems clean not set up to skim easy



  • dropping water level

    are there any main vents on the dry returns to relieve any vacuum left when the burner cuts off? they are also needed to let the air out without any resistance. are the vents hissing as it starts to steam? when you flush the lwco, just after cut-off, does it suck air?--nbc
  • Dan HolohanDan Holohan Moderator Posts: 11,877
    This happens with boilers that have narrow sections.

    When the boiler is making steam, the water in the boiler can sit higher than the water in the gauge glass because of the tight quarters and the rising steam. When the burner shuts off, the steam in the boiler water collapses, causing the water in the boiler to fall. That leaves the water in the gauge glass a bit higher than what's in the boiler, so it falls to compensate, and then bounces back, but at a lower level.

    One of the things that made a steam boiler a steam boiler in the old days was the width of the section. Higher efficiency standards have taken a lot of the water and metal out of modern steam boilers. Check to make sure you're not overfiring. That will make it worse.
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  • smcsmc Member Posts: 11
    water level

    thanks will check gas psi. scott
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 2,470
    Boiler Water Level

    If a system is large in comparison to the boiler, (or if the boiler is under a heavy load) I have seen boilers shut off on pressure (or have the burner cut from high fire back to low fire) and due to the load the steam leaves the boiler quickly

     Because the boiler water has been heated to the corresponding higher pressure, the remaining water flashes into steam and causes the water level to drop. Also causes an unstable water line due to the high velocity of the steam leaving the boiler.

    I'm sure that's as clear as mud.

  • Dropping waterline

    How far is the waterline dropping? If it drops so far as to trigger the lwco, then the slow returns have created a problem, and will have to be flushed or replaced.

    If the drop is a couple of inches, then it is due, as Dan says to the collapsing steam bubbles reducing the apparent water volume in the boiler. To further illustrate what he has said, in my own 1,050,000 btu boiler, the volume of water needed to raise the waterline from the lwco-cut-off level to normal is only about 3 gallons, which shows how narrow the waterways in the sections are.--NBC
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