This banging sound is caused by the surge of pressure in the pipe when the water is stopped or asked to change direction quickly.

Devices such as air chambers, as shown in Figure 1, air cushions, water capacitors, shock absorbers or other water hammer arresters are now being installed in new homes to avoid the problem of water hammering.  They work by providing an air cushion, built into the water system that basically absorbs the shock wave created by the surge of pressure in the pipes when the water is turned off.

If you have not experienced water hammering in the past, but it has started you might have a waterlogged air chamber.  Most of the water hammer prevention devices can become filled with water and hence will no longer serve the purpose they were designed for.

You can attempt to clear the water hammering arrester by performing the following tasks:

1. Turn off the water at the main valve entering the home.

2. Locate the lowest faucet or valve in the home, this could well be the outdoor hose faucet in homes without basements.

3. Open this faucet.

4. Open the faucet(s) at the highest location in the home.  (Water will start to flow out of the lowest faucet as the water system drains itself.

5. Leave the upper faucet(s) open until there is no water dripping from the lowest faucet or 10 minutes, whichever is the longest.

6. Close all the faucets - upper and lower.

7. Turn the main house valve back on.

Figure 2 - Water being fed to faucet or valve through a 90° elbow

If it is fed by an 90 degree elbow you can replace it with a Tee and a capped 18" to 24" pipe that will create an air cushion, as shown in Figure 3.

Figure 3 - Using a "T" fitting and a cap creates an air cushion

This addition to the water piping should correct the water hammering problem.

You can also purchase manufactured water hammer arresters which conveniently mount on the water line, as shown in Figures 4 and 5.