121 – Water Hammer


Water hammer occurs when this non-compressible liquid, flowing through a pipe at a given pressure and velocity, is stopped abruptly by quick closing valves such as solenoid valves on clothes washers and dishwashers. Single lever faucets can also create these “shock waves.”

Consequently, damage can result to piping, water meters, storage tanks, water heaters, temperature and pressure relief valves and pressure regulator gauges.

This kinetic energy force can be controlled by installation of water hammer arrestors, expansion tanks or pressure only relief valves in cold water make up lines at 25 to 50 pounds less than the temperature and pressure relief valve setting on the water heater. This pressure only relief valve will operate as a sacrificial valve, thereby allowing the T&P valve to perform as a safety device rather than an operating valve in the system. When a back flow preventer, or check valve, is used in the system one of the above methods of water hammer control is recommended.

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