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I am replacing the shower rough valve for my tub/shower and the inlet lines were tee’d with a vertical section of pipe and capped off. Why was this done and do I need to do the same thing when I go to install my new one? I’m using PEX for the new valve if that makes a difference.

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That's an old-school water-hammer arrestor. The air bubble trapped in the extension serves to limit water hammer, at least until it all dissolves.

They work if you drain the pipes (to restore the bubbles) when they stop working, and have no mechanical parts to fail.

Modern hammer arrestors try to prevent the dissolving problem by using a piston to separate the water and air, but then have mechanical parts that can fail.

You can use hammer arrestors of either sort, or choose not to, though you may find yourself retrofitting them if water-hammer is a problem in your house without them.

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  • Any idea how long it takes for the air to dissolve into the water, or what affects it? Sounds weird at first, but I suppose water + air under pressure is almost the recipe to make carbonated water
    – Xen2050
    Commented Dec 24, 2023 at 6:23
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    @xen2050 With the small difference that carbonated water isn't made with air...
    – Gábor
    Commented Dec 24, 2023 at 12:43
  • Normal tap water is full of dissolved air, and will create a bubble in the water-hammer arrestor in normal operation. Unless the water is not full of air, and does not create the bubble. It depends on the water, not on the time.
    – david
    Commented Dec 24, 2023 at 21:39
  • PEX pipes are a tiny bit more flexible than copper pipes, so water hammer is less of a problem. Not no problem, but less.
    – vidarlo
    Commented Dec 25, 2023 at 1:14
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    @Xen2050 about 4-6 months in my house, at least for the toilet where I usually first notice the valve shutoff getting harsh. YMMV, of course.
    – Olivier
    Commented Dec 25, 2023 at 22:32

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