Water hammer is a very loud banging, knocking or hammering noise in the pipes that occurs when the flow is suddenly turned off. It is caused by a pressure or shock wave that travels faster than the speed of sound through the pipes, brought on by a sudden stop in the velocity of the water, or a change in the direction. It's also been described as a rumbling, shaking vibration in the pipes.

You might hear water hammer when the clothes washer stops filling, the sprinkler system shuts off or shifts to another zone, when the dishwasher changes wash cycles, or when a faucet is turned off suddenly. It exerts very great instantaneous pressures that can reach excesses of 1000 psi and over time can potentially damage the system by weakening joints and valves causing leaks or even ruptures in the pipes.

Several factors can contribute to water hammer such as:

• Improperly sized piping in relation to water flow velocity
• High water pressure with no pressure-reducing valve
• Straight runs that are too long without bends;
• Poor strapping of piping system to structure
• No dampening system in place to reduce or absorb shockwaves

Older houses were often fitted with air-filled risers connected to the plumbing at various points to absorb the shockwave. These air chambers can become waterlogged over time. If you aren't sure if your house has air chambers there is a test you can perform to check by draining the plumbing to allow the risers to refill with air. To do this turn off water at the main, then fully open the lowest faucet on the system (probably the garden hose) to release water trapped in the pipes. Open a few of the highest faucets next. This will facilitate drainage and allow air in the pipes, refilling the risers. When the lowest faucet stops dripping, close all faucets. Turn the main back on. When you turn on the faucets for the first time there will be some vigorous spitting until the pipes are flushed of air.

If this procedure got rid of the air hammer, you have waterlogged risers but this fix is temporary as they will become waterlogged again. Moreover old air chambers can become full of muck, debris and bacterium that are unhealthy. New risers, called water hammer arresters do not allow water inside.

Due to the health risks of polluted air-chambers, the plumbing code in some areas does not allow new structures to be fitted with air chambers. If you are building a new house, using adequate diameter pipe and water hammer arresters at the valve and faucet junctions will ensure you will not have to deal with water hammer in the future.

If eliminating water hammer from an older house, locating the offending valve or faucet and installing a water arrester at that location is one solution, but the procedure might require breaking into a wall to reach the problem point. You can also try installing low-flow fixtures to reduce the velocity of the water traveling through the pipes. If your water pressure is more than 60 - 80 psi and your system does not have a pressure regulator, having a licensed plumber add one could also reduce water hammer. A setting of 40 psi will likely allow sufficient pressure while mitigating the likelihood of damaging water hammer.

Though it may be a little troublesome to address the problem, the potential cost of not addressing it is much greater. Fixing a plumbing system that has ruptured or even structural repairs as a result of water damage from leaking valves or joints is much costlier. Water hammer arresters run anywhere from $15 - $125 each, depending on capacity, and they come in different diameters. Consult with a knowledgeable salesperson before you buy.