Water Hammer Arrestor Chart
Sizing water hammer arrestors by the PDI-WH201 standard, which rates them in six letter sizes A through F by the water-supply fixture units on the branch they protect. Total the fixture units downstream of the arrestor and read the size below. Arrestors go at quick-closing valves, where the sudden stop of flow sends the shock wave that hammers the pipe.
PDI arrestor sizes A–F
| PDI size | Fixture units (WSFU) | Typical connection |
|---|---|---|
| A | 1–11 | 1/2" |
| B | 12–32 | 3/4" |
| C | 33–60 | 1" |
| D | 61–113 | 1-1/4" |
| E | 114–154 | 1-1/2" |
| F | 155–330 | 2" |
Placement is half the job
Sizing gets the arrestor big enough; placement gets it to work. Water hammer is the shock wave that travels back up the pipe when a valve slams shut and the moving column of water stops instantly — so the arrestor has to sit near that quick-closing valve to absorb the wave before it hammers fittings and joints. The rules:
- Install at quick-closing valves — solenoid/electric valves, washing machines, dishwashers, and flush valves — as close as practical to the point of quick closure.
- On a multiple-fixture branch, place the arrestor at the end of the branch, between the last two fixtures.
- A single arrestor covers a branch up to about 20 ft; longer branches need additional units.
- Step up one size where the branch pressure exceeds roughly 55–65 psi (vendor-dependent).
Why the sealed arrestor won
Older capped air chambers waterlog — the trapped air dissolves into the water within weeks, leaving the chamber ineffective — so codes no longer accept them for quick-closing valves. PDI-WH201 arrestors keep a permanently sealed air cushion behind a piston, so they cannot waterlog and are maintenance-free.
The fixture units used here are water-supply fixture units — the same WSFU that size the water piping itself. Total them for the branch downstream of the arrestor using the fixture unit calculator, or read them per fixture from the WSFU chart.
Common questions
How do you size a water hammer arrestor?
By the fixture units on the branch it protects, using the PDI-WH201 letter sizes A through F. Total the water-supply fixture units downstream of the arrestor, then pick the size whose range covers it: A covers 1–11 FU, B covers 12–32, C covers 33–60, D 61–113, E 114–154, and F 155–330. Cold and hot branches are totaled separately.
Where do water hammer arrestors go?
At quick-closing valves — solenoid valves, washing machine and dishwasher connections, and flush valves — as close as practical to the point of quick closure. On a branch serving several fixtures, put the arrestor at the end of the branch, between the last two fixtures. A single arrestor covers a branch up to about 20 feet; longer branches need more than one.
Why not just use an air chamber?
Because air chambers waterlog. The trapped air pocket dissolves into the water within weeks, leaving the chamber full of water and useless — which is why codes no longer accept capped air chambers for quick-closing valves. A PDI-WH201 arrestor keeps its air cushion permanently sealed behind a piston, so it never waterlogs and needs no maintenance.
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