Why in some houses do the pipes knock when the hot water is run?

Answer 1

Call e has explained it pretty well to you. But to add to it where he/she is telling you to add a tee for an air chamber you should install an expansion tank. It will do much better than just an air chamber. Go to a home improvement or plumbing supply store and tell them the capacity og your heater. Most are threaded at the inlet. Put an adapter on a small piece of pipe, onto the tee. Install the expansion tank. should solve your problem.

Answer 2

There is air trapped in the pipes. It is called water hammer.

This is impossible to remove, but you can eliminate the problem, by installing an air cushion which is essentially a sealed section of pipe installed on your hot water pipe.

You need to look at the hot water pipe in the basement, and see how much vertical space you have above it.

Get a Tee connector of the same diameter as the pipe.

The straight-through part of the Tee will be used to fit your pipe, and the other section will get the cushion part.


Solder a section of straight pipe to the T section, then add an elbow so that it points upwards.

Now add a six inch or longer piece of pipe to the upward-pointing elbow.

Close off the end of this small pipe with an end-cap.

Solder or use copper glue to fix all parts together.

Now cut your main hot water pipe at the chosen spot and remove an additional three-quarters of an inch of it. This is to allow space for the Tee to fit in.

Fit the Tee into place with the sealed section pointing upwards,
and solder/glue it securely.

That is all you need to do. The air that is now trapped in the vertical section will cushion the shock of the air in the pipes, and eliminate the noise problem forever.

Answer 3

because the pipes are old and they are expanding when the hot water runs through them.

Answer 4

Because the pipes in the walls were never properly secured.