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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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Niko
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1 Answers1

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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.

Ecnerwal
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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 Dec 24 '23 at 06:23
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    @xen2050 With the small difference that carbonated water isn't made with air... – Gábor Dec 24 '23 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 Dec 24 '23 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 Dec 25 '23 at 01:14
  • Apparently it's complicated. I found a link Solubility of Air in Water and it could be up to 20% air absorbed into water (O2 is more soluble than N2, not sure about CO2), and varies a LOT by temp & pressure. Deaeration is interesting, I guess you wouldn't want air bubbles (& vapor lock?) in steam systems, so "dearation" devices are common "where fresh water is supplied to the system... [or] on the hot sides of heat exchangers in heating distribution systems." – Xen2050 Dec 25 '23 at 02:21
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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 Dec 25 '23 at 22:32
  • @david it depends very much on the source of the water. Well water typically has little dissolved air, so in the days of non-bladder pressure tanks, snifter valves were commony used to keep the pressure tanks from losing their bubble (while the snifter valves worked - they are troublesome enough that bladder tanks to keep air and water separated have become standard.) – Ecnerwal Dec 26 '23 at 23:38