Water hammer happens in closed pipes when a fast closure (or opening) of a tap or a motorized valve occurs, or a pump suddenly stops. The phenomenon consists of an overpressure (or underpressure) due to the fast stop (or acceleration) of the fluid. This pressure variation starts at the closing (or opening) device and propagates upstream. This pressure variation can lead to noise (similar to a hammer hitting the pipe) up to breakage of the devices installed in the system. Water hammer arrestors, installed as close as possible to the closing (or opening) device, dampen the pressure, keeping it at low levels, avoiding problems to the whole system. Typical points of installation are: under sinks, close to whashing machines, after the pressure reducing valves. The use of water hammer arresters is suggested by the UNI 9182 regulation “Cold and hot water supply and distribution systems. Design, testing and management criteria”.