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Knocking pipes! Won't stop...

I have a single pipe steam system (set to  approx. 1.0) that has been functioning properly this season (tech came out to give it a fall tune-up). This morning the pipes were knocking so bad that it became mind-numbing and I turned the system off.

The site glass is now completely full and won't drain as I tried draing some water from the tank. When the furnace initially fires up, the outgoing pipe sounds like water is boiling in it instead of the boiler.

Any ideas / suggestions for remedy?

Thanks In Advance,



  • Chris M_2
    Chris M_2 Member Posts: 67
    Sounds like the boiler's flooded.

    Keep it powered off and drain that boiler until the water level can be seen in the site glass, about 3/4 the way up. Then run it again and see if the banging stops.  Also, don't walk away from the situation before you find out what caused that water level to go up above the site glass. If the boiler's filled too high, there's no headroom to make steam, and the boiling water gets bounced up into the mains and eventually the radiators. I'd get that tech back out there asap.
  • Ray  Binder
    Ray Binder Member Posts: 11
    knocking pipes

    Better check that sight glass and make sure there is no sludge in the either the valves or the nipples comming out of the boiler. If its clogged it could still show water even though the boiler is empty. I agree, it sounds like your boiler is overfilled. Got an automatic fill valve? is it working properly?
  • Scott Reid
    Scott Reid Member Posts: 6
    Problem solved

    I drained the system even more (probably 12 galllons in all) and the water level came down in the site glass and it seems to be working properly now.

    Thanks for the suggestion.
  • water level

    Keep an eye on the sight glass to make sure it doesn't creep back up. Many years ago I had the same thing happen and it turned out I didn't close the boiler fill valve fully and it was seeping water into the boiler, maybe the tech didn't close it tightly enough..

    If you have an automatic fill valve it might not be closing properly. Also if you have a tankless water heating coil it could be developing a leak.

This discussion has been closed.