Water Hammer: Why Your Pipes Bang and How to Stop It

That loud bang or shudder in the wall when a tap shuts off, or when the washing machine finishes filling, is water hammer. It happens when fast-moving water is stopped suddenly and the shock travels back through the pipes.

It’s common, it’s usually fixable, and it’s worth fixing. Every bang puts a spike of pressure through your pipes, joints and flexible hoses, and over time something gives.

What causes water hammer?

Water in a pipe is heavy and it’s moving. When a valve closes quickly, all that water has to stop at once, and because water barely compresses, the energy has nowhere to go. It turns into a pressure wave that runs back up the pipe, hits the next bend or fitting, and bounces. The bang you hear is the pipe being kicked sideways by that wave, and the rattle after it is the wave bouncing back and forth until it dies out.

The faster the water was moving and the faster the valve closed, the harder the hit. That’s why the usual culprits are almost always the same few things.

High mains pressure

A lot of homes across the Hills District and Hornsby Shire sit on high mains pressure, particularly lower-lying streets fed from a reservoir up the hill. Higher pressure means faster water, and faster water hits harder when it stops.

If the banging is house-wide and your taps blast out when you open them, pressure is the first thing we’d check.

Quick-closing valves

Washing machines and dishwashers fill through solenoid valves that snap shut in an instant. So do some fridge water lines, irrigation controllers and pool top-up valves. Quarter-turn mixer taps can do it too if you flick them off hard.

If the bang only happens when one appliance finishes filling, that appliance’s valve is doing it. The appliance isn’t faulty; it’s just exposing a system that can’t absorb the shock.

Loose pipe clips

Pipes are meant to be clipped firmly to the frame at regular spacing. Under older homes in Castle Hill, Baulkham Hills and Pennant Hills, clips have often rusted through, worked loose, or were never put in where the pipe was extended later. A loose pipe is free to jump, so a small pressure spike becomes a loud knock against a joist or the back of a wall.

Loose clips don’t cause the hammering, but they make it much louder and let the pipe wear against whatever it’s hitting.

Worn tap washers and valves

A worn washer or a loose jumper valve inside an older tap can flutter as water passes, which sounds like a machine-gun rattle while the tap is running rather than a single bang when it closes. Strictly speaking that’s chatter rather than water hammer, but people describe it the same way and it has a similar fix. Our tap maintenance guide covers it.

Why have my pipes suddenly started banging?

If the house was quiet for years and has started banging, something changed. The usual reasons:

• A new washing machine or dishwasher with a faster-closing valve than the old one.
• Sydney Water work in the street has changed the pressure, or the pressure limiting valve at your meter has failed and is no longer holding it down.
• A pipe clip has let go, especially after work under the house, or where a pipe has been knocked.
• A renovation: new tapware, a new pipe run with fewer clips, or a section of pipe that’s been rerouted with more tight bends.
• An old air chamber has waterlogged. Some older homes had a capped vertical pipe behind the laundry to cushion the hit. Over the years the air in it dissolves into the water, and once it’s full it stops working.

If it started after a specific event, tell your plumber. It usually points straight at the cause.

Is it dangerous to my pipes?

It won’t burst a pipe on the first bang. But it’s a repeated shock load, many times a day, on every joint in the line, and that adds up.

What we see fail after years of it:

• Braided flexible hoses under sinks, basins, toilets and the washing machine. These are the weak point in most homes, and a burst flexi hose can put a lot of water through the house in the time it takes you to get home from work.
• Soldered and compression joints that start to weep, often inside a wall where you don’t see it until there’s a stain or mould.
• Tapware cartridges and appliance valves that wear out early.
• Pipes rubbing through where a loose pipe has been knocking against a timber or masonry edge for years.

On the Upper North Shore, in suburbs like Wahroonga and Turramurra, we still find old galvanised pipe in the ground. That pipe is already thin from corrosion on the inside, and hammer is often what finally opens it up.

If a hose or joint has already let go, turn the water off at the meter and follow our burst pipe or leak steps while you wait for a plumber.

How to stop water hammer: what you can safely do yourself

Some fixes are simple and don’t involve touching anything under pressure.

• Close taps gently. With mixer taps, ease them off rather than slapping them shut. It sounds trivial, but on a borderline system it can be the difference.
• Find the noise. Run each tap and appliance in turn with someone listening under the house or in the roof. Knowing where it’s loudest saves time when a plumber comes.
• Re-clip loose pipes you can reach. If you can see a pipe under the house that moves when you push it, add a proper pipe clip to the frame. Use a clip sized for the pipe, and don’t overtighten plastic clips on copper.
• Pack a pipe off anything it’s hitting. A short length of pipe lagging where it touches a joist cuts the noise and stops it wearing through.
• Check your flexi hoses. Look under every sink and basin, and behind the washing machine and toilet. Any rust on the braid, kinks, bulges or damp at the ends means replace it, and those are the hoses taking the most punishment.
• Try draining the system if your house has an old air chamber. Turn off the water at the meter, open the highest and lowest taps in the house until they run dry, close them, then turn the water back on slowly. It doesn’t always work, and it won’t last forever, but it costs nothing.

Fixes that need a licensed plumber

Anything that involves cutting into pipework or changing how water enters the house is licensed work in NSW.

Pressure limiting valve

If mains pressure is high, a pressure limiting valve fitted just after the water meter brings the pressure down for the whole house. It’s often the single most effective fix, and it takes load off every tap, hose and hot water system at the same time. The plumbing code, AS/NZS 3500, limits the pressure allowed inside a home, and many older houses were built before these valves were routinely fitted.

We explain how they work and how to tell if yours has failed in our pressure limiting valve guide.

Does a water hammer arrestor work?

Yes, when it’s the right fix. An arrestor is a small sealed cylinder with a piston and an air cushion, fitted close to the valve causing the problem. When the valve snaps shut, the piston takes the shock instead of the pipe.

They work best on a single problem appliance, like a washing machine or dishwasher, and they’re usually fitted at the taps behind it. They don’t fix a whole-house pressure problem, and they don’t fix a loose pipe, so it pays to find the cause first.

Re-clipping and rerouting inside walls

When the knock is inside a wall or ceiling, getting to it means opening something up. Sometimes a section of pipe needs to be re-run with proper support and fewer sharp changes of direction.

Replacing worn valves and old pipe

Worn stop taps, failed non-return valves and corroded galvanised pipe are all common sources, and all plumber jobs.

When to call a plumber

Book a plumber if the banging is getting louder, if it’s coming from inside a wall, if you’ve found a damp patch or a weeping joint, or if you think pressure is high. If a flexi hose or pipe has already burst, it’s an emergency: turn the water off at the meter first, then call.

We work across the Hills District, Hornsby Shire and the Upper North Shore, and we’ll find what’s causing the noise before we suggest a fix. There’s no call-out fee, and you get a fixed price before any work starts (how we price). If water is already going where it shouldn’t, our 24/7 emergency plumbers can be with you fast.