What Is Water Hammer and How to Fix It in Your London Home

What Water Hammer Actually Is

Water hammer is the noise and shock wave that travels through a pipework system when moving water is suddenly stopped. When a tap is closed quickly or a solenoid valve (such as in a washing machine or dishwasher) snaps shut, the kinetic energy of the flowing water has nowhere to go and creates a pressure wave that reverberates through the pipe — producing the familiar bang or thud.

London properties are particularly prone to water hammer for two reasons: mains water pressure in many London boroughs is relatively high, increasing the force of the pressure wave; and older properties often have unsecured pipe runs where movement is amplified into noise at clips, joists, or wall surfaces.

Types of Pipe Noise and What They Indicate

Not all pipe noise is water hammer. A single loud bang when a washing machine valve closes is classic water hammer. A ticking or creaking noise as hot water runs through pipes — particularly copper pipes under wooden floors — is thermal expansion, an entirely different phenomenon and generally harmless. A persistent vibrating or humming sound from a pipe when a tap is running suggests a loose or worn tap washer or a partially closed isolating valve causing turbulence.

Intermittent banging from a cold water storage tank in the loft, occurring when the float valve shuts off refill, is ballcock hammer — a close relative of water hammer and addressed differently.

DIY Fixes Worth Trying

The first and easiest intervention is to check whether any isolating valves on supply pipes to appliances are fully open. A half-open isolating valve creates turbulence and can cause vibration-type noise that mimics water hammer. Turn them fully open or fully closed — never leave them at an intermediate position.

For classic water hammer from appliances, fitting an in-line water hammer arrestor on the supply pipe to the washing machine or dishwasher is an effective fix. These screw onto the appliance connection and contain a sealed air chamber that absorbs the pressure wave. They cost £10 to £25 and can be fitted without tools beyond an adjustable spanner.

Adding additional pipe clips to unsecured runs — particularly in lofts, under floors, and in airing cupboards — reduces the movement that translates pressure waves into noise at surfaces. Clips should be at 1.2m intervals for 15mm pipe and 1.8m for 22mm pipe.

When to Call an Engineer

Water hammer that has been present for years and recently worsened warrants investigation. New water hammer in a previously quiet system can indicate a failing check valve allowing backflow, a pressure reducing valve (PRV) that is no longer functioning correctly, or deterioration in an internal component of a tap. These require a plumber to diagnose.

Banging accompanied by visible movement in visible pipework, or water hammer so severe that it trips a pressure relief valve, indicates system pressure problems that go beyond the noise itself. High dynamic pressure (pressure spikes during flow) can stress joints and fittings over time, eventually causing leaks. A plumber with a pressure gauge can confirm whether your mains pressure is outside the acceptable range (generally 1 to 3.5 bar at the point of use) and fit a PRV if needed.

Water hammer in London properties in communal blocks may also be caused by a neighbouring property or by communal supply pipework — in which case the building manager or freeholder is the correct first contact.