What Is Water Hammer?
The washing machine finishes its fill cycle and a loud bang shakes the laundry wall. You turn off the kitchen tap and something thumps hard inside the cupboard. Fast, sharp, and louder than it has any right to be.
That's water hammer. It's one of the most common plumbing complaints we hear from Sydney homeowners, and the good news is it's almost always fixable. Often without a plumber, sometimes for free.
This guide covers what causes it, whether it's actually doing damage, and the DIY fixes worth trying today before you decide whether a professional needs to get involved.
Water hammer (technically hydraulic shock) is the banging, thumping, or clanging sound that occurs when fast-moving water is suddenly stopped or forced to change direction inside your pipes.
The Physics in Plain Terms
Water moving through your pipes carries momentum. When a valve snaps shut (a tap turned off quickly, a washing machine solenoid closing mid-cycle), that momentum has nowhere to go. It travels back through the pipe as a pressure wave, the pipe vibrates, strikes surrounding framing or joists, and you hear the bang.
Think of it like a freight train hitting a buffer at full speed. The train can't stop instantly. All that kinetic energy has to go somewhere. In your pipes, it becomes a pressure spike that can hit well above the 500 kPa legal limit.
What It Sounds Like
Water hammer isn't always a single loud bang. Depending on the cause, it can sound like:
- A sharp knock when a tap is turned off
- Rapid knocking or rattling after a washing machine or dishwasher finishes filling
- A thudding that seems to travel along the wall rather than come from one spot
- A vibrating hum during flow (slightly different from true water hammer, usually loose pipes or high pressure)
What Causes Water Hammer?
Most cases come down to one of five causes. Getting the right fix depends on knowing which one you're dealing with.
Sudden Valve Closure
The most common trigger by a long way. Quarter-turn mixer taps, solenoid valves in washing machines and dishwashers, toilet fill valves: they all shut off water almost instantly. No gradual deceleration. The water goes from full speed to zero in a fraction of a second, and the shockwave follows. Modern appliances are especially prone to this because their solenoid valves are engineered for speed, not gentle closure. If the bang happens specifically after your washing machine, dishwasher, or a mixer tap, this is almost certainly it.
High Water Pressure
High mains pressure makes everything worse. More pressure means more momentum, which means a more violent shockwave when the water stops. Under Australian Standard AS/NZS 3500, the maximum allowable water pressure at household outlets is 500 kPa. Many properties, particularly in hilly areas, regularly exceed this. Perth's Hills and Foothills are a well-known example. Mains pressure there can hit 700–800 kPa or higher overnight. But it's not just Perth. We see pressure-related water hammer in homes across Sydney, Melbourne, and Brisbane too.
If your taps splash aggressively when first turned on, or the banging happens at multiple fixtures around the house rather than just one, high pressure is likely in the mix. A pressure gauge from Bunnings attached to an outside tap will tell you exactly where you stand.
Waterlogged Air Chambers
Older homes were often built with air chambers: short vertical pipe sections near taps and appliances that trap air and absorb pressure spikes. Over time, those air pockets get displaced by water and the chambers fill up. Once waterlogged, there's nothing left to cushion the shockwave. Water hammer returns. This is one of the most fixable causes. And the fix is free.
Loose or Unsecured Pipes
Pipes that aren't firmly anchored will physically move when a pressure wave hits. That movement is what creates the banging sound as the pipe strikes wall framing, joists, or other pipes nearby. It's especially common in older homes where brackets have corroded or never been installed properly, but it can develop gradually in any home as years of vibration work them loose. If the noise seems to travel along the wall rather than coming from one fixed point, loose pipes are often the culprit.
Worn Tap Washers or Valves
A worn tap washer or damaged internal valve can vibrate rapidly under pressure, producing a chattering or hammering sound even while water is flowing, not just when it stops. This one is localised. It's happening at one specific tap, not throughout the house. Replacing the washer or tap often clears it entirely — our dripping tap guide walks you through the process. Issues with bathroom plumbing fixtures like mixer taps and shower valves are a common source of this.
Is Water Hammer Dangerous?
An occasional bang isn't an emergency. But persistent or severe water hammer is worth taking seriously, and not just because of the noise.
Repeated pressure spikes stress joints, fittings, and appliance connections over time. Joints weaken. Seals deteriorate. Fittings that were fine under normal conditions start to fail. In worst-case scenarios (usually older homes with copper or galvanised pipes and compression fittings), sustained hydraulic shock causes burst pipes. It also chews through the lifespan of washing machines, dishwashers, and hot water systems faster than they should go.
If water hammer has already caused any water leak problems at joints or connections, fix those promptly. A slow leak inside a wall will cause far more damage than the water hammer that started it.
Caution
If water hammer is accompanied by visible pipe movement, water staining on walls or ceilings, or a sudden drop in pressure, call a plumber. The system may already be damaged.
How to Fix Water Hammer Yourself
Work through these in order. A lot of water hammer cases are fixed at step one or two, without spending a cent.
Step 1: Drain and Reset the Air Chambers
The free fix. Works well for homes built before the 1990s where air chambers have become waterlogged over time.
