Technical Foundation
What Is a Water Hammer Arrestor?
A water hammer arrestor is a plumbing device installed near a fast-closing valve — solenoid valves on washing machines and dishwashers, quick-closing fixture valves — to absorb the pressure spike (water hammer) created when flowing water is suddenly stopped. Piston-type arrestors use a sealed piston riding against a compressible gas charge inside a chamber; bellows-type arrestors use a sealed, flexible bellows that compresses to absorb the shock, with no piston seals to wear over time.
Arrestors are sized not by pipe diameter alone but by PDI-WH201 fixture unit rating, which accounts for the number and type of fixtures the arrestor is protecting and the flow velocity involved — an arrestor undersized for the fixture unit load may fail to fully absorb the pressure spike, while a grossly oversized one is simply wasted cost. Air chambers (a capped vertical pipe stub) are a low-cost legacy alternative, but they lose their air charge and effectiveness over time as it dissolves into the water, unlike sealed mechanical arrestors.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Mechanism | Piston-type vs. bellows-type | Bellows have no dynamic seals to wear out; piston types are common and cost-effective |
| PDI-WH201 Fixture Unit Rating | Rated capacity in fixture units (A through F sizes) | Standardized sizing basis — the correct way to size an arrestor, not pipe diameter alone |
| Connection Type | NPT threaded, sweat/solder, push-fit | Must match the installation method and existing pipe material |
| Mounting Location | Air chamber replacement, branch line, fixture supply stop | Closer proximity to the fast-closing valve gives more effective shock absorption |
| Certification | ASSE 1010, PDI-WH201 compliance | Confirms tested performance versus an uncertified generic shock absorber |
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Applications
Where Water Hammer Arrestors Get Used
Water hammer arrestors are specified anywhere a fast-closing valve — solenoid, flush or quick-acting fixture valve — creates pressure spikes that would otherwise stress piping and generate noise.
Residential Plumbing
Undersink and washing machine arrestors installed at solenoid valve locations to eliminate pipe-banging noise and joint stress.
Multi-Family & Commercial Restrooms
PDI-rated arrestors sized to fixture unit banks of flush valves and lavatory faucets in high-traffic restroom groups.
Hospitals & Institutional Facilities
Arrestors specified on quick-closing solenoid and flush valves to protect piping and reduce noise in patient-adjacent areas.
Commercial Kitchens & Food Service
Arrestors protecting piping from repeated water hammer caused by pre-rinse spray valves and dishwasher solenoids.
Laundry Facilities
High-capacity arrestors sized for banks of commercial washing machines with frequent solenoid valve cycling.
New Construction & Retrofit Plumbing
Arrestors specified per PDI-WH201 during design, or retrofit-installed to resolve reported water hammer noise complaints.
FAQ
Water Hammer Arrestors Questions, Answered
How is a water hammer arrestor sized — by pipe diameter or something else?
Arrestors should be sized using the PDI-WH201 standard, which rates capacity in fixture units based on the number and type of fixtures being protected, not simply by matching the arrestor's connection size to the pipe diameter. A residential undersink arrestor is typically sized for a single fixture, while commercial applications require calculating total fixture units on the branch line and selecting a PDI size letter (A through F) rated for that load.
What's the difference between a piston-type and bellows-type arrestor?
A piston-type arrestor uses a moving piston sealed against a pressurized gas charge to absorb the shock, and is widely used and cost-effective, though the dynamic seal is a wear point over very long service life. A bellows-type arrestor uses a sealed, flexible metal bellows with no sliding seal, which some specifiers prefer for longer expected service life or lower long-term maintenance, generally at a higher purchase cost.
Why not just use an air chamber instead of a mechanical arrestor?
An air chamber (a capped section of pipe extending vertically from the branch line) works by compressing trapped air to absorb the shock, but that trapped air gradually dissolves into the flowing water over time, and once it's gone the chamber fills with water and stops absorbing shock — this is why old air chambers often become ineffective after months or years without any visible failure. Sealed mechanical arrestors (piston or bellows) don't lose their gas charge to the water stream and maintain rated performance over a much longer service life.
Where should the arrestor be installed relative to the valve causing the water hammer?
As close as practical to the fast-closing valve itself — a washing machine or dishwasher solenoid valve, or a flush valve — since the arrestor is most effective at absorbing the pressure spike near its source, before it travels far enough down the pipe run to build momentum and stress fittings and joints. Installing arrestors only at the far end of a long branch line, away from the actual valve, is a common mistake that reduces their effectiveness.
Does every fixture need its own arrestor, or can one arrestor protect multiple fixtures?
It depends on fixture type and piping layout — a single, properly PDI-sized arrestor can protect a bank of similar fixtures fed from a common branch line (e.g., a restroom's flush valves), but individual appliances with their own solenoid valves, like washing machines, typically get a dedicated arrestor at each supply connection since the shock originates at that specific valve.
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