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Water Hammer After a New Shower Mixer? Here's What's Happening
Water Hammer After a New Shower Mixer? Here's What's Happening, a Prime Plumbing & Gasfitting plain-English guide to water hammer after new shower mixer for Melbourne homeowners. Below we cover what works, what doesn't, and when to call a licensed plumber.
Quick answer: A new wall-mounted or lever-action shower mixer closes the water flow much faster than the old screw-down twin pillar taps it replaced. On older Melbourne plumbing, that fast closure produces a shockwave (water hammer) the original supply line wasn't designed for. The fix is usually a water hammer arrestor at the cold branch behind the mixer, a 30-minute install that doesn't require ripping out the new mixer or the tiles.
If you've recently renovated a bathroom and now hear a loud bang every time you shut the shower off, you're not imagining it, and it's a much more common pattern than the renovator who fitted the mixer probably told you.
Why the new mixer triggered the hammer
Old twin-pillar shower taps (the ones you turn separately for hot and cold) close slowly. You can't shut them in less than half a second, physically impossible with a hand on a screw-down mechanism. That slow closure gave the water column time to decelerate gradually, and the supply line never saw a real shockwave.
A modern wall-mounted lever-action mixer does it differently. The internal cartridge closes the water in well under a tenth of a second when you flip the lever down. That's an order of magnitude faster than the old taps, and the water column going through the supply line at peak flow has all that momentum to shed somewhere.
Where does it go? Into a shockwave that travels back through the pipe.
If the supply line was sized and supported for the old slow-closing taps, it now suddenly has to handle a much sharper pressure spike on every shut-off. That's water hammer. (For the underlying mechanics, see what causes water hammer.)
What we typically find when we attend the callout
When we look at a "new mixer + new water hammer" situation on a Melbourne renovation, it's usually one of three patterns:
Pattern 1, Original supply line, new fast-closing valve
Most common. The renovator replaced the tap-set but didn't touch the supply line. The line is the original 1980s/90s copper, sized fine for the old taps but now subject to the new fast closure.
Real example: On a recent Hawthorn callout, the homeowner had spent a small fortune on a renovation and the new shower mixer triggered a bang loud enough to hear in the bedroom. Inspection showed the supply line was the original copper from when the house was built, totally fine for slow-closing taps, but underspec'd for the new cartridge mixer's fast shut. We fitted a hammer arrestor at the cold supply branch (concealed behind the wall plate so you can't see it) and the bang stopped. Cost: well under what the new mixer itself cost.
Pattern 2, Mains pressure was always high, just never noticed
Slightly less common but worth checking. If the incoming mains pressure is over 500 kPa, your old taps were managing the pressure with their slow closure. The new mixer can't.
The fix isn't an arrestor, it's a pressure-limiting valve at the meter, which makes every fixture in the house close cleaner. See how to fix water hammer for the broader fix list.
Pattern 3, Pipe was unsupported and the new flow rate is amplifying the noise
Sometimes the renovator opened up the wall and the supply pipe behind it lost support during the work, the original clip is gone, the pipe sits loose against framing. Now any shockwave makes the pipe knock against framing. The bang is partly from the shockwave and partly from the mechanical knock.
This one's diagnosis-only, fitting an arrestor without re-securing the pipe will reduce but not eliminate the noise. We'd recommend re-securing as part of the fix.
What you can do before booking a plumber
A few quick checks that help us scope the fix accurately:
- Listen for which side. Does the bang happen when you turn the shower OFF, or when it's running and you change temperature? "Off" suggests the cold supply or hot supply specifically. "Mid-flow" suggests the cartridge itself.
- Check other fixtures. Does the dishwasher or washing machine also bang now? If yes, it's probably mains pressure, not the shower specifically.
- Check the warranty. Some mixer warranties (Methven, Phoenix, Dorf, Caroma) cover early cartridge failures. Water hammer can stress cartridges, so it's worth checking before the install is too old.
- Don't ignore it. The bang itself isn't dangerous, but cumulative shockwaves on a brand-new mixer can wear the cartridge faster than it should. Cartridge replacements aren't cheap.
