Pipes banging when the water shuts off
A loud bang or thud in the walls when a faucet shuts off or the washing machine stops filling is called water hammer. Water moving through the pipes stops all at once and slams against the closed valve. Besides the noise, it strains joints, hoses and appliance valves over time. The fixes are cheap and usually quick. A ten-dollar pressure gauge tells you more than anything else here. Start there.
What causes water hammer
Appliances like washing machines, dishwashers and ice makers use electric valves that snap shut in a fraction of a second. Single-handle faucets can do the same thing if you close them fast. The faster the water was moving, and the higher your water pressure, the harder the bang.
A water hammer arrestor is a small fitting with an air cushion sealed inside. Screwed on near the valve, it absorbs the shock. Older houses often have air chambers, capped pipes behind the walls, that did the same job until they filled with water.
Why pipes bang
Washers, dishwashers and ice makers.
Over 80 psi makes it much worse.
Older homes lose their cushion.
Rattle and knock against the framing.
| You hear | Likely cause | Fix |
|---|---|---|
| One bang when water shuts off | Water hammer | Arrestors; check pressure. |
| Knocking while water runs | Loose pipes | Add straps or foam. |
| Ticking as hot water flows | Pipes expanding | Loosen tight straps. |
| Banging from the heating | Boiler or radiators | A heating technician. |
Let's fix it
- Find when it happensTurn on each faucet and appliance one at a time and shut it off. A single bang right as the water stops is water hammer. Rattling or knocking while the water is running usually means a loose pipe. Ticking as hot water flows is the pipe expanding.
- Check your water pressureScrew a pressure gauge onto an outside faucet or the washing machine's cold valve, with everything else off, and open the valve. Normal is about 40 to 60 psi. Anything over 80 psi is too high and makes water hammer much worse.
- Drain the air chambersIf your house has air chambers, turn off the main water valve and open the highest and lowest faucets in the house. Let them run until the pipes are completely empty, which lets air back into the chambers. Close the lowest faucet first, then the highest, and turn the main back on slowly. Open each faucet briefly to let the sputtering air out.
- Add arrestors at the washerTurn off the washing machine valves, unscrew the hoses, and screw a water hammer arrestor onto each valve. Screw the hoses onto the arrestors, hand-tight plus a quarter turn with pliers. Do the same at a dishwasher if it's easy to reach.
- Secure loose pipesIn the basement or crawl space, have someone run water while you look for pipes that shift or knock. Add pipe straps or foam insulation where they rattle against wood. Leave a little room on hot water pipes so they can expand.
- Test itRun the washing machine and, just for the test, shut off each faucet quickly. The bang should be gone or much softer. If the pressure was high, a plumber can adjust or replace the pressure reducing valve.
Watch it done
- No bang, or a much softer one, when faucets shut off quickly
- The washer runs without banging
- Pipes don't knock or shift in the basement
- Pressure reads about 40 to 60 psi
If it still bangs
If the banging only started recently, the pressure reducing valve may be failing. A pressure reading that's high or jumps around confirms it.
If only one fixture bangs, an arrestor can be installed right at that fixture's supply valve. Under a sink it's often a simple screw-on job.
What a pro typically charges (a rough estimate; prices vary by area): A plumber typically charges $150–$400 to install water hammer arrestors, and $300–$600 to replace a pressure reducing valve.
Find a plumber near youSomething else going on?
Water hammer questions
Is water hammer dangerous?
It isn't an emergency, but the pressure spikes strain joints, hoses and appliance valves over time, so fix it when you get the chance.
What is normal house water pressure?
About 40 to 60 psi. Over 80 psi is too high and usually needs a pressure reducing valve.
Where do water hammer arrestors go?
As close as possible to the valve that shuts quickly, like the washing machine's hot and cold valves.
Why did my pipes start banging suddenly?
Often a failing pressure reducing valve, or air chambers that have filled with water.
Why do my pipes bang when the washer fills?
The washer's valve closes fast, and the moving water slams against it. Water hammer arrestors at the washing machine valves usually fix it.