What Is Water Hammer, and Why Are Your Adelaide Pipes Banging?

What water hammer actually is

Water has mass, and water moving down a pipe has momentum. Close a valve slowly and that momentum bleeds away harmlessly. Close it in a fraction of a second and the momentum has nowhere to go, so it converts into pressure at the face of the valve. The resulting spike travels back up the pipe as a wave at well over 1,000 metres per second, reflects off the next fitting, and runs back and forth through the system until friction absorbs it.

The arithmetic is the part that makes people take it seriously. Stopping water travelling at just 1 metre per second instantaneously produces a pressure rise in the order of 1,200 kPa. Normal household supply pressure sits somewhere between 350 and 500 kPa, so a single bad shut can momentarily triple or quadruple the load on every joint, every tap seat, every flexible hose and every solder connection in that run.

Why Adelaide mains pressure is usually underneath it

Every water hammer event starts from whatever pressure the system was already sitting at, so the higher the standing pressure, the harder every bang lands. SA Water commits to delivering a minimum pressure at the meter. There is no equivalent obligation to cap the maximum, because controlling pressure inside the property line is the owner's responsibility, and it is done with a pressure limiting valve at the meter.

The current Australian standard for plumbing installations caps static pressure at any outlet in a new installation at 500 kPa. Homes plumbed before that requirement came in, which covers a large share of Adelaide's pre-1990s housing stock, frequently have no limiting valve at all. Suburbs sitting low relative to the reservoir that feeds them can run well above 500 kPa, and the owner has no way of knowing without putting a gauge on an outside tap.

A pressure limiting valve is a consumable, not a permanent fitting. They typically last 10 to 15 years and then drift out of adjustment or fail open, which is a common reason a house that was quiet for a decade suddenly starts banging with no other change.

The 5 causes, and what each one sounds like

Older homes were often plumbed with air chambers: short capped lengths of vertical pipe that trapped a pocket of air to act as a cushion. The design works, but the air dissolves into the water over a few years and the chamber waterlogs, at which point it does nothing. That is why a house can be quiet for 15 years and then start banging without a single thing being changed. Draining the system down restores the cushion temporarily. A modern arrestor with a sealed bladder or spring does not waterlog and is the permanent version of the same idea.

Pipe material changes what you hear rather than whether it happens. Copper is rigid and transmits the impact efficiently, so hammer in a copper house is loud and sharp. PEX and other flexible plastics expand slightly under the spike and absorb part of it, so the same fault in a newer house is a duller thud.

Find your cause in 10 minutes

1. Write down exactly what you were using when the bang happened, and where in the house the noise seemed to come from. Timing is the single most useful piece of information, and it is free.
2. Run the washing machine and the dishwasher one at a time with nothing else running. If the bang tracks a specific appliance, the solenoid valve on that appliance is the trigger and the fix is local.
3. Shut every mixer tap in the house quickly, one by one. If they all bang, the cause is pressure, not any single fitting.
4. Fit a pressure gauge to an outside tap. Any hardware store carries them and they screw straight onto a hose bib. Take the reading with nothing else running, then again while a tap is open.
5. Open the cupboards under the sinks and look at how the pipes are held. A run that can be moved sideways by hand is a run that is striking something during every event.
6. Flush each toilet and listen at the end of the refill rather than at the start. Knocking that only appears in the last second of a fill is the cistern inlet valve.

The fixes, simplest first

1. Re-clip and cushion the offending run. Proper saddles with a rubber or felt liner, at the spacing the standard requires. On an accessible run under a sink or in a subfloor this is a small job with a large effect on the noise.
2. Fit mini arrestors at the washing machine and dishwasher inlets. They absorb the spike at the point it is generated rather than after it has travelled the house.
3. Replace worn tap internals. Jumper valves and seats damaged by previous hammer events chatter, which adds a second noise on top of the first.
4. Replace a chattering cistern inlet valve with a slow-closing type. This is a small part and it removes the cause rather than muffling it.
5. Fit or replace the pressure limiting valve at the meter. This is the structural fix when the gauge reads high, and it protects every hose and fitting in the house at the same time.

Banging that is not water hammer

This is the section worth reading before booking anyone, because several household noises get labelled water hammer and are nothing of the sort. Fitting arrestors to any of these achieves precisely nothing.

• A tick, crack or creak starting a few minutes after a hot shower and fading over 10 minutes. That is thermal expansion: a hot copper pipe growing against a joist or a tight penetration. It arrives after the water use, not at the instant of shut-off, and in most cases it needs nothing done to it at all.
• A hum, shudder or squeal while a tap is part open, which stops the moment you open it further. That is a worn washer or jumper valve vibrating in the flow. It is a tap repair, not a pressure problem.
• A rumble or kettling noise from the hot water unit itself, worst when it is heating. That is sediment on the bottom of the cylinder, and it is a hot water job.
• A single dull knock from the roof space on a hot afternoon with no water running anywhere. That is roof sheeting or framing moving, not plumbing.
• A rattling downpipe or gutter bracket in wind or rain. It is worth ruling out before anyone opens a wall.

The test that separates them is timing. Water hammer is instantaneous and it is always tied to something stopping: a tap closing, an appliance valve shutting, a cistern finishing its fill. If the noise arrives after the water use rather than at the end of it, look elsewhere.

What happens if you leave it

Water hammer is one of the few plumbing faults that is genuinely progressive. Each spike works soldered joints and threaded connections, degrades tap seats, and fatigues the braided hoses under the basins. Failure comes from repetition, and it lands at the weakest point rather than the noisiest one, which is why the leak rarely appears where the banging was loudest.

The failure that matters most is a flexible braided hose letting go under a vanity while nobody is home. That is a burst supply line running at full mains pressure into a cabinet, hour after hour, and the damage it does is on another scale entirely from the valve that would have prevented it. Fixing the hammer is worth doing for the noise. It is worth more for what it prevents.

Marc is a licensed plumber and gas fitter with 20 years of experience and carries a pressure gauge on every job, so getting a reading is usually a matter of asking. You get a text with an arrival time before he sets off, and the price is discussed up front before anything starts.

Questions we get asked

References

• Standards Australia: AS/NZS 3500 Plumbing and Drainage
• SA Water: supply pressure and the meter
• Government of South Australia: plumbing regulation