An arrestor absorbs the shock from quick valve closures, using a sealed air cushion to stop pressure spikes that cause banging and pipe damage.
Pipe banging isn’t normal. That thud after a tap snaps shut is a pressure surge in the line. Plumbers call it water hammer. A water hammer arrestor turns that surge into a safe stop; fixtures, hoses, and valves stay calm.
What A Water Hammer Arrestor Does In A Home
A water hammer arrestor acts like a shock absorber for supply lines. A sealed gas pocket sits behind a piston or diaphragm. When a fast valve snaps shut, the piston moves, the gas compresses, and the pressure spike fades. Lines stay quiet and joints see less stress.
Here’s a quick map of symptoms, likely causes, and how an arrestor helps.
| Symptom | Likely Cause | How An Arrestor Helps |
|---|---|---|
| Loud bang after a quick shutoff | Momentum from running water hits a closed valve | Piston moves, gas compresses, spike fades |
| Shudder in exposed pipes | Shock wave bounces along the run | Cushion absorbs energy before it reflects |
| Loose washing machine hoses | Repeated surges strain rubber or braided lines | Dampens each start and stop cycle |
| Valve stem wear | Hammer pounds seats and seals | Reduces impact that chews through parts |
| Leaks at threaded joints | Micro-movement fatigues threads and tape | Smoother stops reduce movement |
That sealed cushion does the work. Old air chambers could flood and stop working. Modern mechanical units hold their charge and keep working year after year when sized and placed right.
How The Device Works Inside
When flow stops suddenly, the pressure wave moves at near sonic speed for water in pipe. The wave slams into elbows, tees, and fixtures. The arrestor sits near the source and gives the wave a spring to push against. A few milliseconds later, the line settles. No banging. No rattle.
Pressure Spike In A Nutshell
Engineers use the Joukowsky relation, which ties pressure rise to density, wave speed, and the change in velocity. Faster stops and higher speeds spike harder. The cushion inside the arrestor softens the stop so the wave fades quickly.
What’s Inside The Can
Wave speed depends on pipe wall stiffness and water temperature. Copper and steel transmit a sharp hit. PEX softens it a little yet still passes a jolt. The sealed can adds the spring those plastics lack.
Most units use a sliding piston with O-rings or a flexible diaphragm. The sealed gas pocket never mixes with water. That separation keeps the cushion intact and keeps debris out of the gas side. The metal shell takes the mechanical load while the moving part soaks up the shock.
What Does A Water Hammer Arrestor Do For Appliances
Dishwashers, ice makers, and washing machines use solenoid valves that close fast. Each snap sends a jolt. An arrestor at the stop, on a branch, or at the machine inlet cushions that jolt so hoses and fittings last. Kitchens with high-flow spray heads benefit too.
Placement That Makes Sense
Install close to the quick-closing valve. Point of use arrestors at the stop suit a sink, dishwasher, or fridge. For fixture groups, place a branch unit at the manifold end or on the riser. Follow the label for orientation and limits. Local code and the listing set the rules.
Sizing Basics That Matter
Sizing uses fixture unit counts and branch length. A small unit near a single valve is common, while long branches or stacks need a larger can. Certified models are labeled with a size code so a designer or plumber can match the branch. That label links to a sizing chart from the standard.
Here’s a sizing snapshot. A single dishwasher or ice maker often uses a small point-of-use model. A laundry pair on long risers may need a mid-size branch unit. A bank of flush valves or lab faucets calls for a larger can sized by a table that totals fixture units and branch length.
Don’t oversize wildly. The goal is a springy cushion, not a dead end tank. A matched size gives the smoothest stop with the