When I shut off a water faucet or right after the toilet tank refills after flushing, I hear a vibrating sound in my pipes. It last between one and two seconds.

On the interwebs, I found out that it's called "water hammer" and that it can be fixed by draining my plumbing system: by shutting off the water main and opening up all the faucets and flushing all the toilets in the house.

Well, I did that and it helped for two, maybe three days. What do I do next (other than call a professional)?

  • How long did you wait after you drained it before you turned the water back on? Commented Sep 12, 2010 at 21:00
  • 20 - 30 minutes
    – vitule
    Commented Sep 12, 2010 at 21:01
  • 5
    My guess is that draining the lines in your house introduces some air into the system. That air softens the water hammer, acting as a cushion. After a few days the air is gone, so your water hammer is back. The water hammer arrestors work by providing that cushion on a permanent basis.
    – user558
    Commented Sep 13, 2010 at 2:00

5 Answers 5


This normally happens near quick-acting valves in your plumbing system, like those used in a clothes washing machine. Those valves are opened by a solenoid and get slammed shut by water pressure, and all that water coming to such an abrupt stop can rattle already-loose pipes, cause them to become loose, or in the worst case damage them over time and cause leaks. That's why water hammer arrestors are commonly installed near washing machine outlets (it may be required by code, I'm not sure). They look like this:

water hammer arrestor

They work by maintaining a reservoir of air inside that vertical tube. When the column of water is stopped abruptly by a valve, the air acts as a cushion, compressing so that the water can slow down more gradually and impart less inertial force to your pipes.

These are fairly easy to install even for someone with no plumbing experience. I believe the one pictured is designed to be mounted inline with a sink or toilet connection, and they also make models with standard garden hose style threads for a washer connection. You shouldn't need to install these at every fixture; one set (hot and cold) for each bathroom/kitchen should be plenty.

Since your problem seems to occur on (relatively) slower-acting valves that shouldn't be generating an unacceptable amount of water hammer, you may also just have a section of pipe that's not mounted properly. If you're only noticing the banging sound in one area, it may be possible to open up the wall there and install some pipe clamps to keep things from moving around and making noise. This would be a more difficult job than installing arrestors (but potentially cheaper if you do it yourself).

  • 10
    Actually the valves in washing machines are opened by a solenoid valve. They are closed by a spring, and also by the water pressure itself. The more pressure, the faster these close, the more sound that is created. Commented Sep 15, 2010 at 16:00
  • @Brad Gilbert: good point, I'll edit my answer. Commented Sep 15, 2010 at 17:37
  • 4
    I believe I saw This Old House do a DIY arrestor by just installing 12-18" of copper pipe on a T-fitting with a cap on the end.
    – Freiheit
    Commented Jul 11, 2011 at 18:55
  • My house had the DIY type arrestors. When I remodeled the bathrooms, I switched them out for the purpose made parts from Home Depot. To be honest, the Home Depot parts seem to work a little better. Commented Jul 12, 2011 at 19:28
  • 4
    The major problem with DIY arrestors is over time, the air will leak out and be replaced by water, and once they are filled with water you have two issues: the arrestor will effectively be useless, and it'll also be a dead-leg that can harbour bacteria which will then contaminate everything downstream (and possibly upstream, if the water sits unused for long enough). Actual arrestors have a piston inside that keeps the air contained.
    – gregmac
    Commented Sep 4, 2013 at 20:39

@Mike Powell, That's a great answer if it is actually water hammer. If it turns out to be the toilet valve that is causing the vibration then the valve will need to be replaced. Take the lid off the toilet, then push down on the float until the water starts to fill the tank. Then let go of the float. Do you hear the vibration? Do you hear any form of noise coming from the valve itself? Or is the noise just in the pipes? For the valves that have a float extended on a long steel rod, this float can gradually close the valve while the tank fills and cause a whining noise as the valve closes. If there is loose piping in the walls, this flow "restriction" can case a vibration.


I had this problem. My pipes started making a very loud banging. I tried the drain the lines trick but it didn't help. I then put a water pressure gauge on my faucet and found my water pressure to be up over 100psi. I followed the steps for reducing the water pressure on the water pressure reducing value but then noticed that after running the water for just a few seconds the pressure dropped to almost nothing. I then replaced the water pressure reducing valve, which in my case was faster to actually replace then it was to go buy. Now I have no banging and constant pressure. Moral of the story, go buy a $10 water pressure gauge and make sure your pressure isn't high. I can't believe with my luck the pipes didn't break.

  • You replaced the gauge right? I didn't know that was an option but sure enough, some of them have one. In my case I replaced the whole pressure reducing valve, problem went away. Weird...
    – rogerdpack
    Commented Feb 8, 2019 at 14:37

We bought a front end loader washing machine and had the problem from day one. It wasn't bad but worsened with time. We put in water hammer arrestors that seemed to work for the most part for about a year, and then, over about a week, seemed to give out completely. We replaced the water hammer arrestors with new ones, but that didn't help. We then bought two $8 copper water pressure regulators from Walmart (usually made for RVs) preset to 40 psi. Installed and it completely solved the problem! Would higly recommend the 10 minute install. These are one-way values, so make sure the flow is in the correct direction.


I had water hammer in bathroom commode. Removed top and adjusted the screw to allow more water inside tank before float shut off water value. Hammer sound stopped and tank filled quicker.

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