Water Hammer Arrestor Sizing per PDI WH-201
Why this matters
The bang heard when a dishwasher solenoid snaps shut, or when a quarter-turn ball valve closes hard, is a pressure spike that can hit 5 to 10 times static pressure in milliseconds. On an 80 psi system that is 800 psi at the valve, well above the burst rating of most consumer-grade copper or PEX fittings. Repeated spikes loosen fittings, crack solder joints, and eventually rupture a supply line. The single-piston, factory-charged water hammer arrestor sized per PDI WH-201 absorbs the spike correctly; an old open-air capped stub of pipe (the air chamber method) waterlogs in months and stops working. Most plumbers know they need an arrestor; many do not know how to size one or where to put it, so the install fails inspection or trips a callback inside a year.
What WH-201 actually specifies
The Plumbing and Drainage Institute (PDI) Standard WH-201 is the accepted US standard for engineered shock absorbers in plumbing systems. It defines:
- Six size classes labeled A through F
- A sizing table based on fixture-unit load (water supply fixture units, WSFU) on the branch
- Performance testing: 10,000 cycles of quick-closure shock at 60 psi minimum, with no measurable charge loss
- Marking requirement: every PDI-listed arrestor stamps the class letter and listing number on the body
A copper-and-PEX stub of dead pipe with a cap is not WH-201 compliant. Sediment Wal-Mart "Sioux Chief MiniRester" units typically are listed; verify the WH-201 mark on the body.
Sizing the unit
Use the WH-201 table. WSFU is summed for all fixtures downstream of the arrestor (the fixtures whose flow can be shock-stopped by the arrestor).
| Class | Capacity (WSFU) | Typical use |
|---|---|---|
| A | 1 to 11 | Single lav, single fixture |
| B | 12 to 32 | Single dishwasher branch, washer hookup, multi-fixture residential branch |
| C | 33 to 60 | Group of fixtures, light commercial branch |
| D | 61 to 113 | Commercial restroom group |
| E | 114 to 154 | Large commercial group |
| F | 155 to 330 | Industrial / large multi-fixture |
For most residential work, a single Class B at the dishwasher and a single Class B at the washing machine is the right answer. A whole-house single arrestor at the meter is wrong; the arrestor needs to be close to the quick-closing valve to do its job. Distance limits per WH-201 commentary: install within 20 ft of the quick-closing valve on the same branch.
WSFU quick reference
ASME A112.1.2 / 2018 IPC values:
| Fixture | WSFU (private) |
|---|---|
| Dishwasher (residential) | 1.4 |
| Clothes washer (residential) | 1.4 |
| Bathtub w/ shower | 1.4 |
| Lavatory | 0.7 |
| Kitchen sink | 1.4 |
| Toilet (1.6 gpf) | 2.2 |
| Bidet | 1.0 |
| Shower head | 1.4 |
| Hose bibb | 2.5 |
A typical residential bath group (toilet, lav, tub-shower) sums to 4.3 WSFU. Class B covers it.
Installation rules
- Orientation. Install vertical (piston-up) where possible. Horizontal install is permitted by most makers but verify in the IO sheet. Never invert (piston-down); sediment from the branch collects above the piston and the arrestor jams.
- Location. As close to the quick-closing valve as practical, on the same branch. Mounting at the meter or hot-water heater does nothing for a dishwasher slam at the far end of the house.
- Tee fitting. Use a standard tee, full branch size. Never bush the connection down (1/2 in branch teeing to a 3/8 in arrestor port chokes the arrestor's response).
- No isolation valve. Do not put a valve between the branch and the arrestor; a closed valve disables the unit and no one ever opens it back up.
- Access. Arrestors are sealed and theoretically lifetime, but a piston seal can fail. Locate behind a removable access panel for the dishwasher (toe-kick) and washer (laundry box). A built-in-the-wall arrestor with no access becomes a sheetrock cut on the day it fails.
Built-in washer/icemaker boxes
Manufactured washer outlet boxes (Sioux Chief OxBox, Oatey Quadtro) integrate Class B arrestors on each supply leg. These are WH-201 listed and are the recommended install for the laundry. The same applies to icemaker outlet boxes for the fridge: integrated arrestor preferred over a separate inline unit.
Common installation mistakes
- Single arrestor for the whole house. Doesn't work. Spikes propagate from the valve outward; the arrestor must be near the source.
- Air chamber (capped dead leg) instead of a unit. Air dissolves into water within weeks. The dead leg eventually fills with water and bangs as hard as no arrestor.
- Undersized on a multi-fixture branch. A Class A on a branch serving washer + dishwasher + utility sink will bottom out and not absorb the spike. Sum WSFU first.
- Installed before pressure regulation. On a high-pressure street (>80 psi), install the PRV first, then size arrestors against the regulated pressure. An arrestor sized for 60 psi cannot absorb spikes from an unregulated 120 psi system.
- Buried in a slab or behind a wall with no access. When the piston seal fails (and it eventually will), the access cost can exceed the system cost.
Verification after install
There is no inspection test that proves an arrestor is working; the unit is sealed. Verification is by absence of audible hammer at the quick-closing valves after install. If hammer persists:
- Check installed location is within 20 ft and on the same branch
- Verify static pressure is under 80 psi (install PRV if not)
- Confirm class size against actual WSFU load
- Inspect the orientation (some piston-style units fail when mounted upside-down)
References
- PDI Standard WH-201: Water Hammer Arrestors
- ASSE 1010: Performance Requirements for Water Hammer Arresters
- ASME A112.1.2: Air Gaps in Plumbing Systems (WSFU values)
- IPC 2021, Section 604.9: Water Hammer
- IAPMO Uniform Plumbing Code 2021, Section 609.10
- Sioux Chief Hydra-Rester Engineering Catalog
- Oatey Quadtro Washing Machine Outlet Box IO Sheet