WATER SERVICES

Water hammer: causes, fixes and compliance

Water hammer is the bang you hear when a fast-closing valve stops moving water dead and the pressure has nowhere to go. It is not just noise, it stresses the pipework, and the fix is part of doing the job properly.

What water hammer actually is

Water moving through a pipe carries momentum. Shut a valve quickly and that momentum has nowhere to go, so it spikes the pressure and slams back through the line as a shock wave. You hear it as a bang or a knock. Over time those repeated surges loosen fittings, stress joints, and wear out appliance valves.

Why it happens, including flexible hoses

The trigger is almost always a fast stop in flow: a quick-turn tap, a washing machine or dishwasher solenoid snapping shut, or a lever mixer flicked off. Modern flexible hose connections with quarter-turn fittings close fast, which makes them a common source. Higher water velocity in the pipe makes the surge worse, which is why keeping velocity within sensible limits is part of the picture.

Arrestors and the velocity idea

A water hammer arrestor gives the surge somewhere to go: a small sealed cushion that absorbs the shock instead of letting it ring through the pipe. Fit it near the cause, the fast-closing valve. The other half of the job is design: sizing pipe so water velocity stays within the limits the current standard sets, because slower-moving water carries less momentum to begin with. Check the standard for the figures rather than guessing them.

The fix that passes inspection

• An arrestor fitted close to the offending valve, not at the far end of the run.
• Pipe sized so velocity stays within limits, not undersized to save a bit of copper.
• Fixings and supports done properly so the pipe is not free to move and knock.

Photograph the arrestor in place and the supported run, and note where you fitted it. Elemetric keeps that with the job.

Common questions

What causes water hammer?

A sudden stop in flow, usually from a quick-closing valve, a solenoid in an appliance, or a flicked-off mixer. The moving water hits the closed valve and sends a pressure surge back through the pipe, which is the bang you hear.

Do the newer flexible hose connections cause water hammer?

They can. The faster a tap or valve shuts, the bigger the surge, and modern quarter-turn and lever fittings on flexible connections close quickly. Where hammer shows up, an arrestor and sensible velocity sort it out.

General information for licensed tradespeople, not legal or regulatory advice. The licensed plumber remains solely responsible for compliance. Refer to the current AS/NZS 3500 standards and the Building and Plumbing Commission (formerly the VBA) for authoritative requirements.