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loud knocking sound

i have steam heat in my home. i hear a very loud knocking sound at times. i was told that i should bled the lines from time to time ibut  dont see this helping very much is there something else i should be doing or trying?


  • Jamie Hall
    Jamie Hall Member Posts: 18,679
    edited November 2010
    This really should be over on the steam heat board, but...

    what you are almost certainly hearing is classic water hammer.  There are a number of possible causes for this, depending on a number of possible things!

    Easiest thing to check: pressure.  On the boiler, you will find a pressuretrol (most likely) which has a scale on it.  That scale indicates the "cut in" pressure for the boiler, and should be set at about 0.5 (it measures in psi).  Insides the box there is another dial, with is the differential; it should be set at 1.0. 

    Second thing to check: water level.  It should be visible in a sight glass on the front of the boiler; typically in residential boilers it should be about half way up the glass, although anywhere from a quarter to three quarters is OK.

    Having given those two tips... we will need to know:

    Has it always done this, or is it new?  If it is new, when did it start?  What was changed just before it started?  Is the boiler new?  Are there two pipes to each radiator, or only one?  Are the steam pipes in the basement insulated?  Does the knocking sound happen only at the beginning of a cycle, in the middle, or at the end?  (or all the time, heaven forbid!).

    That's for starters... the more information we have, the more we can help! 

    I might add -- bleeding the lines won't do a thing, for or against.  And... it shouldn't, and needn't do that.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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