Eliminating Water Hammer

Question:

My water pipes bang every time I turn the water on or off. I have hammer arresters, but somehow, the air cushion has leaked out. How can I re-establish the air cushion in the pipes?

—Nick M. Pavicic, via email, None

Answer:

Senior editor Daniel S. Morrison replies: Water hammer is the banging you hear when you quickly turn on or off a water faucet or valve. It happens because of a quick change in the velocity of water in a long pipe. A good analogy is that of a speeding train smashing into a mountain: The engine stops, but the caboose keeps right on coming. Water in a pipe can’t simply fall off the track and pile up as a train can. Instead, the liquid sends shock waves back through the water pipes. The shock occurs where the water moving back from the front of the line meets the water rushing forward from the back of the line.

Drawing by: Dan Thornton

The pressure exerted by these shock waves is around 60 times the velocity of the water, which can translate to 600 psi. This is much more than a domestic-water system is tested and rated for, so these shock waves can be destructive to water pipes. The faster the water valve closes, the greater the shock to the pipe. The easiest way to stop the shock at a faucet is simply to turn off the water more slowly: say, over the course of 1 second rather than 1/4 second. But solenoid valves in dishwashers, washing machines, and ice makers can’t be adjusted to close slowly. Instead, you need some sort of air cushion.

A common way to supply an air cushion is to add a vertical section of capped pipe to the supply line of a fixture. The size of the cushion can be figured in The Plumbing and Drainage Institute’s Standard PSI-WH201 Water Hammer Arrester Standard (www.pdionline.org). But even correctly sized air cushions lose their charge, as yours did; follow the steps at right to get it back.

The best remedy is to install water-hammer arresters with a permanently sealed chamber of pressurized air or gas. These arresters should be as close to the valve as possible.

To re-establish the air cushion in a plumbing system, follow these steps:

1. Turn off the water at the meter.

2. Turn off the water to each toilet, washing machine, and dishwasher.

3. Turn on the hot and cold water at the sink, shower, and tub closest to the meter.

4. Turn on the hot and cold water at the sink, shower, and tub farthest from the meter (second or third floor).

5. When the water lines are empty, turn off all the faucets.

6. Turn on the water at the meter.

7. Turn on the water at the toilets, washing machine, and dishwasher.

8. Turn on every faucet (hot and cold), and flush every toilet. Brace yourself for some powerful banging and cloudy water both during and after this last step, but the cushion then should be re-established and the hammering gone.