I am in a newly built house (completed in 2020). For the first 8 months or so, the plumbing worked great. We had a toilet gasket warp and so the tank would slowly leak, and I started noticing a persistent thunking every time that tank refilled.

Well, I changed the gasket to stop the toilet tank issue but I started noticing that thunking occurs anytime ANY water in the house is run. I believe this to be water hammer as I can't think of anything else it could be.

We had a plumber come from the company that built the house. He suggested installing water hammer arrestors on the washing machine... So he did that and it didn't change a thing.

I'm assuming for a water hammer arrestor to work it'd need to be on the main line where the water hammer is actually happening?

It seems to be difficult to pinpoint the exact location, as the hammering happens with any faucet in the house that's used (sink, shower, tub, toilet, appliances...)

The one last-ditch attempt I made at a fix was replacing the PRV on the mainline. That also has done nothing...

Anyone have any ideas as to what could be causing this and any possible solutions for a whole home issue like this?

EDIT: I just want to clarify. It seems to me like hammer arrestors should be part of the code or something... I wouldn't expect this to be happening in brand new plumbing, so is it possible the plumbers messed up by not including hammer arrestors in the original plumbing?

  • Our house has no water hammer arrestors (unless they are hidden) but we have never had significant water hammer from the clothes washer which is our only electrically operated valve. At one point I set up a spring loaded shutoff on an outside faucet and got major water hammer at the kitchen sink. We just naturally don't slam shut our manual valves so no detectable water hammer there. Apr 13, 2021 at 0:45
  • 2
    Have you checked the pressure in your home?
    – Kris
    Apr 13, 2021 at 0:57
  • Ball and float valves can oscillate if you get a ripple in your tank, so if things started with that toilet I'd inspect the heck out of it while it's thunking. I found a website listing worn stop valves as a cause, but they don't state why. You could also have trapped air somewhere in your system. This gives oscillations in pressure a way to store energy. Water isn't very compressible, but air is. Flow velocity higher than 1.5m/s can be a cause as well
    – K H
    Apr 13, 2021 at 10:54
  • They also mention inadequately strapped or sized pipe as a potential cause so if your strapping has been shook loose it shakes more from that point on. It matters where exactly the sound is coming from. Does it come from the same place regardless of what tap you run to make it happen? Does it continue if you leave the tap running or fade out?
    – K H
    Apr 13, 2021 at 11:01
  • 1
    Water hammer typically occurs at an elbow which has several feet of straight water line behind it. The more linear feet of water you have then the harder your water will hammer the elbow when you abruptly close a tap.
    – MonkeyZeus
    Apr 13, 2021 at 12:06

3 Answers 3


Water hammer happens when there is too much pressure in the line when the valve shuts. Ironically, it is a problem when the water supply doesn't have enough air, not water. Since water can't be compressed, all of the pressure is transmitted to the pipe as mechanical force when the valve is closed, causing it to knock and rattle. Reducing the water pressure can avoid the noise and may even go unnoticed with today's eco-friendly fixtures, but it is better to add arrestors so it can compress shock.

To reduce the water hammer effect, there needs to be a different place for the pressure to go. Water hammer arrestors work by creating a compressible gap of air that can absorb the shock of the water hitting the suddenly closed valve. They are usually installed at each fixture above the valve so that there is the least amount of shock on the line; however, a whole house water arrestor can work too as long as it can absorb excess water pressure. An arrestor placed on the highest fixtures in the system can have the best effect as long as there is a path from each fixture to the air gap.

If it is new construction, there may be a PEX manifold which makes life easy. But if not, to find the best place(s) to add a local arrestor:

  1. Turn the water off and open every faucet to drain the system.
  2. Close all the faucets and turn the water back on.
  3. Turn on one faucet at a time, from the top floors down, until only water comes out, not air. Pay attention to how long the air takes to come out and make sure to do both hot and cold.
  4. The fixtures that take the longest to flush air have the longest path and need the arrestor. If lower floors also spit air after upper floors are flushed, there is probably a siphon and they need arrestors too.

If the walls are closed on both sides, a quick test would be to add an external arrestor at the problem. One like this mini arrestor can attach past the shutoff valve at the fixture.

My home was built in the 60's with all copper plumbing (even the sewer!) and we had water hammer that would shake the whole house. I had local arrestors on the washer and a few toilets that helped, but the tub and shower were still loud. When we renovated the upstairs bathrooms I added 6" buffers to both and the problem has now completely gone away.

  • I appreciate the detailed response and this gives me a lot to play around with. One question I had in all of this, is shouldn't plumbers be required to leave extra pipe with an air gap to act as a hammer arrestor? This house was just built in 2020... you'd think they'd have a simple prevention for water hammer in the codes by now. Is it possible the plumbers messed up somewhere?
    – eerick
    Apr 20, 2021 at 19:21
  • extra pipe to trap air is no longer code as the air gap is not reliable. Hammer arrestors also need to be accessible as the diaphragm can fail over time and stop working
    – redlude97
    Apr 20, 2021 at 22:46

As a plumber here in British Columbia it is required by code to have hammer arrestors, for the dishwashing machine and the laundry machines. Typically anything with a fast acting valve, which means it closes quickly when the water is not needed. Improperly strapped piping is what causes them to thump and move around. They do make hammer arrestors that thread right onto your shut off for your appliance. That are easy to add


Had water hammer when turning off every fixture in the house. Installed arrestors on hot and cold close to noise. No go. Finally heard small noise at upstairs shower. Changed delta series 1400 shower cartridge. That fixed it. No water hammer in house.

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