Water hammer — pipes banging when taps are turned off

Water hammer is a pressure shockwave that travels through the pipework when a fast-closing valve (tap, solenoid valve, or dishwasher fill valve) abruptly stops water flow. The kinetic energy of the moving water column has nowhere to go and causes a hammering noise — sometimes felt as a vibration through the walls.

The most common culprits in a domestic system are: quarter-turn lever taps (which close in a fraction of a second), washing machine and dishwasher inlet solenoid valves, and ballcock valves in cisterns that slam shut.

Check whether the pipes are adequately clipped to the joists or walls — loose pipes will rattle and amplify the hammer effect. Pipes that run through joists with too much clearance will knock against the timber. Adding additional pipe clips or packing the holes with foam can reduce noise significantly.

If the noise comes from the washing machine or dishwasher connection, fit an inline water hammer arrestor — a small device (£10–£20 from any plumbers' merchant) that screws inline on the supply hose and absorbs the pressure shock with a spring-loaded piston.

For whole-house water hammer, a pressure-reducing valve (PRV) set to reduce mains pressure to 2.5–3 bar can help. High mains pressure (above 4 bar) is a common root cause — a plumber can fit a PRV on the incoming main for £100–£200.

On a gravity-fed system, ensure the ball valve in the header tank is not "chattering" — an old or stiff ball valve can vibrate as the tank fills, causing a hammering or machine-gun noise. Replace the ball valve (£5–£15 part) if it is more than 10 years old.