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Knocking Radiator in 120 yr. Old House

We put new hardwood flooring down last winter and reattached 2 radiators once the job was complete. One works just fine the other knocks. It never knocked before, in the 15yrs. I've owned this house. My plumber & I went thru the entire house to make sure all the air valves were zone accurate. We also pitched the knocking radiator, which made the knocking worse.

On my plumbers' suggestion, I contacted a company listing themselves as Boiler Specialists. They checked everything I had already checked and had no new suggestions. They said the knock wasn't that loud and I should learn to live with it!  It's loud. We can hear it throughout the entire house. It wakes all of us up each morning at 6am sharp.

BTW-The boiler runs great. It's not low on water & I even put a cleaner in it a few months ago.

Any ideas?


  • BobC
    BobC Member Posts: 5,306
    Pipe slope?

    If new hardwood flooring was laid I assume the radiators are now 3/4" higher than they were before. Usually that doesn't cause a problem because it just increases the slope of the piping a bit but I would check all the piping I can get to for proper slope. Don't trust your eyes, use a level.

    I suspect you have water trapped somewhere and that is causing the steam to collapse. Does the sound occur at the start, the middle, or the end of the heating cycle?

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • Lafayette
    Lafayette Member Posts: 7
    Knocking Radiator

    It knocks for apprx 5 mins non-stop each morning with the first blast of heat at 6am. During the day, there are only intermittent, faint knocks.
  • JStar
    JStar Member Posts: 2,752

    Definitely a newly mis-pitched pipe.

    Where are you located? We can suggest a real steam specialist that would NEVER recommend "living with it"!
  • New floor

    Could the pipe be rubbing against the new floor boards as it expands?

    For the moment, I would not program any setback in the thermostat. Setbacks will not save much if any fuel, and in this case are exaggerating the knock.

    I presume you have checked everything else, such as pressure, main venting, in order to minimize wet steam production?--NBC
  • Lafayette
    Lafayette Member Posts: 7
    Live With It ??

    I live in Clinton Hill, Brooklyn. I'm a little leary to have yet another person come to check on this problem. I'm currently unemployed (company closed) and can't keep paying for non-existent results. Hoping to resolve this situation myself.
  • JStar
    JStar Member Posts: 2,752

    Well, my first step would be to disconnect the radiator and see how and where the valve settles. That could lead you in the right direction.
  • Lafayette
    Lafayette Member Posts: 7
    New Floor

    Yes, the main vent & pressure are all in proper working order and the knocking is definitely a metal knock, not wood, so no interference with the new flooring. Thanx for the ideas. Everything helps!
  • Lafayette
    Lafayette Member Posts: 7

    Hate to go back to that, but I think you're right. Just in case, can you email your contact info. Maybe the new year will have a job waiting for me & I'll be able to call that specialist afterall.

    Thanks for your help.
  • Steve Whitbeck
    Steve Whitbeck Member Posts: 669

    Can you lift the pipe end of the radiator without any more effort than the weight of the radiator? Lift carefully...

    If you can then raise the radiator on one inch shims at ALL four corners to see if the knock goes away.

    If this doesn't work then it could be sludge build up in the pipe that is slowing the return of the condensed water back to the boiler. The noise is from steam hitting standing water in the pipe or radiator.

    Did they replace the riser pipes coming through the floor when they put the radiators back? If they did they installled one that was too long.
  • Lafayette
    Lafayette Member Posts: 7

    The risers weren't touched at all. I don't think we have an inch to lift the rad, but I'll get my plumber back over here to give it a try. Thanks.
  • Jamie Hall
    Jamie Hall Member Posts: 18,924
    Amost always...

    (but not always!) a knocking -- particularly a fairly heavy knocking -- early in a cycle after a setback, when the system has been cool for a while, is water trapped in a runout.  You don't mention anywhere whether this is one pipe or two pipe; not that it makes a really big difference.

    What makes it a little tricky is that the knocking, though heard at the radiator, may be from at least a little distance down the pipe.  What I would suggest is first, if this is one pipe steam, make sure that the radiator valve is fully open (two pipe it doesn't matter).  Then go back along the pipes from the radiator until you get to a steam main and make sure that every single section of pipe is pitched properly (for example, suppose there is a longish runout in the basement.  You raised one end of that when you did the floor.  Is that runout fixed in the middle on something, and when you raised one end did you get a sag in the other end?  That would do it -- it doesn't take much).

    Then come back if that doesn't help...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
This discussion has been closed.