Water hammer is the loud bang or repeated thumping that runs through the pipework when a fast-closing valve, typically a quarter-turn ceramic disc tap, a washing-machine solenoid, or a dishwasher fill valve, abruptly stops a moving column of water. The kinetic energy in the column has nowhere to go, so it converts to a pressure spike that travels back through the pipe and hits the next fixed point, usually a poorly clipped run under a floor or behind a wall. A single bang is annoying. Repeated hammering accelerates joint fatigue, loosens compression fittings, splits old soldered joints, and over years is a recognised cause of pinhole leak development on copper pipework.
Diagnosing the source is half the job. We isolate fixtures in turn while listening with an acoustic stethoscope to identify which appliance triggers the hammer, then trace the pipework run to find the unrestrained section that is moving under the pressure spike. The fix is usually one of three: fit a shock arrestor (a sealed gas-charged cylinder) close to the offending valve, add pipe clips where the run is unrestrained, or fit a pressure-reducing valve where the underlying mains pressure is above 4 bar and shock loads are inevitable. On older properties with mixed copper and lead pipework the fix can include re-routing a short section onto modern WRAS-approved copper or push-fit with proper expansion allowance.
Where this service applies
- Loud bang when a kitchen quarter-turn lever tap is closed quickly
- Repeated thumping when the washing machine or dishwasher solenoid fills
- Knocking from inside walls or under floors during normal use of the cold supply
- Shudder or vibration on the shower hose when the head shuts off
- Pressure spikes loosening compression fittings, joints weeping at the back-nut
- Recurring pinhole leaks on older copper pipework suggesting underlying shock fatigue
- Mains pressure above 4 bar where every fast-close fixture triggers shock
- Pipework whining or singing under flow, distinct from hammer but often related
- Newly fitted appliance triggering hammer that was not there before
How we deliver
No surprises, no upselling. The exact path every job follows.
- 1
Symptom replication
We ask you to demonstrate the noise so we hear it under the same conditions you do. Most water hammer reproduces reliably and we can identify the trigger fixture inside the first 10 minutes. - 2
Mains pressure measurement
A calibrated gauge fitted at the kitchen cold tap reads static and dynamic mains pressure. Anything above 4 bar is high enough to make shock load inevitable on fast-closing valves, regardless of clipping. - 3
Acoustic trace of the run
Stethoscope used along the pipework run to locate the unrestrained section that moves under the pressure spike. Most domestic hammer noises come from a 200 to 400mm length of pipe with no clip in it. - 4
Fit a shock arrestor at the trigger
A WRAS-approved sealed-bellows shock arrestor is fitted close to the offending valve (most often the washing-machine inlet) so the pressure spike is absorbed at source rather than travelling through the pipework. - 5
Add or upgrade pipe clipping
Unrestrained pipework clipped at the recommended 1.2m centres on 15mm copper and 1.8m on 22mm copper. Clips include a rubber liner so the pipe is supported without creating a fresh contact noise. - 6
Fit a pressure-reducing valve if required
Where mains pressure is excessive, a WRAS-approved adjustable PRV is fitted on the rising main downstream of the stop-tap, set to 3 bar dynamic, with a gauge port for future verification. - 7
Retest and verify silence
Original trigger fixture cycled 10 times after the fix. We confirm the hammer has gone before we leave and document the readings before and after for your records.