Water hammer is the bang you hear when a fast-closing valve — a washing machine solenoid, a dishwasher, an ice maker, or a flush valve — stops moving water abruptly and sends a pressure spike back up the pipe. A water hammer arrestor absorbs that spike. This tool sizes one per the industry standard PDI-WH 201.
How it works
Each protected fixture contributes a standard number of Water Supply Fixture Units (WSFU). The tool sums them and looks up the smallest arrestor whose rated range covers the total:
Size Fixture units Typical connection
A 1 – 11 1/2" nominal
B 12 – 32 3/4" nominal
C 33 – 60 1" nominal
D 61 – 113 1-1/4" nominal
E 114 – 154 1-1/2" nominal
F 155 – 330 2" nominalA single washer (4 units) or dishwasher (1.4 units) lands comfortably in Size A. A whole branch feeding many flush-valve water closets can climb into Size D, E, or F.
Example
A laundry box feeding one clothes washer (4 units) plus a nearby utility sink (1 unit) totals 5 fixture units — still Size A. By contrast, a commercial restroom branch with six flush-valve closets at 6 units each is 36 units, requiring Size C.
Why water hammer matters beyond the noise
The pressure spike from water hammer is not just annoying — it is measurable damage over time. Repeated shocks can fatigue pipe joints, loosen compression fittings, crack older lead or copper connections at the wall, and eventually cause pinhole leaks. Solenoid-operated valves in appliances close in milliseconds, far faster than a manually operated valve, which is why washing machines and dishwashers are the most common culprits in residential water hammer complaints.
In commercial buildings with flush-valve water closets, the pressure spike from a single flush is small, but with multiple valves operating simultaneously — as happens in a busy restroom during peak periods — the aggregate shock can be significant. This is why commercial installations with flush-valve plumbing typically need larger arrestors (Sizes C through F) even if each individual fixture generates only a modest WSFU count.
Where to install the arrestor
Proximity to the source is the most important installation rule. The arrestor must be on the same branch as the quick-closing valve, as close to it as practical. Installing it further upstream — for example, on the main supply line — allows the pressure wave to build before it encounters the arrestor, reducing its effectiveness.
For a washing machine, the ideal location is inside the laundry box (the wall-mounted valve box), where many boxes now include dedicated tapped ports for an arrestor. For a dishwasher connected under the sink, a 1/2 inch arrestor on the hot-water supply beneath the cabinet works well.
The arrestor must be installed in a position where it will not be permanently submerged, and in a location accessible for inspection. Modern piston-type arrestors are sealed for life and require no maintenance, but they can fail (a stuck piston) and should be accessible if a new knock appears after years of silence.
Arrestors versus old air chambers
Older plumbing used capped stub pipes (air chambers) to provide a cushion of trapped air. Air chambers work initially, but the trapped air gradually dissolves into the water and the chamber waterloggs, becoming useless. A piston-type arrestor uses a sealed gas chamber separated from the water by a piston, which maintains the air cushion indefinitely and never waterloggs. If you are replacing an old installation that used air chambers, a properly sized piston-type arrestor is the correct upgrade.
Always mount the arrestor near the offending quick-closing valve, keep access reachable, and confirm the figure against the specific arrestor maker’s PDI rating, which is the authoritative number for that product.