Water hammer is the bang that shudders pipes when a fast-closing valve, a dishwasher solenoid, a washing machine valve, a single-lever faucet snapped shut, stops moving water abruptly. The water column's momentum converts to a pressure spike that hammers joints, loosens hangers, and over time cracks fittings. Arrestors, sealed chambers with a compressible air cushion or a piston, absorb that spike. Sizing and placing them correctly is what makes the bang stop for good.
The physics in one paragraph
Moving water has momentum proportional to its velocity and the length of the pipe run. Stop it instantly and that momentum becomes pressure: the spike magnitude grows with flow velocity and pipe length, and inversely with the valve's closing time. Long pipe runs feeding fast solenoid valves are the worst case, which is why dishwashers and washing machines are the classic hammer sources. The arrestor works by giving the pressure wave somewhere to go, compressing a trapped air charge or driving a piston against a spring, instead of slamming into the pipe walls.
Sizing the arrestor to the fixture
Arrestors are rated by fixture units, a sizing system that accounts for the flow and valve characteristics of the fixtures they protect. Match the arrestor size to the fixture units of the valve it serves: an arrestor sized for a single lavatory faucet is inadequate for a washing machine's solenoid valves. Manufacturer sizing charts map fixture types to arrestor sizes; use them rather than guessing.
When one arrestor serves a branch with multiple fast valves, size for the total fixture units on that branch. Undersizing is the common failure: the arrestor absorbs part of the spike and the remainder still bangs. There is no meaningful penalty for modest oversizing, so when between sizes, go up.
Placement: close to the valve
The arrestor must sit as close as practical to the fast-closing valve it protects, because the water column between the valve and the arrestor still hammers. The ideal is within a few feet, on the supply line just before the valve. An arrestor installed twenty feet away at the water heater protects almost nothing at the dishwasher; the long pipe run between them still sees the full spike.
Orientation matters by type. Piston-type arrestors generally install in any orientation; older air-chamber types must stay vertical to keep their air charge from migrating. Verify the type you bought and follow its orientation requirements. Install with isolation valves where practical so a failed arrestor can be replaced without draining the system.
Why air chambers fail and pistons endure
The old field-made air chamber, a capped vertical pipe stub, works until its air charge dissolves into the water, which it inevitably does over months. Then it becomes a dead leg of water that hammers as if it were not there. Manufactured piston arrestors seal the air charge behind a piston with O-rings, keeping it separate from the water indefinitely, which is why they are the modern standard.
If an existing system has old air chambers that have gone dead, replacing them with piston arrestors at the same locations usually cures chronic hammer. Do not just add more air chambers; they will die the same death on the same schedule.
System-level hammer reduction
Arrestors treat the symptom at the valve; system design reduces the cause. Lowering supply pressure to the minimum the fixtures need reduces spike magnitude directly, since the spike rides on top of static pressure. Securing loose piping with proper hangers and strapping stops the visible shuddering and prevents joint fatigue. And where hammer persists despite arrestors, check for a failed pressure regulator or thermal expansion issues that raise the baseline pressure the spikes build on.
Test after installation by operating the offending valve at full flow and closing it fast. Silence is the pass criterion; any remaining bang means the arrestor is undersized, misplaced, or failed. Water hammer is a solved problem when the arrestor matches the fixture and sits next to the valve.
Size the arrestor to the fixture units, put it within a few feet of the fast valve, prefer sealed piston types over air chambers, and keep system pressure sane. The pipes go quiet, the joints stop taking a beating, and the 2 AM dishwasher cycle stops announcing itself through the walls.
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