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Question DetailsAsked on 12/2/2013

what does it mean when pipes bang when toilet is flushed or whenshower or faucets are turned on ?

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3 Answers


Knocking water pipes seem to be a mystery to many homeowners. Although there is nothing seriously wrong with your pipes, the knocking sound is an unnecessary irritant. There are several things that could cause the noise, like loose pipes rattling against the wall, too high water pressure, too hot of water, and the most commone is water hammer. The fix for water hammer is relative easy and you should be able to fix it in less than 20 minutes.

Try this method before calling a plumber.

To eliminate the sound, start by turning off all the faucets in the house and then turn off the main water valve in your house.

Next, turn on all of the faucets and flush every toilet in your home to drain the water. Do this for as long as it takes to stop any water from coming out. When you are certain everything has been drained as dry as possible, shut off all of the faucets.

Then go back to your main valve and very slowly turn it back on. The knocking sound should be eliminated.

Let me know if this works. If not I can explain other fixes.



Answered 5 years ago by Homefront Inspection


The response by HomeInspection will work IF the plumbing was installed correctly in the first place, with water hammer cushion standups (explained later). The problem - which CAN be serious because the vibration from the water hammer can break pipe joints over time (especially as they age and pipes corrode) comes when there is a sudden stopping (or occasionally starting) of forceful water flow, causing a shock wave to travel through the pipes. This causes the pipes to move whereever there is slack - resulting in a "bump". Think a bus or train full of passengers and how a "wave" of motion and bumping passes through them when the vehicle stops or acdelerates suddenly.

This issue most commonly occurs with washing machines and dishwashers turning on and off, because their solenoid-operated valves commonly turn on and off quite very fast. Pulsating type sprinklers with high flow rate and a large hose can also cause this effect, as can automatic sprinkler system valves operating. However, using other faucets can sometimes also cause it, especially in a house with no provision for water hammer in the piping.

The permanent solution is to have an air cushion built into the system - commonly a foot or more of pipe extended up above the shutoff valves at each faucet or takeoff point, which holds air that acts as a cushion. In areas without a significant mount of entrained air in the water this air can be absorbed by the water so the standup pipes become full of water and no longer stop the shock - the comment by HomeInspection on totally draining the system refills these standups with air - see Figure 1 in this article -

There are also mechanical ones to achieve this - here is an example -

Usually the pipe extension above the shutoff valve does the job well, but some washing machines need the mechanical ones installed if they have fast operating high-flow valves.

Some plumbers now install the mechanicval type at every valve, but my experience is that is just introducing more things to fail - the standup pipes, or larger diameter air chambers where headroomm is limited, have worked fine every time I have run into this problem, at least when you dealing with household water pressures. Highrise and industrial high-pressure systems can be a totally different story.

Answered 5 years ago by LCD


I want to thank Phil for answeing this question. We had the issue and used his suggestion and it worked wonderfully!! THANKS SO MUCH!!!!

Answered 3 years ago by cindysme

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