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

Water hammer is the loud bang or thud you hear in your pipes when a tap, washing machine or dishwasher shuts off suddenly. Water moving through your pipes has momentum; when the flow stops instantly, that momentum has nowhere to go and slams into the closed valve as a pressure shockwave, which can spike to several times your normal water pressure. The noise is annoying, but the repeated shockwaves are the real problem: over time they loosen joints, stress fittings and can eventually cause a leak or burst pipe.

The fix ranges from tightening a pipe clip to installing a water hammer arrestor, and most causes are straightforward to identify. Here's how.

What Causes Water Hammer?

Four causes account for nearly every case:

• Fast-closing solenoid valves. Washing machines and dishwashers shut their water off electronically and instantly. This is why the bang so often happens mid-cycle in the laundry. Older tap washers closing against high pressure do a slower version of the same thing.

• High water pressure. The higher your mains pressure, the faster the water moves and the harder it slams. Anything much above 500 kPa makes hammer dramatically worse, and pockets of the Northern Rivers network run high, particularly lower-lying streets in Ballina and Lennox Head where the mains pressure has less elevation to overcome.

• Loose or unsecured pipes. A pressure wave in a well-clipped pipe is a dull thud. The same wave in a loose pipe lets it physically jump and rattle against timber, which is the machine-gun knocking some homes get behind walls.

• A failed or waterlogged arrestor. Newer homes often have small air chambers or arrestors already installed. When these fail or fill with water, hammer appears in a house that never had it before.

How to Stop Water Hammer

Work from cheapest to most involved:

1. Close the offending tap slower. If hammer only happens on one manually operated tap, slower closure confirms the diagnosis instantly.

2. Slightly close the appliance's isolation tap. For washing machine or dishwasher hammer, turn the tap that feeds the hose in by a quarter to half a turn. Reducing the flow rate reduces the slam. This is the single most effective free fix.

3. Secure loose pipes. Where pipes are accessible, under the house, in the ceiling, refit or add pipe clips so nothing can move. Leave a little room for thermal expansion; don't clamp hot water pipes dead rigid.

4. Install a water hammer arrestor. A small sealed device with an air cushion inside, fitted at the offending appliance or fixture, that absorbs the shockwave. Screw-on arrestors for washing machine taps are a simple job; inline arrestors within the pipework are a plumber's job.

5. Fit a pressure limiting valve. If your whole house hammers and pressure is the underlying cause, a pressure limiting valve at the meter brings the entire system down to around 500 kPa. (The opposite complaint, low water pressure, starts with the same gauge test.) This also extends the life of your hot water system, flexi hoses and tap washers, all of which age faster under high pressure.

Is Water Hammer Dangerous?

To you, no. To your plumbing, yes, cumulatively. Every bang is a pressure spike travelling through every joint, fitting and hose in the system. The most common casualties are flexi hoses under sinks and vanities, which are already the leading cause of home water damage claims in Australia, and older fittings behind walls where a slow failure goes unnoticed. If a long-standing hammer problem has recently been joined by damp patches, an unexplained jump in your water bill or the sound of running water when everything is off, treat it seriously; that's leak detection territory, and finding it early is the difference between a repair and a restoration.

When to Call a Plumber

Call if the hammer is house-wide, if it's coming from inside walls where nothing can be secured, if it appeared suddenly without any change to your appliances, or if the quarter-turn trick and a screw-on arrestor haven't fixed it. House-wide hammer usually means a pressure problem or a failed valve, and both are quick to test with a gauge.

Jesse at JF Plumbing sorts water hammer across Lennox Head, Ballina, Byron Bay and the wider Northern Rivers, from a single noisy laundry tap to whole-of-house pressure regulation. With over 8 years of plumbing experience, his first step is a pressure test at the tap, because guessing at hammer wastes everyone's time and the gauge doesn't lie.

FAQ

Why do my pipes bang when the washing machine runs? The machine's solenoid valve shuts the water off instantly, sending a shockwave back through the pipe. Partially closing the machine's isolation tap or fitting a screw-on arrestor usually fixes it.

Can water hammer burst a pipe? Not usually in one hit, but repeated pressure spikes fatigue joints, fittings and flexi hoses over time, and those are what eventually fail. Long-standing hammer is worth fixing for that reason alone.

How much does a water hammer arrestor cost? Screw-on arrestors for washing machine taps are inexpensive hardware-store items. Inline arrestors and pressure limiting valves cost more because they involve cutting into the pipework, but they solve the problem at its source.

Why has water hammer suddenly started in my house? Something changed: mains pressure in your street increased, an existing arrestor failed, a pipe clip let go, or a new appliance with a fast solenoid valve arrived. Sudden onset is usually the easiest kind to diagnose.

Pipes shouldn't announce themselves. JF Plumbing services the whole Northern Rivers. Meet Jesse and the team or call 0412 230 635.