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

Water hammer is the bang you hear when fast-flowing water hits a sudden stop — a washing-machine valve closing, a single-lever tap slammed shut, a fast solenoid on a shower. The fixes are clip loose pipes properly, fit hammer arrestors on fast-closing appliances, and check / reduce your mains pressure if it's running above 4-5 bar.

What water hammer actually is

Water is, near as makes no difference, incompressible. When a column of moving water is stopped suddenly by a closing valve, it can't squash — instead it generates a pressure shock wave that runs back up the pipe at hundreds of metres per second. The spike can briefly hit several times the static system pressure. You hear that energy as a bang, and you feel it as the pipework jumps.

Common causes in UK homes

• Fast-closing solenoid valves — washing machines, dishwashers, ice-makers and some shower mixers all use them. They snap shut in milliseconds.
• Single-lever (mixer) taps slammed off — particularly common in kitchens.
• High mains pressure — Thames Water and Affinity in much of London / north of London can run 5-7 bar at the boundary in some streets. Without a PRV, that's hammering through your whole house.
• Unclipped pipes — copper or plastic that's loose against joists or studs amplifies the bang. A properly clipped pipe still has water hammer; you just don't hear it.
• Failed air gaps / lost air pockets — older systems had vertical "stand pipes" full of trapped air to absorb shock; once they fill with water they don't work any more.
• Failing toilet ballcock — old brass Portsmouth ballcocks judder and hammer as they close. A modern bottom-entry float valve fixes it.

Diagnose what's banging

1. 1

   Identify the trigger

   Does it bang when the washing machine fills? When a tap closes? At random when nothing's running? The trigger tells you where to look.

2. 2

   Walk the pipework

   Put your hand on accessible pipes (under the sink, in the airing cupboard) while someone runs and stops the trigger tap. You'll feel where the shock is loudest.

3. 3

   Check mains pressure

   If you have a hose tap with a gauge fitting, take a static reading. Above 4-5 bar at the kitchen cold tap is high — a PRV will quieten the whole house.

4. 4

   Check pipe clipping

   Look in the loft, under floors and at the back of units. Pipes should be clipped at roughly 1.2-1.5 metre intervals on copper and 0.5-1 metre on plastic.

How to fix it

1. Clip loose pipework

The cheapest, most effective starting point. Single-clip a loose pipe against the nearest joist or stud with a proper saddle clip. Two or three clips often stop the bang entirely. Don't bury the pipe in foam — clipping rigidly is what works.

2. Fit hammer arrestors at fast-closing appliances

A water-hammer arrestor is a small in-line device with a sealed gas-charged or spring-loaded piston that absorbs the pressure spike. Common formats:

• Mini-arrestors that screw between the appliance valve and the inlet hose — five minute fit on a washing machine or dishwasher.
• In-line arrestors that a plumber pipes into the cold and hot feeds permanently.

Fit one to each of cold and hot at the appliance — that catches both directions of shock.

3. Bring mains pressure down with a PRV

If your static mains pressure is above 4-5 bar at the kitchen tap, you're feeding the whole house a constant overdose. A pressure-reducing valve on the rising main brings it to a stable ~3-3.5 bar, dramatically reducing hammer and extending the life of every fitting in the house. A failed PRV — see our low pressure guide — can also cause hammer when it sticks open intermittently.

4. Replace old ballcocks

A juddering Portsmouth ballcock in a toilet cistern or cold tank is a classic source of slow-cycle hammer. Replacement with a modern diaphragm-type float valve is a 20- minute job and removes the trigger entirely.

What not to do

• Don't shove foam or rags around a banging pipe — it muffles the noise but doesn't fix the cause. The pressure spike is still damaging the joints.
• Don't crank a PRV right down — you'll just choke flow. 3-3.5 bar is the right target.
• Don't ignore a sudden new bang. If it's just started, something has changed — often a failed appliance valve or a PRV that's let go.

Frequently Asked Questions

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