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When quickly closing a water tap, I've often noticed a loud "bang" sound - this doesn't occur if closing the tap slowly. What causes this banging sound to happen?

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  • No place in the pipe to absorb the pressure of rapidly moving water stopping very suddenly. This is typically solved by having a tee hanging off the line filled with air - the air is compressible and can absorb the shock of the water stopping.
    – CrossRoads
    Mar 5, 2020 at 20:03
  • youtube.com/watch?v=xoLmVFAFjn4 This maybe? Saw this video yesterday. Mar 5, 2020 at 20:11
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    Look up "water hammer"
    – MonkeyZeus
    Mar 5, 2020 at 20:21

1 Answer 1

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As Sheetmetal85 mentioned in the comments, you are hearing "Water Hammer". This is probably happening in one of two ways:

  • When shutting the water off quickly, the force of all the moving water is banging against your faucet causing a "thunk"
  • When shutting the water off quickly, the force of the moving water causes your pipes to shake causing some noise not directly next to the faucet (this could be due to unsupported pipes, some pipe straps that have worn down, really anything keeping pipes from remaining stationary)

Start by looking for something that could be shaking, including the sink itself, when the water is shut off quickly, and tying down whatever is doing the shaking. Other than that, the next best solution is a thing called a water hammer arrestor, which basically acts like a cushion to absorb the force of the water and prevent pipes from shaking.

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  • Worth noting that a water hammer arrestor can go anywhere convenient in your system. Often the easiest place is at a laundry sink. Mar 5, 2020 at 21:13
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    A thermal expansion tank can go anywhere in a system, but guidance from Plumbing & Drainage Institute indicates that an arrestor works best when located near the quick-closing valve. That makes sense: to absorb the energy of rushing water it needs to be in front of said water. (source: PM Engineer Magazine )
    – Greg Hill
    Mar 5, 2020 at 22:25
  • Thanks, @GregHill for the info and link. I learn something new every day. Mar 6, 2020 at 1:46
  • I see a lot of kitchen sinks with water hammer arrestors coming out of the top of the sink, in the place where you'd normally expect a spray hose to come out. I'm not sure that I "love to see it", but I've never heard water hammer from any of these sinks, so I guess it must work. Mar 6, 2020 at 4:04
  • @TungstenWizard It seems unusual that somebody would pay a premium to have a water hammer dressed up in chrome and installed above the sink. The device you've seen might actually be a dishwasher air gap, which prevents water going down the sink drain from backflowing into the dishwasher's tub.
    – Greg Hill
    Mar 6, 2020 at 17:21

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