The short answer
Water hammer is the bang or thud you hear when water moving through a pipe is stopped suddenly, usually by a tap, dishwasher or washing machine valve closing fast. The water slams into the closed valve and the shock travels back down the pipe. High water pressure and loose pipes make it worse. A pressure limiting valve, arrestors and pipe clips fix it.
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It’s the washing machine that gives it away. The rinse cycle finishes filling, the valve clicks off, and somewhere in the wall there’s a thud like someone dropped a boot in the roof. Most households get used to it. Some start explaining it to visitors.
It’s called water hammer, and it can be fixed. The table matches the noise you’re hearing to its usual cause.
| What you hear | When | Most likely cause |
|---|---|---|
| One loud bang or thud | Washing machine or dishwasher finishes filling | Fast-closing appliance valve |
| A bang when a tap is shut quickly | Mixer taps, especially quarter-turn | Fast-closing tap, often with high pressure |
| Banging all over the house | Most taps and appliances | Water pressure above 500 kPa |
| A rattle or knock in the wall | After a bang | Loose, unclipped pipes moving |
| A shudder or hum while the tap runs | Tap half open | Worn jumper valve or washer, not water hammer |
What causes water hammer
Water hammer happens when water moving through a pipe is stopped suddenly. The moving water slams into the closed valve, and the shock travels back along the pipe as a pressure wave. The bang is that wave hitting the pipework.
Water is heavy and it doesn’t compress, so a pipe full of it moving at speed carries a lot of momentum. A valve that closes gently lets the water slow down. A valve that snaps shut in a fraction of a second doesn’t.
That’s why modern houses tend to bang more than old ones. Washing machines and dishwashers fill through solenoid valves that close almost instantly, and a quarter-turn mixer tap can go from full flow to shut in one flick of the wrist. The old screw-down tap took four turns to close, and the water had time to slow down.
Why high water pressure makes it worse
The higher the pressure in the pipe, the faster the water moves and the harder the shock when it stops. A house with high mains pressure can bang every time anything shuts off, where the same fittings at lower pressure would be quiet.
The Australian plumbing standard, AS/NZS 3500.1, limits the static water pressure at any outlet inside a building to 500 kPa. Victoria’s plumbing regulator sets out in its practice note on maximum static water pressure within a building that a pressure limiting valve is required where the supply exceeds 500 kPa, and that applies to new buildings, extensions and renovations alike.
Mains pressure isn’t the same everywhere. It depends partly on how far your house sits below the water authority’s supply, so a home at the bottom of a hill can see noticeably higher pressure than one at the top of the same street. A plumber can test it with a gauge on an outside tap in a few minutes, and it’s the first thing worth checking if the whole house bangs.
Is water hammer doing any damage?
It won’t hurt you, but it’s hard on the plumbing. Each bang is a spike of pressure well above the normal working pressure, and it lands on every joint and valve in the line.
- Flexible braided hoses under sinks and basins
- The inlet valves on washing machines and dishwashers
- Soldered and compression joints in copper pipe
- Tapware cartridges and valves
- Pipe clips, which work loose and let the pipe move more next time
None of that fails overnight. But a joint that’s been hit a few thousand times is more likely to start weeping inside a wall, and a hidden leak is a much bigger job than a noisy pipe. If you’re already seeing damp patches, our burst pipes and leak detection page covers what happens next.
How to stop water hammer
The fix depends on the cause, which is why it’s worth working out which kind of banging you’ve got before buying anything.
| Fix | What it does | Best for |
|---|---|---|
| Pressure limiting valve | Holds the house supply at a safe pressure | Banging all over the house, pressure above 500 kPa |
| Water hammer arrestor | A small chamber that absorbs the shock at one fitting | A washing machine or dishwasher that bangs |
| Re-clipping pipes | Stops loose pipe hitting the frame | Rattling after the bang |
| Replacing a worn jumper valve | Stops a vibrating tap | Knocking only when a tap is half open |
A pressure limiting valve goes on the supply, usually near the meter, and holds everything downstream at a safe pressure. It’s the right fix when the whole house bangs, and it takes strain off every tap, hose and appliance at the same time.
An arrestor is a small sealed chamber fitted at the tap that feeds an appliance. When the valve snaps shut, the chamber takes the shock instead of the pipe. It’s the usual cure when only the washing machine or dishwasher bangs.
Loose pipes make any bang louder, because the pipe jumps and hits the timber around it. If you can get under the house or into the roof, look for copper pipe that isn’t clipped or has pulled away from its clips. Refixing it won’t stop the pressure wave, but it can stop the knock that follows it.
You can also help by closing mixer taps a little more gently. Slowing the handle over the last bit of its travel gives the water time to stop.
Fitting a pressure limiting valve or an arrestor is licensed work in Victoria, because both are connected to the water supply. If you’d like a sense of what a plumber’s visit costs before you book one, our guide to how much a plumber costs in Melbourne has the current rates.
Knocking that isn’t water hammer
One noise gets mistaken for water hammer and has a different cause. If a pipe shudders, hums or knocks while a tap is running half open, and goes quiet when the tap is opened fully, the likely cause is a worn jumper valve or washer vibrating inside the tap.
That’s a small repair on the tap itself. The same worn part is often what makes a tap drip, which is why the fix overlaps with how to fix a leaking tap.
Banging when the hot water is running, especially from a storage unit, can also be something else again: a build-up of sediment in the tank. If the noise comes from the hot water unit itself rather than the pipes, have a look at our hot water systems page.
Read this next: How to fix a leaking tap