Water hammer — pipes banging when taps are turned off
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Most likely cause & what to check
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.
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Frequently asked questions
Is water hammer damaging my pipes?
Occasional water hammer is unlikely to cause immediate damage, but frequent severe banging creates stress on pipe joints, fittings, and appliance valves over time. It is worth fixing both for comfort and to prevent long-term joint failures — particularly on older properties with soldered copper joints.
Can I fix water hammer myself?
Some causes are DIY-fixable: securing loose pipes with clips, draining and refilling the system to clear air locks, or adjusting a float valve in the loft tank. Fitting a water hammer arrestor on a washing machine connection is also a straightforward DIY task. A faulty or worn tap valve is more complex and may require a plumber.
What is a water hammer arrestor and do I need one?
A water hammer arrestor is a small device containing a piston or air chamber that absorbs the shock wave when water flow stops suddenly. They are particularly effective on washing machine and dishwasher connections where the solenoid valve closes rapidly. They cost £10–£30 and can be fitted without a plumber in most cases.