You turn off a tap and hear a loud bang in the pipes. Or the pipes clunk when the washing machine fills. This is water hammer.
Water hammer is loud and unsettling, but it’s not always dangerous. Understanding what causes it and when to worry helps you know whether to fix it immediately or let it ride.
What water hammer is
Water flows through a pipe at speed. When a tap closes quickly, the water stops suddenly. The moving water doesn’t stop instantly. It slams against the closed tap, creating a shock wave that travels backward through the pipe. This shock wave bounces around inside the pipe, causing the banging you hear.
Think of a train stopping suddenly. The momentum creates a jolt. Same principle.
Why water hammer happens
Old taps with fast-closing valves cause it. Modern taps close slower and reduce hammer. High water pressure makes it worse. The faster the water moves, the harder it hits when it stops.
Washing machines and dishwashers cause hammer. They fill quickly then shut off abruptly. The sudden stop creates a shock wave.
Loose pipes make the banging louder. If a pipe isn’t properly secured, the shock wave makes it rattle and clang against whatever’s nearby.
Is water hammer dangerous
Usually not immediately, but it can be a warning sign. A single bang when you turn a tap off is normal. Repeated banging every time something happens, or constant knocking in the pipes, suggests pressure is high enough to stress the system.
Over years, this stress can weaken joints and fittings. A valve might crack. A solder joint might fail. A leak could develop. Water hammer isn’t dangerous in itself, but chronic hammer creates conditions for damage.
How to stop water hammer
Install an arrestor. This is a small chamber filled with air. When the shock wave hits it, the air compresses and absorbs the impact, stopping the bang. Installation costs GBP 150-300 and solves the problem permanently.
Reduce water pressure. If pressure is above 3 bar, lower it to 2.5 bar or below. A pressure-reducing valve costs GBP 100-200 to install. Lower pressure also reduces water hammer intensity.
Secure loose pipes. If pipes rattle against walls or fixtures, strap them down. Pipe clips and rubber-lined clamps stop vibration. Cost: GBP 20-50 for materials.
Replace old taps with slow-closing ones. Modern taps have a damping mechanism that closes over a second or two instead of instantly. This eliminates hammer. Cost: GBP 100-300 per tap.
If you have a water hammer arresto already and it’s still banging
The arrestor may have lost its air charge. Over time, water leaks into the air chamber and the device stops working. It needs recharging or replacement. A plumber can do this (cost: GBP 50-100 for recharging, or GBP 150-200 to replace).
Pressure may have increased. If pressure rises above what the arrestor is rated for, it won’t absorb all the shock. Check your pressure (use a pressure gauge available at any plumbers’ merchant for GBP 10-20). If it’s above 3 bar, a pressure-reducing valve helps.
When to call a professional
If banging is violent, frequent, or getting worse, call a plumber. Persistent water hammer indicates high pressure or damaged pipes. A professional can test pressure, inspect pipes, and recommend permanent fixes.
If water hammer started recently and you’ve made no changes, a pipe might be damaged. Don’t ignore this. Call a plumber to investigate.
If banging is accompanied by leaks, reduced pressure, or visible pipe damage, it’s a plumbing emergency. Call immediately.
FAQs
1. Is water hammer normal?
An occasional bang when you shut a tap fast is normal. Frequent banging every time something closes, or continuous knocking, is not. It suggests high pressure or system stress. Have it checked.
2. Can water hammer damage my pipes?
Not from a single bang. But chronic banging puts stress on joints and fittings. Over time, this can crack valves or fail solder joints, leading to leaks. Fix recurring hammer to prevent damage.
3. How do I stop water hammer?
Install a water hammer arrestor (GBP 150-300). Reduce water pressure with a regulator (GBP 100-200). Strap loose pipes to stop rattle (GBP 20-50 DIY). Replace old taps with slow-closing ones (GBP 100-300).
4.Why does my washing machine cause water hammer?
It fills quickly then shuts off abruptly. The sudden stop creates a shock wave. Installing an arrestor on the fill line solves this specifically (GBP 150-250). It stops just when the machine fills, not affecting other taps.
