Glossary
Water Hammer Arrestor
A sealed chamber that absorbs pressure spikes when water flow stops suddenly in a pipe.
A water hammer arrestor is a small cylindrical device installed on a supply line, usually near the fixture that causes the hammer. Inside is a piston or diaphragm separating water from a pressurized air chamber. When a solenoid valve (like on a washing machine or icemaker) shuts off fast, the sudden stop sends a pressure wave backward through the pipe. The arrestor lets that spike compress the air charge instead of slamming the pipe walls and fittings.
They come screw-on or solder-on, sized by water pressure and flow rate. The air side needs recharging occasionally—pressure drops over time as air dissolves into the water or seeps out. When that happens, the device fills with water and stops working, and you'll hear banging again. Precharge pressure is typically 2/3 of your static line pressure.
Code requires them in most jurisdictions when you have quick-closing valves. They're cheap insurance against burst pipes, fitting failures, and the noise that drives homeowners crazy. Hammer arrestors are not the same as expansion tanks, which handle thermal expansion in closed hot-water systems—keep that straight on the job.