Yes, you need water hammer arrestors if a specific valve causes a bang when it shuts and your pressure and pipe straps are fine. Local code may also require them.
When water hammer arrestors are worth installing
An arrestor is a small sealed device that absorbs the pressure surge created when flowing water is stopped suddenly. It treats a symptom of fast valve closure, so it helps most when the cause cannot be removed.
Use this rule: if the bang happens at the moment a specific valve shuts, your pressure is in a normal range, and the pipes are properly strapped, add an arrestor at that valve. If any of those three is off, fix that first. An arrestor on a house with excessive pressure or loose pipes only masks a problem that keeps stressing joints.
You may also be required to add one regardless of noise. That depends on your local code, covered below.
What causes water hammer and how to find the source
Water in a pipe has momentum. A solenoid valve in a washing machine, dishwasher or ice maker snaps shut in a fraction of a second, and the water slams into the closed valve and sends a shock wave back through the pipe. Higher pressure makes the hit harder. Quick-closing flush valves and single-lever faucets can do the same thing.
Listen for when and where the noise occurs. It tells you which fixture to treat.
| What you hear | Likely source | First step |
|---|---|---|
| Single bang when the washer fill stops | Washer solenoid valves | Treat the washer supply lines |
| Bang a few seconds into a dishwasher cycle | Dishwasher fill valve | Treat the dishwasher supply |
| Bang when a faucet is shut off fast | Fixture valve plus pipe movement | Check pipe straps and pressure |
| Knocking that rattles whole walls | Loose or unsupported pipes | Inspect pipe supports first |
| Bang after the whole house is quiet, from anywhere | High incoming pressure | Test with a gauge |
Run each appliance alone, with other fixtures off, so you can match one noise to one valve. Low-flow devices such as ice makers and coffee machines seldom justify an arrestor on their own.
Check pressure and loose pipes before buying anything
Screw a hose-bib pressure gauge onto an outdoor spigot or the washer connection, open the valve, and read it with no fixtures running. About 40 to 60 psi is the usual target for homes. Readings approaching 80 psi are a warning sign, often from a failed pressure-reducing valve (PRV), and that is a fix rather than an arrestor job.
Next, look where pipes run through framing. Pipes that can shift will knock against joists every time a surge hits. Add proper straps or cushioned clips at the spacing your local code or the pipe maker specifies.
Finally, check for an old air chamber, a capped vertical stub of pipe behind the fixture. It works by trapping air, but the air slowly dissolves into the water and the chamber floods. Draining the system by shutting off the main and opening the highest and lowest faucets can restore the air. If the banging returns within weeks, a sealed arrestor is the more durable fix.
Are water hammer arrestors required by code?
Yes, in most places. The International Plumbing Code (section 604.9) and the International Residential Code require arrestors where quick-closing valves are used. Adoption and amendments vary by jurisdiction, so your local rules decide what applies to your home.
UPC-based codes, such as California’s, require arrestors to meet ASSE 1010 or PDI-WH 201 and be installed as close as possible to the quick-acting valve. Codes also say to follow the manufacturer’s installation instructions, and they are not always specific about the number or location. That leaves the final call to your local building department (the authority having jurisdiction).
Call that office before new plumbing work, because they decide whether you need a permit and an inspection.
Where water hammer arrestors go and how many you need
Install each arrestor as close as possible to the valve that causes the noise, per the manufacturer’s instructions. For a washer, that is usually at the hose connection or inside the wall box. For a dishwasher or ice maker, it is on the supply stop or just upstream of it.
Types and sizing
- Point-of-use units thread between the supply stop and the appliance hose, which makes them the easy retrofit.
- Inline units are sweated, pushed or crimped into a branch line, typically behind an access panel.
- Sizing follows PDI letter sizes A through F, chosen by the fixture units on the branch. The manufacturer’s chart gives the number, and your local code may amend the fixture-unit values.
PEX does not prevent water hammer. It flexes a little, but a solenoid slam still produces a surge, and loose PEX runs can still knock. If the line is hot, such as near a water heater, confirm the unit’s temperature and pressure rating first. Sizing the heater itself is a separate question, covered in our guide on what size water heater you need.
DIY or plumber?
Screw-on point-of-use arrestors are a reasonable DIY job. Inline units mean cutting into pressurized supply pipe, which is better left to a licensed plumber, especially where a permit is required.
- Shut off the water supply to the fixture or the whole house.
- Open a faucet to relieve pressure and drain the line.
- Install the arrestor at the valve per the maker’s instructions, in the orientation it allows.
- Restore the water slowly and check for leaks.
- Run the appliance several cycles and listen.
If you are already working on the main for a PRV or a whole-house water filter, have the plumber test pressure at the same visit. Persistent hammer can loosen joints over time, so a leak near the noisy fixture is a reason to call someone promptly.
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Frequently Asked Questions
Do washing machines need water hammer arrestors?
Not always, but they are the most common culprit because the solenoid valves close fast. If your washer bangs when the fill stops, an arrestor at the hose connection is the standard fix, and code in your area may require one.
Can I put an arrestor on an old air chamber instead of replacing it?
Yes. Many plumbers replace a waterlogged chamber with a sealed arrestor of the right PDI size. Follow the manufacturer’s instructions for the fitting and orientation, and have a plumber do it if it means cutting into the supply line.
How long do water hammer arrestors last?
Sealed piston-type arrestors are made to be maintenance-free, but lifespan depends on the model and your water conditions. If the banging comes back after years of quiet, the unit may have failed and should be replaced.
Will one arrestor protect the whole house?
No. An arrestor works best close to the valve that causes the surge, so a single unit at the main will not quiet a distant washer or dishwasher. Treat each noisy fixture or branch separately.
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