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I ordered up a pair of water hammer arresters for my washing machine and installed them. The house still shakes, rattles, and rolls when the washer pulses the water. I saved a bit of money by ordering restocks, since it seemed to me that a device with no moving parts except air molecules was a pretty safe bet. Was I wrong?

  • "no moving parts except air molecules" isn't quite right - there has to be something flexible/movable separating the water and the air, otherwise the air slowly gets absorbed by the water and carried out. – Zhentar Jan 19 '15 at 15:13
  • Where did you install them? – Steven Jan 19 '15 at 16:34
  • "In the laundry room" -- The laundry as a fitting with a gate-valve lever to shut off both temps and two 3/4" threaded connectors (one for each temp), I put one item on each. – bmargulies Jan 19 '15 at 18:07
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In my experience, these things just don't work. I used to rave about them, and tell everybody to use them to stop water hammer. I was wrong.

In theory they should work, but in practice they do not. Modern washing machines have fast acting solenoids, which can turn off the water very quickly. Water hammer arrestors designed to fit on washing machine supply lines, are simply too small to absorb enough of the energy to prevent water hammer.

Your best bet is to secure the plumbing better, and/or buy larger arrestors.

To answer your question directly... There's probably nothing wrong with the devices you've received, aside from being poorly designed and undersized.

  • They aren't poorly designed or undersized, just misunderstood. They are quite amply sized for the purpose of limiting water hammer to safe levels (less than 150psi), but if your plumbing isn't well secured it doesn't take very much to knock it around loudly. – Zhentar Jan 19 '15 at 19:49
  • @Zhentar I'd have to see some calculations to the contrary before I was convinced, since my experience has shown them to be ineffective. – Tester101 Jan 19 '15 at 21:11
  • The pressure increase = 0.0135 * water velocity * pipe length / valve closing time. Assuming 3gpm through 1/2" pipe, 100 ft straight pipe to the washer, and a very fast closing 0.25 second solenoid, and we get an 86psi pressure increase. If your flow pressure is 60psi, that comes to 146 psi and technically you don't need an arrestor at all (though you've got no margin of safety). Using those same parameters with the Sioux Chief water arrestor sizing estimate, and it recommends at 2 cubic inch air chamber - and the AA arrestors are about 2.5. – Zhentar Jan 20 '15 at 17:29

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