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I couldn't find an answer to this question using Google. I think it's because I am not sure how to ask the question properly. I am not a plumber but I have seen in my new construction house with cpvc and I have seen in the past the same thing done with copper pipe.

When a water line comes out of a wall, just before it comes out there seems to be an area where the pipe goes up a little more and is then capped off. What is the reason for this? Why not just bend 90 degrees out to the source?

Is this necessary with PEX tubing too?

example from my house

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    Just curious: where are you located? (I ask because I'm interested in local variations on code interpretation/compliance.) – Aloysius Defenestrate Dec 22 '17 at 20:01
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    @AloysiusDefenestrate I am in Illinois about 60miles south of Chicago. – eaglei22 Dec 22 '17 at 20:03
  • Thanks. I've been busted in the past with arrestors similar in design to this. (Bay Area, California...) – Aloysius Defenestrate Dec 22 '17 at 21:05
  • That is strange I learned to do them in the north bay area (sonoma county) but that was in the 70' s. – Ed Beal Dec 23 '17 at 16:38
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Sometimes we put in up stubs, they fill with air and act as a surge arrestor (helps to reduce pipe chatter).

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    Oh okay, so there is always air trapped in that stub up area, the air never finds its way out the exit point? – eaglei22 Dec 22 '17 at 19:51
  • It is capped no place for the air to go. – Ed Beal Dec 22 '17 at 19:55
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    The air slowly dissolves into the water. Once in a while, as you notice water hammer, you should drain the whole supply system to refill the air reservoirs. Fancier water hammer arrestors cost more and try to prevent this, but they add complexity (which can fail), while this (classic) type is cheap and simple, so long as you drain the pipes once in a while (might be yearly, depending on various factors.) – Ecnerwal Dec 22 '17 at 20:29
  • @Ecnerwal thank you. Actually in one bathroom I do sometimes here a subtle knocking once in a while, maybe this will help. – eaglei22 Dec 22 '17 at 20:34
  • Real water hammer arrestors are required by code, at least in some states (Not just empty stub filled with air but an actual device on top). – Steve M Jan 21 '18 at 16:45

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