- Turn off the main water supply at the stopcock, usually near the water meter.
- Open all taps in the house, starting at the highest point and working down, to drain as much water out of the pipes as possible.
- Flush all toilets.
- Leave everything open for 20–30 minutes.
- Turn the main supply back on slowly and close the taps one by one, starting from the lowest point.
As the pipes refill, air re-enters the chambers and the cushioning effect is restored. This fix can hold for months to years, though you may need to repeat it if the water hammer comes back.
Tip
Set aside a Sunday morning for this. The process takes 30–40 minutes and it's always worth trying before spending money on anything else.
Step 2: Secure Loose Pipes
Check all the pipes you can actually reach: under sinks, in the laundry, in the roof space or subfloor. Look for loose, broken, or missing brackets. Pipe straps and clips cost next to nothing at Bunnings and take minutes to fit. Space them every 1–1.5 metres along horizontal runs and at every change of direction.
Caution
Don't use steel straps on copper pipes. Dissimilar metals in contact accelerate corrosion. Use copper or plastic-lined brackets for copper pipework.
Step 3: Check Your Water Pressure
A water pressure gauge from Bunnings runs $20–$40. Attach it to an outdoor tap, run the tap fully, and take a reading.
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Under 500 kPa — within the acceptable range under AS/NZS 3500
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Over 500 kPa — high pressure is contributing to the problem. A pressure limiting valve (PLV) should be considered
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Over 700 kPa — this is high enough to be actively damaging pipes and appliances. A PLV is a priority, not optional
A PLV fits at the water meter and regulates all incoming pressure to a safe level. It's a professional installation and one of the most effective long-term fixes for pressure-related water hammer. The added bonus: it extends the lifespan of every water-using appliance in the house.
Step 4: Install a Water Hammer Arrestor
A water hammer arrestor is a small device (available from Bunnings and plumbing suppliers for $30–$80) that contains an air-charged chamber sealed by a piston. It installs near the source of the problem (behind the washing machine, under a sink, near the dishwasher) and absorbs the pressure spike before it travels through the pipes.
DIY-fit versions with compression fittings are widely available. Look for models rated to at least 150 PSI for washing machines and dishwashers. Unlike old-style air chambers, they don't become waterlogged, so they stay effective indefinitely.
DIY Fix Comparison
| Fix | Best For | Cost | Difficulty |
|---|---|---|---|
| Drain and reset air chambers | Older homes with waterlogged chambers | Free | Easy |
| Secure loose pipes | Pipes that rattle or move | $10–$30 | Easy |
| Check and reduce water pressure | High pressure contributing to noise | $20–$40 (gauge) | Easy to check; plumber needed for PLV |
| Install water hammer arrestor | Appliances and fast-closing valves | $30–$80 per unit | Moderate |
| Pressure limiting valve (PLV) | Whole-house high pressure | $300–$600 installed | Licensed plumber required |
When to Call a Licensed Plumber
Try the DIY steps first. Call a licensed plumber if:
- Water hammer persists after draining the system, securing pipes, and fitting arrestors
- Your pressure reads above 500 kPa and you need a PLV installed
- The noise is coming from pipes inside walls or under a slab you can't access
- Water hammer has been going on for years and you're worried about joint or fitting damage
- You notice any signs of a leak: damp patches, staining on walls, or a water bill that's crept up unexpectedly
- The banging is accompanied by a drop in pressure or changes in flow
As NSW Fair Trading advises, any work on the water supply system (PLV installation, pipe repairs, mains connections) must be carried out by a licensed plumber. Fitting an arrestor at a washing machine tap with a compression fitting is generally fine as DIY. Touching the mains line is not.
247 Local Plumbers is available 24 hours a day, 7 days a week across Sydney and Melbourne with a $0 call-out fee. We diagnose and fix water hammer issues including PLV installation, pipe securing, and arrestor fitting. Call 1300 138 780 or book online.
If water hammer has caused a burst pipe or active leak, our emergency plumbing service aims to have a licensed plumber at your door within the hour.
How to Prevent Water Hammer
Once it's fixed, a few habits keep it from coming back:
- Turn taps off gradually. Not dramatically slowly. Just don't snap them shut. Two extra seconds makes a real difference to the pressure spike.
- Check water pressure once a year. Mains pressure can shift over time, especially in areas with ageing infrastructure or significant elevation changes. A $20 gauge check is cheap preventative maintenance.
- Keep up with appliance maintenance. Worn solenoid valves in washing machines and dishwashers are a common trigger for water hammer that appears or worsens suddenly. Worth checking when you service the appliance.
- Recheck pipe brackets every few years, especially in older homes where years of vibration may have gradually worked them loose again.
Wrapping Up
Water hammer can be a minor annoyance fixed in an afternoon for free, or a symptom of pressure problems that need a professional. Start at step one, work through the DIY fixes in order, and be honest about what the pressure gauge tells you.
If the banging persists after you've been through all of it, give 247 Local Plumbers a call. $0 call-out fee, upfront pricing before any work starts. Call 1300 138 780 and we'll work out what's going on.