The fix: hammer arrestor at the cold supply branch
For "new mixer + new hammer" specifically, the standard fix is a water hammer arrestor fitted at the cold supply branch behind the mixer. The arrestor has a sealed internal piston or air chamber that compresses to absorb the shockwave when the cartridge closes.
The arrestor sits behind the wall plate so you can't see it. The install:
- Turn off water at the meter
- Drain the affected line at the lowest point
- Open the access panel or wall plate
- Cut and fit the arrestor as a tee at the cold supply branch (or both hot and cold if both lines hammer)
- Pressure test
- Close and re-fit the wall plate
Total time on-site: 30-60 minutes per mixer, depending on access. We don't usually need to disturb the new mixer itself or the tiling.
Service area
Prime Plumbing & Gasfitting fixes water hammer issues across Melbourne's eastern, south-eastern, inner-east and bayside suburbs. See all suburbs we service
Book a fix
New mixer banging every time you shut the shower? Call 0475 407 670 or send your address through the contact form.
- BPC #103414: Plumbing Industry Commission licensed
- Type A Gasfitter: registered with Energy Safe Victoria
- 4.8 stars on Google
- Fixed-price written quote before any work
- Compliance Certificate issued where the work requires one
For the broader picture: see what causes water hammer and how to fix water hammer.
About the author
Alister Williams is the founder and lead plumber of Prime Plumbing & Gasfitting. He's been licensed under BPC #103414 for over a decade, holds Type A Gasfitter endorsement with Energy Safe Victoria, and is a member of the Master Plumbers Association of Victoria.
What this fix does NOT need
A few things you don't need that the internet sometimes recommends:
- Tearing out the new mixer: not required. The mixer isn't the problem; the supply line is.
- Replacing the supply line: rarely required for a single mixer. We'd only suggest this if the supply line was already at end-of-life from age.
- Returning the mixer to the manufacturer: it's not a defective mixer, it's working as designed. Manufacturers don't accept water hammer as a fault.
- Draining the whole house: this is American advice for systems that use field-built air chambers. Australian residential plumbing doesn't typically use those.
When to call us
Book a water hammer diagnosis if:
- Bang started after a recent bathroom or kitchen renovation
- You can hear it from another room
- You've checked it's not just one tap
- The new mixer is still under warranty (we want to fit the arrestor before the cartridge wears prematurely)
When to Call a Licensed Plumber
Some plumbing problems are DIY-friendly; others need a licensed professional under Victorian law.
Anything involving gas, sewer drainage, hot water units, backflow prevention, or work behind tiled walls must be handled by a licensed plumber or gasfitter.
Frequently asked questions
It normally appears within the first week, as soon as you start using the new fixture daily. If it's appearing months later, the cause is more likely mains pressure that's drifted up (water authority adjustments) rather than the renovation itself.
Possibly, depending on contract terms. Most renovation contracts include "leave the plumbing in working order", water hammer might fall under that. But many tradespeople will argue it's a side-effect of the new fixture's design, not their installation. Worth asking; not always worth fighting over given the fix is typically a 30-minute job.
Anything connected to the mains supply line in Victoria requires a licensed plumber. If a DIY install leaks and damages the property, your insurance may deny the water-damage claim. The cost saving from DIY isn't worth the insurance risk, verify any plumber's licence at bpc.vic.gov.au.
No, a hammer arrestor sits on the supply line, not on the mixer itself. The mixer warranty isn't affected. In fact, fitting an arrestor often extends the cartridge's working life because the cartridge isn't repeatedly shock-loaded.
Indicative range, we quote in writing first after we've seen the access. A single arrestor install on an accessible cold supply branch is the smallest scope; multiple arrestors or pressure-limiting valve at the meter is more involved. We'll quote the cheapest viable option alongside any related work that should be done at the same time.
Before You Book
A quick checklist to share with your plumber when you book:
- When did the issue start?
- Is it isolated to one fixture or multiple areas?
- Are there any visible leaks, smells or unusual sounds?
- Have you turned off the relevant isolation valve?
If you're unsure, Prime Plumbing & Gasfitting can inspect the issue and explain the options before any work starts. Call 0475 407 670 or request a quote online.
