When I turn off the cold taps in my house there is a loud banging noise from behind the wall in the downstairs bathroom (which is where the cold water supply enters the house). This started happening some months ago now and I can't think of anything that happened plumbing-wise just before it started.

Anyway, I found that I can temporarily fix this 'water hammer' by turning off the water supply where it comes into the house and then draining the cold water system by running all the downstairs taps. When they run dry I turn the water back on at the main stop-tap and the banging noise is gone!

After a few days the banging starts to happen again - quietly at first but growing to the loud banging again after about a week.

What could this be?


My guess is that this has been caused by some bracket (or similar) that was holding a pipe behind a wooden wall coming loose. I'd much rather fix the cause and to install an arrestor in an appropriate place I'd have to mess with the wall anyway I think.

The only thing I can remember happening around the time this began was some very windy weather. When I heard the first bangs one night I assumed it might have been something banging in the neighbour's garage, which shares a wall with my bathroom.

2 Answers 2


Water hammer, no air quotes needed. Noise in pipes from the momentum of moving water, suddenly stopped. Draining the pipes is providing temporary air bubbles which cushion it.

Water hammer arrestors, either in the form of a few feet of capped vertical pipe at the end of the run to hold a more lasting bubble, or spring/plunger/diaphragm/piston units sold under that name will cure it. Shutting the water off more slowly will also cure it, where possible (automatic valves on washing machines and dishwashers are notorious for quick action causing water hammer.)

I have no association with any of the places these images come from:

image from plumbingmart.com

Here's one where you can see it, so you might avoid opening walls if you did it this way: image from kirsner.org

You may have arrestors in place which have failed, given the "randomly started up a few months ago" part of this. Unfortunately you're probably going to have to open walls to find, replace, or add them.

  • I don't see any arrestors.
    – Ben
    Apr 15, 2015 at 15:12
  • 1
    They are often hidden in the walls; thus my mentioning that you may have to open walls (but no guarantee that there are any hiding in there.) However, you could just add some in a more exposed manner if that works for you aesthetically (some folks don't want to see things, some don't mind.) NOT having any might be a direct cause of any "bracket ripping loose" as you have speculated in your edit - in which case, both fixing the bracket AND adding arrestors would be the good call.
    – Ecnerwal
    Apr 15, 2015 at 15:18
  • Thanks for your help. :) I'll see if I can fit an exterior arrestor first I think, to see if that actually stops it. If it does then when we get to modernising that bathroom we can get behind the wooden panelling and see what's what.
    – Ben
    Apr 15, 2015 at 15:26
  • 1
    Depending how fancy the neighbor's garage is, and how well you get on with them, it might be easy to take a look from that side with simple drywall repair and no need to rip open the wooden paneling. ...or it might be unthinkable.
    – Ecnerwal
    Apr 15, 2015 at 15:35

It's called "water hammer", and it's caused by the physical momentum of the water in the pipes being dissipated when the flow stops suddenly.

By draining the system, you're creating an air bubble somewhere in your plumbing that provides a cushion for the momentum, eliminating the banging, but this bubble is leaking away somehow, which causes the banging to resume.

A permanent cure would be to connect the bottom end of a vertical pipe that's a few feet long to a point near the service entrance in your home. Make sure the pipe is full of air and cap off the top end permanently. This will provide a permanent air bubble that should eliminate the banging for good.

  • That placement will not work. Water hammer is quite common even in well-systems which have a HUGE permanent air bubble at the supply end (in the form of a pressure tank) - the WH cushioning bubbles need to be at the far end of the affected lines.
    – Ecnerwal
    Apr 15, 2015 at 11:51
  • @Ecnerwal: All I can say is, it works in my house just fine.
    – Dave Tweed
    Apr 15, 2015 at 12:17

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