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Plumbers, I have a peculiar water-hammering issue and I'm wondering where to install arrestors.

It's triggered only at the hot valve in 2nd floor bathroom, when we shut off a whole bunch of different water outlets in the house. It's loud and hard, causes the hot shower valve to get loose, with water running, and the only way to stop it is to turn that particular valve off. The hammering valve is on a tiled wall and breaking open that wall is the least favorite option for me.

It's exactly a single hard "thunk" when we shut a bunch of hot valves, or run machines: the washing machine, hot sides of the two sinks right next to the shower (all these on second floor), kitchen sink hot valve, bathroom sink hot valve, toilet flush and the dishwasher (all these on first floor).

Exactly at the moment of the "thunk" we feel in the valves we shut, or run the machines, the pipe behind the hot shower valve starts hammering, it gets loose, water starts running, and we need to run to that bathroom to turn the loose valve off. Fun fact tho: the toilet right next to the hammering shower valve never triggers the hammering. But the hot valves in the sinks next to the toilet do trigger it.

Edit: Now the "hammering" loud vibration starts randomly without a trigger, at night or day, out of the blue. In this case, I'm not sure it's still "hammering"? And I wonder arrestors will help. I'd really appreciate suggestions, thanks!

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  • You're calling it "water hammer", but I'm not sure that you're using the correct term and as a result you're trying to fix the wrong problem. Water hammer sounds like a single hard 'thunk' when a water valve shuts off. Water hammer is not an ongoing vibration while the water is running.
    – brhans
    Commented Feb 7, 2022 at 14:26
  • Oh, thanks, I mis-described, sorry will edit the original post. Yes, it's exactly a single hard "thunk" on the valve being shut off, triggered by the manual shutting off of the hot valves I listed, plus the machines, and the toilet flush downstairs. Exactly at the moment of the "thunk" we feel in the valves we shut, the pipe behind the hot shower valve starts hammering, and we need to run to that bathroom to turn the loose valve off. Fun fact tho: the toilet right next to the hammering shower valve never triggers the hammering. But the hot valves in the sinks next to the toilet do trigger it.
    – Samil Can
    Commented Feb 7, 2022 at 16:56
  • Make sure you put the hammer on what flows straight through the tee. Usually that means there's always two street elbows attached to the 'cross' of the tee to make the hammer vertical, leaving the output of the tee sticking straight out from the wall.
    – Mazura
    Commented Feb 7, 2022 at 19:06
  • Thanks for the suggestion, and excuse my lack of information on the topic. Could you rephrase this for someone who's not familiar with the terms? :)
    – Samil Can
    Commented Feb 7, 2022 at 23:07
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    It sounds as if the water hammer event causes that shower valve to open a little bit, and some kind of resonance causes sustained noise until you go and re-close that shower valve?
    – Greg Hill
    Commented Feb 7, 2022 at 23:29

1 Answer 1

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A hammer arrestor is effective only when it provides a path for the stopping water flow to take. So you'll want to install an arrestor near each point of use where shutting the water off is causing the hammer.

For example, if the bathroom sink H and C are causing hammer, you'll want two arrestors, one on each of the H and C lines right there at the sink. Putting an arrestor on the kitchen sink will not help the bathroom.

One thing about water hammer, however, is that a hammer in one place can reverberate to other locations and sound like the problem is there also. But it's just the shock wave traveling through the water to the rest of the home.

Having hammer like you describe is unusual. Is it possible you have excessively high water pressure? You might test it and if it's high, a pressure reducer might be a better choice than hammer arrestors all over the place.

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  • Thanks, the pressure is 50 to 60 psi so I feel like it's within the range of normal? I went around the house, listening to the walls, and I'm pretty sure hammering is only right behind the shower hot valve, nowhere else. While I hate the idea of breaking down the shower wall, I'm wondering whether it's a faulty shower pipe/faucet installation also? Is there a way to check and eliminate that possibility? If it will solve it, I will install arrestors everywhere, but it's awfully specific that multiple outlets trigger hammering only in that particular valve, what do you think? Thanks!
    – Samil Can
    Commented Feb 7, 2022 at 18:09
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    Yes, 50-60 psi is typical.
    – jwh20
    Commented Feb 7, 2022 at 19:07
  • Is is possible that a water hammer arrestor on the hot supply to the lavatory that is right next to the shower would have the best effect. Do you know how the water is routed to your bathroom? In our "master" bath the plumbing for the lavatory, toilet and shower is all in one wall. Hot line emerges from the slab in the wall behind the toilet and has a T, left end 5 ft to the lavatory, right 5 ft over to the shower. But there are many other arragements. Any idea what yours is? Commented Feb 9 at 23:34

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