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Low-water cutoff triggering when water level is nearly at top of glass

jmortara Member Posts: 1
I had a new gas boiler installed when I bought the house in 2011. The low-water cutoff and refiller are from that date, I believe. 3-story townhouse, 2000 sq ft, about 8 or 9 radiators in our 1-pipe steam system. I have the boiler serviced at the beginning and end of each heating season by the same plumber who installed it.

The boiler is directly under our room, and I can hear the refiller being triggered, even though I've set the water level myself after flushing—to the manufacturer's specs at the halfway point.

If I drain the boiler with the power on, I can see the refiller triggering when the level is about 1 inch above the bottom of the glass. But I'd like to know why I can hear the refiller triggering when the water level is already toward the top of the glass. I think this is part of the cause of audible surging in the near-boiler piping. Would there be any legitimate reason a refiller would be triggered when the water level is already high? Within 1-2 days of setting the water at halfway, I have a flooded boiler every week.

Thanks for any help!

Jason in Brooklyn


  • JohnNY
    JohnNY Member Posts: 3,230
    Could be a bad low water cut off or probe. There are other very techy reasons like water quality issues that interfere with the conductivity of the condensate, but more likely, it's your 10-year-old probe or circuit board.
    What does your plumber say?
    Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
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    EBEBRATT-Ed Member Posts: 15,529

    Likely that the gauge glass and valves need to be cleaned removed and cleaned. And the low water cutoff as well.

    How long since the boiler was serviced? It's just like a car, needs service
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    My guess is a clogged pigtail, below the pressuretrol, which is allowing the pressure to rise beyond the1.5 psi max. This high pressure can push the boiler water up into the returns, starving the boiler of water, each time the boiler fires.
    When the burner is cut off, the water returns, creating an overfill.
    Pictures of the return piping would help, as would an accurate low pressure gauge (0-3psi).—NBC
  • ethicalpaul
    ethicalpaul Member Posts: 5,702
    But I'd like to know why I can hear the refiller triggering when the water level is already toward the top of the glass.

    It’s not clear to me if you’ve seen it trigger when the water level is high, or if you are just assuming it’s high when it triggers.

    it’s common for poor piping or water issues to cause boiler water to be carried to the main, making the level drop during a call for heat.

    watch some calls for heat to see what’s happening 
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
  • jmortara
    jmortara Member Posts: 1
    Thanks for the quick replies! I really appreciate that.

    As I mentioned the boiler is serviced twice a year. During one of these, I think in the fall, my plumber takes various parts out and cleans them.

    I haven seen this occur once while flushing the boiler multiple times. I had filled it up manually again almost to the top of the sight glass, and the refiller kicked in just for a few seconds. I thought it was odd but was dealing with other things and didn't focus on it.

    We keep the temps around 70 at night and around 74 during the day. Normally I only hear this when just waking up in the morning, still in bed, when the thermostat kicks in on a cold day, so haven't seen it much. The boiler is directly below and the refiller makes a very distinctive sound, for a specific duration. It's also usually this hour that I hear the sloshing in the pipes after a few mins of the boiler doing its first firing of the day. I have timed the sound of the refiller and it triggers for just about 1 min, not quite exactly.

    When I let the cycle complete, and see that the thermostat is no longer calling for heat, I have gone downstairs to check, and the sight glass is over the top. If I drop the water level this all just happens again in the next day or two. The water in the glass might start a bit rusty but I flush it all out 3 times and can get relatively clear water in the glass. A friend of mine who is a buildings system engineer gave a blanket statement that the controls are designed to be replaced every 5 years—not sure if that makes sense to me but it's possible. The pressure gauge is 0-30psi—I'm aware that that's not a very accurate range when I assume the pressure should have been set to around 2psi (I read Dan's book). Maybe we'll replace that as well with one of those fine-tune ones with the ounce markings.

    I will ask my plumber what he thinks, but I want to have an informed understanding of what might be the issue. He's great, but knowing him, he might decide to replace parts without really doing a deep diagnosis, and that doesn't help me understand the system any better.
  • The Steam Whisperer
    The Steam Whisperer Member Posts: 1,215
    I'd check on all the above suggestions, but have seen another one too. The electronic water feeders we use will feed a small amount whenever the power it cycled to them. I think this tends to be a problem with Dunkirk boilers especially. Thier factory wiring is very poorly set up and can also easily lead to the boiler operating controls being bypassed if a more advanced thermostat is installed that requires continuous power. I brought his problem to them over a decade ago and I don't believe they ever made any changes.
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  • ethicalpaul
    ethicalpaul Member Posts: 5,702
    From your description (sloshing in the pipes), and from this far distance, I find it plausible that water is getting carried into your main. What you can do is some Friday night set the hold temperature on the thermostat to 70 and then when you wake up set it manually to 74, then go watch what happens during the call for heat. You'll see if the water level drops during this long call for heat and then if you send us pictures of the piping near the boiler we can let you know how things look.
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
  • Bronxtech
    Bronxtech Member Posts: 18
    You didn't mention if the water level was bouncing all over the place, which can cause feeder to fill. 
    Also like Johnny said probe could be mucked up, and I think you said water is dirty/muddy.
    Have plumber flush returns and boiler. Svce probe, put in chemical for water, gauge glass n washers also,  . Then see operation.