What causes the banging noise in my home's pipes?

The problem of pipes banging on a wall is often called "water hammer." Check out this HowStuffWorks podcast to learn more about this phenomenon.

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Episode Transcript

Speaker 1 (00:00):
Welcome to brain Stuff from how stuffworks dot com where
smart Happens. Hi. I'm Marshall Brain with today's question, what
causes the loud banging noise in my homes pipes? The
problem of pipes banging in the wall is often called
water hammer. It can happen on a normal faucet, but

(00:20):
it's more common on pipes attached to a washing machine
or a dishwasher. In both of these machines, the valve
is electrically operated. It's called a solenoid valve. Solenoid valves
close very quickly, so imagine the following scene. The washing
machine is filling, so the valve is open. Water is
rushing through the pipe towards the washing machine. When the

(00:43):
washing machine is full, the valve slam shut very abruptly.
In the pipe, there may be five or ten pounds
of water that had been moving at a significant speed,
and now it suddenly comes to a complete halt. The
water's momentum carries the entire phipe system forward, and it's
the shifting of the pipes that makes the noise. If

(01:05):
it's a long, straight pipe carrying the water, the amount
of motion and therefore the noise can be especially significant. Obviously,
this is not good for the pipes. Each time you
hear the noise that means your pipes have been stressed
and flexed. The normal solution is to put a shock
absorber in the pipe near the valve. A simple shock

(01:26):
absorber would be a foot long piece of vertical pipe
attached near the valve. This pipe is filled with air.
Now when the valve closes, the moving water rushes up
into the vertical pipe and compresses the air so it
slows down gradually. The problem with a simple vertical pipe
is that the bubble can get dissolved into the water,

(01:46):
so a sealed system using a rubber bladder can be purchased.
This solution also allows you to orient it other than vertically.
Another approach is to use a spiral of copper pipe.
When the water hits the spiral, the coil absorbs the
shock like a spring. Do you have any ideas or
suggestions for this podcast? If so, please send me an

(02:08):
email at podcast at how stuff works dot com. For
more on this and thousands of other topics, go to
how stuff works dot com.