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I am plumbing a small bathroom. The total number of supply lines is five -- shower (hot and cold), sink (hot and cold) and toilet.

I want to place hammer arrestors but I'm not sure if I need 5 hammer arrestors, or if I can just place two for the sink, or if I should place them as the pipes enter the room, and protect the whole room that way.

All the diagrams on the internet indicate placement as close as possible to the faucet.

The total length of pipe from where the pipe enters the room until the furthest faucet is only about 10 feet. Total including verticals.

I'm attaching my poorly drawn sketch of the plumbing. The arrestors are "orange". Should they g near each tap or can they go as the pipes enter the room?

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EDIT: Photo of the finished work

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Put them near the sink.

Water hammer is caused by the inertia of the moving water in the water lines trying to keep moving when a valve shuts it off suddenly - so placing them at the far end of lines is most effective.

In "typical" bathroom fixtures of the current era, the sink is also the most likely to be a "quick" shutoff (which is the most prone to causing water hammer.) Valves that shut more slowly allow the water to slow down as the valve is closing, whereas those that close quickly get to all the way shut while the water in the pipes is still moving.

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  • many shower valves can be shut just as quickly as the faucets... But when dealing with water hammer for 6" pipes and flows of 30 litres a second then the consequences are a bit more evident :) – Solar Mike Nov 2 '20 at 16:27
  • ...and if that applies, you may need to add another set at the shower. – Ecnerwal Nov 2 '20 at 16:29
  • Thanks. I guess I was trying to avoid having to cut the pipes I had already installed since I messed up and forgot to install the hammer arrestors. but if there's one thing I learned from my dad, it is "do it right the first time" – JDS Nov 2 '20 at 22:01
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I would put them before the first tee in the supply line ie as close as possible to the cause of the water hammer, that protects more of the supply side.

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