Water Hammer: Why Your Pipes Bang & How to Fix It

Updated July 2026 · 7 min read · Geelong Emergency Plumbing

Water hammer is the percussion instrument nobody wanted in their wall — a bang, clang or thud that happens whenever a tap or solenoid valve closes quickly. It's alarming, it's annoying, and it's telling you something specific about your plumbing that's worth understanding before the repeated pressure shocks start doing what repeated shocks do to pipe joints over years.

What Water Hammer Actually Is

When flowing water in a pipe is stopped suddenly — by a tap, a dishwasher valve, a washing machine solenoid — the moving water's momentum has nowhere to go. The pressure spike at the point of closure can reach several times normal mains pressure in a fraction of a second, and it travels as a shock wave back through the pipe system. That shock hitting a fitting, a bend or an unsecured section of pipe creates the bang. The physics is the same as a water pipe version of slamming a door on a moving crowd.

It's worth separating from the two things it's sometimes confused with: ticking pipes (thermal expansion and contraction of copper, normal, not water hammer), and vibrating pipes during flow (usually a loose washer or worn fitting that chatters at certain flow rates — different cause, different fix).

Common Causes

• High mains water pressure — the force of the shut-off scales with the pressure. Homes above ~500 kPa (Geelong's mains can run higher in some areas) are more susceptible; a pressure-limiting valve is the fix if pressure is the root cause.
• Solenoid-valve appliances — dishwashers, washing machines and irrigation systems have fast-acting solenoid valves that close almost instantaneously, creating sharp hammer. Older appliances with worn valves are particularly sharp.
• Unsecured pipes — pipes that aren't held firmly by clips or brackets amplify the shock wave into physical movement and noise against the wall framing.
• Air chambers depressurised — some older plumbing used air chambers (short vertical pipe stubs above the supply line) as buffers. Water logs these chambers over time, eliminating the air cushion.

Fixes: From Easy to Definitive

1. Re-charge waterlogged air chambers

If your system has air chambers, turn off the main supply, open the lowest tap in the house to drain the lines, and leave it for a few minutes. When you restore supply, the air is pulled back into the chambers before the water refills. Costs nothing; works if air chambers are the mechanism. May need repeating every year or two.

2. Secure loose pipes

If the banging is localised to a specific wall and sounds like physical impact, unsecured pipes are likely amplifying the shock. Adding pipe clips — plumbing-supply stores stock the right sizes for copper and PVC — to accessible pipes (under sinks, in cupboards, in the roof space) absorbs movement. Pipes in walls can be braced at access points or dampened with rubber-backed clips.

3. Slow the closing appliances

Some appliances allow valve-closing speed adjustment; many modern dishwashers have a service menu option. More practically: an inline hammer arrestor fitted to the washing machine or dishwasher supply is a $15–$30 part that absorbs the shock at the source. Plumbers fit them routinely and they're the cleanest solution for appliance-specific hammer.

4. Install a pressure-limiting valve (PLV)

If mains pressure is above 500 kPa (a plumber can test this with a pressure gauge — 60 seconds at the hose tap), a PLV on the incoming supply is the root-cause fix. It limits pressure to a set point (typically 350–450 kPa), reducing hammer and extending the life of every fitting, appliance and flexi hose in the house. Cost installed: $400–$900 depending on access. It's also the fix that addresses the long-term risk of high-pressure water damage that water hammer is a symptom of.

5. Expansion tank / dedicated arrestors

For systems with check valves or backflow prevention where pressure has nowhere to go during thermal expansion, a dedicated expansion vessel absorbs the overpressure. Relevant for some hot-water configurations — the plumber who diagnoses the pressure will identify whether this applies.

When Water Hammer Becomes Urgent

Occasional, minor water hammer is an annoyance. Repeated, severe water hammer — especially in older homes with soldered copper joints — progressively stresses joints until they begin to weep or fail. If the hammer has been present for years and you're starting to see any damp patches or unexplained staining near pipe runs, get the pressure tested and the fix implemented before the joint decides to make the point more dramatically. A leak detection service can check for early-stage joint stress.

If you're unsure whether pressure is the underlying cause, a simple test: buy or borrow a pressure gauge that fits a garden tap (available at plumbing suppliers for $20–$40), attach it to an outdoor tap and check the static reading. Under 500 kPa is acceptable; above that and the pressure-limiting valve conversation is warranted for both the water hammer and the long-term health of every fitting, flexi hose and appliance in the house. The reading takes two minutes and the gauge earns its cost with one accurate diagnosis.

FAQs

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