Quick answer
A water hammer arrestor is a small device with a sealed air or gas cushion behind a piston. When a fast-closing valve, like a washing machine or dishwasher solenoid, stops the water suddenly, the arrestor absorbs the pressure spike that would otherwise bang the pipes. They are installed close to the valve causing the hammer, often screwed onto the washer's hot and cold valves. High house pressure makes hammer worse, so check that it is under 80 psi, and secure loose pipes. Call (888) 217-6781 to reach a plumbing company taking calls in your ZIP.
Follow the short menu, then your 5-digit ZIP. The line goes quiet for a few seconds while it connects; stay on. If it does not connect, call once more: companies taking calls change minute to minute.
Written & reviewed by Marcus ReedIICRC WRT
Reviewed November 4, 2026· Next review May 2027
Water hammer is the bang or thud you hear when a valve closes suddenly. Moving water has momentum; when a solenoid valve slams shut, the water stops all at once and the pressure spike shakes the pipes. See water pipes banging.
How an Arrestor Works
An arrestor is a short chamber with a piston and a sealed air or gas cushion. When the spike arrives, the piston moves and the cushion absorbs it. Older homes used capped air chambers, which fill with water over time and stop working; sealed arrestors do not.
Where They Go
| Fixture causing the bang | Where the arrestor goes |
|---|---|
| Washing machine | Screwed onto the hot and cold hookup valves |
| Dishwasher | On the supply line under the sink |
| Toilet or fill valve | On the supply stop |
| Ice maker or refrigerator | On the supply line |
| Whole house | Near the main, with high pressure fixed first |
Talk to a plumber about this
One call reaches a plumbing company taking calls in your ZIP.
Fix the Pressure Too
High house pressure makes hammer much worse. A gauge on a hose bib should read under 80 psi. If it reads higher, the pressure-reducing valve may have failed. See signs of a bad pressure-reducing valve.
Other Causes of Banging
- Loose pipes rattling against framing, often after the hammer starts.
- Thermal expansion ticking in hot lines.
- A worn valve washer chattering.
The Call
Call when arrestors at the washer do not stop it, the bang is in the walls, or pressure is high. (888) 217-6781 connects you to a plumbing company taking calls in your ZIP. Follow the short menu, enter your 5-digit ZIP and stay on while it connects; if it does not connect, call once more, because the companies taking calls change minute to minute.
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Frequently Asked Questions
What does a water hammer arrestor do?
It absorbs the pressure spike when water is stopped suddenly by a fast-closing valve, so the pipes do not bang.
Where should a water hammer arrestor be installed?
As close as possible to the valve causing the hammer, such as the washer hookup valves, the dishwasher supply, or the toilet supply.
Why do my pipes still bang with arrestors?
House pressure may be too high, pipes may be loose in the walls, or the hammer comes from a different fixture. A failed arrestor can also lose its air cushion.
Sources
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Disclaimer: HearthDry is an independent educational resource. This article is for informational purposes only and does not constitute professional, legal, or insurance advice. Consult licensed professionals before making decisions about your property or insurance claims.
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