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generic question about knocking radiators

I'm a homeowner and I've been given two stories about knocking radiators and I'd like to know which is true (or if both are true).

Basically, I'm trying to find out if my boiler is sending live steam or not as well as figuring out the following:

FIRST STORY: knocking occurs only with live steam and it happens because when the down-running condensate hits the up-going live steam, some of the condensate turns to steam and thus there is over-pressure in the pipe and thus knocking.  This makes sense to me, but then I was told the second story:

SECOND STORY: knocking occurs in ANY radiator (live steam or not) when the condensate pools and then has to be knocked free by the steam pressure, so knocking can be fixed by tilting the radiator to avoid pooling (I have seen knocking decreased significantly by tilting a radiator, so I know there is truth to the last part of this)

Thanks for any help.


  • JStarJStar Member Posts: 2,668

    Water hammer occurs when steam meets water where water doesn't belong. Think of the system as a bad neighborhood, and condensate is a very well dressed, young woman. She needs to leave town and get home before the steam gang can catch her.
    - Joe Starosielec
  • Jamie HallJamie Hall Member Posts: 6,869
    Sorry, but the first yarn

    just isn't so... Be a nice way to get more steam, though, if it worked.

    The second yarn is more or less correct, as is (no surprise), JStar's comment (never thought of condensate that way, I have to admit...).  What happens is that steam is moving in the pipe at a pretty good velocity.  If there is also condensate in the pipe -- whether from the leading edge of the steam or left over, say in a low spot, that high velocity steam will pick up a slug of that water and push it along.  Also at a considerable velocity.  When that slug of water hits a bend, as it might be an elbow, it will create a bang or water hammer -- which can be remarkably loud.  The solution is, of course, to make sure that the water never can pool enough to get picked up as a slug and pushed along.  This means large enough pipes, so that if the condensate is trying to flow one way and the steam the other the condensate can lay low on the bottom of the pipe, out of the way of the steam, and enough pitch so that the condensate can flow along in a very shallow layer.  And avoidance of any sags or dips where pooling might occur.

    You also want to avoid introducing liquid water along with the steam ("wet steam") as might happen from inadequate near boiler piping.

    Simple enough to write down.  Not always quite so simple to put into practice...

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • RodRod Posts: 2,067
    Radiator "Knocking"

    Hi- The noise ("banging") in the radiator is caused by steam collapsing (condensing). When steam condenses to water, it instantaneously reduces its volume to 1/1700 (1 cubic FOOT of steam reduces to 1 cubic Inch of water!)  This creates a huge vacuum void and water on both sides of the void rushes in to fill the void. The banging you hear is the walls of water from both sides colliding with each outer.  Since pooled water (being below 212 F deg.) encourages the steam to collapse and having excessive water creates a condition where a bubble of steam can get trapped between two walls of water, it is beneficial to expedite the condensate (water) leaving the radiator. On a 1 pipe system radiator, the radiator should be slightly sloped towards the inlet steam pipe and it is also important the the valve on the steam pipe is FULLY open as this helps prevent the incoming steam and outgoing condensate colliding.

      As Jamie mentioned "Wet Steam" can be the culprit as the water droplets the steam encourage the steam to collapse. "Dry Steam" is far more efficient.

    - Rod
  • phindsphinds Member Posts: 1

    Very helpful info guys. Thanks

    (I'm the OP, but we had some problems w/ my account so the question no longer shows under my name)

  • RodRod Posts: 2,067
    Steam Collapsing Video

    Hi- Here's a good video of steam collapsing - "Water Hammer"

    Turn up the volume so you can hear the sound.

    - Rod
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