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Old 12-12-2012, 01:10 PM   #1
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Water hammer, expansion tank?


In my new house, I did some pipeline testing due to slow hot water flow in various places. This involved turning the mains on and off. One effect I noticed was that the CPVC pipes would buck violently and with a loud thud when I turned the mains water back on.

Had to help my parents with a valve replacement yesterday and noticed they have an expansion tank mounted horizontally (and perpendicular to the vertical mains run) just downstream from the mains valve. Not sure why.

Somewhere I read that an expansion tank eliminates the water hammer effect. I don't think I need this in normal operation--this only happens when I shut off the cold water and drain the pipes. Also there is no backflow preventer and the hot water expands like crazy (I know, I've run the tank without hot/cold hooked up and it pumped a ton of water out as it heated) but the pressure release valve doesn't open in any case, so I assume that the expanding water is just pushing back out the mains and thus I don't need an expansion tank for that purpose.

Are the banging pipes a problem? Or do I just ignore this as it only occurs if my pipes are empty (i.e. shut off mains, release pressure by running washing machine, turn mains back on)?

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Old 12-12-2012, 06:48 PM   #2
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Water hammer, expansion tank?


An expansion tank does not eliminate water hammer unless it is installed next to the faucet which when turned on or off gives water hammer.

A water hammer arrestor is like a tiny expansion tank, just big enough to cushion the shock as the contents of the pipe heading towards the faucet is stopped dead in its track when you shut off the faucet. It is obviously impractical to have an additional regular expansion tank at every faucet.

If you let water hammer go on for months or years, it is possible for joints between pipe sections to crack open and leak.

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Last edited by AllanJ; 12-12-2012 at 06:53 PM.
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