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Thread: Choosing a water hammer arrester

  1. #1
    DIY Member lahabra's Avatar
    Join Date
    Dec 2007

    Smile Choosing a water hammer arrester

    I went to Loes, they have 2 types

    1. Air pocket 3/4" x 10”…………….$7.00
    2. Piston 1/2" x 6” …………..…….$ 10.00

    I am in favor of the big one because it is simple. I have read on various posts that the air bulb would only work for a short period of time, presumably because the turbulence would push the air out. I don’t subscribe to this thinking. I estimate that when pressurized the pocket would be compressed into the top 1/4 of the chamber. I find it hard to believe that the air would travel 8” down to escape. Also if air bubbles were in the water they could collect in the bulbs and improve there effectiveness.

    I don’t like the piston design because I think the piston could leak and render the device un-effective.

    Am I seeing this wrong?

  2. #2
    Forum Admin, Expert Plumber Terry's Avatar
    Join Date
    Aug 2004
    Bothell, Washington
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    You are seeing it wrong.

    Air chambers will water log very quickly.
    Hammer arrestors are used now, to prevent water logging.

    The one on the left is for a washer.
    The one in the center can be soldered onto a tee.
    The one on the right can be used inline like for a lav supply or an icemaker line.

    The fallacy of air chambers
    It used to be thought that an air chamber, or capped stand-pipe, was an effective solution to controlling water hammer. However, within an air chamber, nothing separates the air from the water. It only takes a few short weeks before the air is absorbed into the water, leaving the air chamber waterlogged and completely ineffective. Laboratory tests confirm that the air is depleted by simple air permeation and by interaction between static pressure and flow pressure.
    Last edited by Terry; 04-08-2009 at 11:25 PM.

  3. #3


    If you’re just going to use an air chamber then just get some pipe a T and an end cap and make your own. The longer the chamber the better you will be. In a few weeks when it shows up again you will not have wasted a lot of money. But if you don’t mind draining your pipes all the way down every few weeks then go for it.

    You should only need 2 for the washer, one for each WC, and one for the dishwasher.

    You are right about the piston leaking; but they leak because someone installed the wrong size for the water supply and presser.


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