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We're doing a minor remodel of our master bathroom, and I have replaced the 30+ year old diverter and nearby pipe with a new one from Delta. Now when any faucet in the house is turned on and off quickly, there is a hammering or banging sound coming from the diverter (2 or 3 bangs). Trying to hold the diverter and nearby pipes firmly still does not reduce the noise, though the diverter is certainly well secured to the brace. The photo below shows the extent of the plumbing I had to do. (Note the wonderful polybutylene at the right which transitions via a Sharkbite to the CPVC.)

Are newer pressure balancing valves subject to this kind of hammering? That seems like a possibility since the old valve certainly did not balance the pressure.

The only other thing I can think of is that I knocked some pipe strap/hanger loose somewhere, but why would it cause the sound to come from the diverter area?

Photo of diverter and pipes

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Have you run the water from the faucet yet, after installing the new plumbing? –  Tester101 Mar 7 '13 at 12:56
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Related: How do I fix a water hammer problem? –  BMitch Mar 7 '13 at 13:12
    
@Tester101 I did run water from the new tub spout stub. –  joshdoe Mar 7 '13 at 13:28
    
While it seems hammer arrestors should fix the problem, I don't understand why I'd be getting hammering from this diverter if I turn a faucet on and off somewhere else in the house. If the water flow suddenly stops at a valve in another bathroom on another floor, how can the pressure wave travel dozens of feet to this bathroom and cause the noise? Wouldn't the noise only be heard at or very near the faucet being operated? –  joshdoe Mar 7 '13 at 13:31
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The energy will flow though the system until it is dissipated, so the hammer can occur almost anywhere. –  Tester101 Mar 7 '13 at 13:38

2 Answers 2

I'm not sure why you're suddenly having trouble with water hammer, the old plumbing may have been installed in such a way as to mitigate water hammer. Since I have no way of knowing what the old plumbing looked like and/or what you changed, I can't say for sure why you're having trouble.

So I'll simply talk about a couple ways of stopping the hammering.

The old way

We [plumbers] used to try to stop water hammer with 12-18" capped risers, and this method worked great, just long enough for us to be on to the next job.

enter image description here

The idea was that air would be trapped in the capped riser, which would provide a "cushion" for the fast moving water. This effect, however, is short lived. The air is eventually absorbed into the water, and the riser becomes completely filled with water.

The new way

Modern water hammer arrestors separate the water from the air, which allows them to function properly for much longer. Most devices either use a flexible bladder, or a piston type design to cushion the water and prevent hammering.

enter image description here

These devices are usually less than 6" long, so they can fit in much tighter places. Cost wise they are comparable to the old method, so they are not going to break the bank. They are available with different connector types, so they can be connected to the existing plumbing in a variety of ways.

enter image description here

As with the old style standpipes, water hammer arrestors should be installed in line with the flow of water. Water hammer arrestors should be installed near all fast acting valves, to prevent water hammer.

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agreed, the newer arrestors will work much longer. –  HerrBag Mar 7 '13 at 13:23
    
Since I solved my problem, but with your help, not sure if you want to update this answer to include checking for debris in the check valves, so I could mark your's as the answer. –  joshdoe Mar 9 '13 at 17:13
    
@joshdoe You should write the solution that worked for you up as an answer, and accept that as the correct answer. There is no problem answering your own question, in fact it's encouraged. –  Tester101 Mar 10 '13 at 12:05
    
@Tester101 Okay, I took your advise and wrote my own answer. I made an attempt at explaining the cause of the noise, perhaps you could comment on it as to its validity. Thanks again! –  joshdoe Mar 11 '13 at 14:58
up vote 2 down vote accepted

While Tester101 provides a great answer for the classic case of water hammer, my situation is slightly different. The key piece of information from my question was that the noise occurred not only when faucets were turned off, but also when they were turned on. Doing a more focused web search on this turned up this post on TerryLove.com, which suggested to inspect the check valves for debris. Previously I had inspected the valve cartridge, but failed to look at the check valve assembly which was behind this cartridge. Indeed, there were small fragments of polybutylene pipe that prevented the check valves from operating properly. Once I cleared out the fragments from the check valve, the hammering noise disappeared!

Attempt at an Explanation

In the static case, when no water was flowing anywhere in the house, the water pressure at the check valves of the cold and hot lines were the same, and because of the debris both check valves were slightly open. When the nearby faucet was turned on quickly, say on the cold side, the following series of events occurred:

  1. Water suddenly begins flowing from the cold line at the sink faucet, causing the pressure to suddenly decrease at the location of the cold check valve at the diverter.
  2. Now the pressure at the hot water line at the diverter is higher than the cold water line, causing water to suddenly flow from the hot line through the check valves to the cold side.
  3. This sudden flow from the hot to the cold side of the diverter causes the hot check valve to slam open, and causes the cold check valve to slam (mostly) shut.
  4. The sudden stop of water flow causes a pressure surge which then causes noise just like traditional water hammer.
  5. Turning the cold faucet off again causes the whole sequence above to occur again, but in reverse.

Another key piece of information is that the noise does not occur when both cold and hot faucets are turned on simultaneously. In this case the pressure at the diverter drops equally for both hot and cold, and thus no water flow.

How to Avoid the Problem

Of course what I should have done is what every decent set of instructions would have included (or what common sense would have told me if I hadn't been in a hurry): flush hot and cold water through the diverter before installing the valve catridge or check valves. This way any pipe fragments or bits of solder will not get stuck in the check valves.

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