Water Hammer: Why Your Pipes Bang When You Turn Off the Water (and How to Fix It)

Water hammer is the loud bang or series of thuds you hear in your pipes when a faucet, washing machine, or dishwasher valve shuts off suddenly. Moving water has momentum; when a valve slams closed, that water has nowhere to go and it crashes into the valve, sending a pressure wave (a “hammer”) back through the pipes. The fix is to give the shock somewhere to go — usually by installing water hammer arrestors — and to bring your home’s water pressure down if it’s too high.

It’s tempting to treat banging pipes as an annoyance. It isn’t. Each bang is a pressure spike well above your normal line pressure, and repeated spikes loosen pipe hangers, stress soldered and glued joints, and wear out appliance valves and faucet seals. Here’s how to diagnose what’s causing yours and stop it.

Is it water hammer or something else?

Several pipe noises get lumped together. Match the sound to the cause:

• A single loud bang or a few rapid thuds the instant a valve closes — water hammer. Most obvious with fast-acting solenoid valves on washing machines, dishwashers, ice makers, and sprinkler systems.
• Rattling or vibrating while water is running — a loose pipe knocking against framing, or a worn washer fluttering inside a faucet. Not water hammer, but it has the same fix for loose pipes (secure them).
• Sputtering, spitting faucets and a hollow knocking — trapped air in the lines, often after work on the plumbing.
• Ticking or creaking from hot water lines — thermal expansion as copper or CPVC heats and rubs on framing. Harmless, though it can be cushioned.
• A whistling or screeching when a valve is barely open — a worn or partially closed valve, not a pressure wave.

What causes water hammer

1. Fast-closing valves

Modern appliances use electric solenoid valves that snap shut in milliseconds. Older homes were plumbed when valves closed slowly by hand, so many have no protection against the shock.

2. High water pressure

The force of the hammer rises with pressure. Residential plumbing is designed for 40–60 psi. If your home’s pressure is above 80 psi — which happens near municipal pumping stations and in low-lying areas — every valve closure hits harder. High pressure also shortens the life of your water heater, faucets, and toilet fill valves.

3. Waterlogged air chambers

Many homes built before the 1990s have short vertical stubs of capped pipe above each fixture. The air trapped in them was meant to act as a cushion. Over years the air dissolves into the water, the chamber fills, and the cushion disappears. Homes that suddenly develop banging after decades of quiet often have waterlogged chambers.

4. Loose or unsupported pipes

A pipe that’s held firmly can absorb a small pressure wave. One that’s loose in its strap or running unsupported through a wall will jump and slam into the framing, amplifying the noise and the wear.

How to fix water hammer

Fix 1: Recharge the air chambers (free, five minutes)

1. Turn off the main water supply to the house.
2. Open the highest faucet in the house, then open the lowest faucet (an outdoor spigot or a basement/first-floor tub) to drain the system. Let it run until it stops.
3. Close the lowest faucet, then turn the main back on slowly. Air refills the chambers as the pipes fill.
4. Close the top faucet once water runs steadily. Test the fixture that was banging.

If this works, it confirms you have air chambers — and that they’ll waterlog again in a year or two. Arrestors are the permanent version of this fix.

Fix 2: Install water hammer arrestors

An arrestor is a sealed cylinder with a piston or bladder that absorbs the shock and can’t waterlog. For a washing machine, screw-on arrestors thread directly onto the hot and cold supply valves — no tools beyond a wrench, about $15–$30 each, ten minutes. For dishwashers, ice makers, and fixtures where the hammer happens inside the wall, a plumber installs arrestors on the supply lines behind the wall or under the sink.

Fix 3: Lower the water pressure

• Screw a $10 pressure gauge onto an outdoor hose bib and turn the water on full. Read it with no other water running.
• Above 80 psi: you need a pressure-reducing valve (PRV) at the main line, or the existing one has failed. This is a plumber job, and it protects every fixture in the house, not just the noisy one.
• If you already have a PRV (a bell-shaped valve near the main shutoff), it may simply need adjusting — or it’s failed after 10–15 years of service.

Fix 4: Secure loose pipes

Where pipes are accessible — under sinks, in the garage, in the attic — add pipe straps or clamps every 6 to 8 feet on horizontal runs, with a rubber sleeve or foam between the pipe and the strap so it can’t rattle. Fill gaps where pipes pass through framing with foam pipe insulation.

When to call a plumber

• Recharging air chambers didn’t work and the banging is coming from inside a wall (arrestors need to go where the hammer happens).
• Your pressure reads above 80 psi. A PRV installation or replacement is a half-day job that pays for itself in appliance life.
• You notice a new leak, a damp spot on a wall or ceiling, or a drop in pressure after a period of banging — a joint may have failed.
• Your home has polybutylene or badly corroded galvanized supply lines. A repiping estimate is the honest next step.

Frequently Asked Questions

Can water hammer damage pipes?

Yes. Each bang is a pressure spike that can exceed 150 psi even in a home with normal 50 psi line pressure. Over time it loosens fittings, fatigues joints, and can burst weak pipe — especially aging galvanized, polybutylene, or poorly glued CPVC. It also wears out appliance solenoid valves and faucet cartridges.

Why do my pipes bang only when the washing machine runs?

Washing machines have the fastest-closing valves in the house and open and close many times per cycle. Screw-on water hammer arrestors on the hot and cold supply valves behind the machine solve it in most cases.

How much does it cost to fix water hammer?

Screw-on arrestors for a washing machine are $15–$30 each and DIY. Having a plumber install in-wall arrestors at a fixture typically runs $150–$400. A pressure-reducing valve installed at the main is generally $350–$800 depending on pipe size and access.

Is water hammer the same as trapped air in pipes?

No. Water hammer is a pressure shock when a valve closes fast, producing a bang. Trapped air causes sputtering faucets and irregular knocking while water runs, and is fixed by bleeding the lines. The two can occur together after plumbing work.

Will a water hammer arrestor fix banging pipes anywhere in the house?

Only near where it’s installed. An arrestor absorbs the shock at the valve that creates it. If several fixtures bang, the underlying cause is usually high pressure, and a pressure-reducing valve at the main is the whole-house fix.