A water hammer arrestor is a small device installed near a quick-closing valve, such as a washing machine's, that absorbs the pressure surge created when water flow stops suddenly. A sealed piston and gas chamber cushion the shock, preventing the loud banging noise and pipe damage known as water hammer.
Water moving through a pipe carries force. When a valve slams shut fast, like the solenoid valve inside a washing machine or dishwasher, that moving water has nowhere to go. The sudden stop sends a pressure shockwave back through the pipe. That shockwave is water hammer, the banging or knocking sound many homeowners hear right after an appliance's fill cycle cuts off. A water hammer arrestor is the fitting installed to soak up that shock before it happens.
Inside the arrestor is a small, sealed chamber holding a cushion of gas. A piston or a flexible bellows keeps that gas separate from the water. When the shockwave hits, the piston moves and squeezes the gas cushion. That action soaks up the pressure spike the way a shock absorber soaks up a bump in the road. Once the surge passes, the piston returns to its resting spot, ready for the next one. The gas chamber is sealed at the factory, so a working arrestor needs no maintenance and keeps working for years. Compare that to the old-style capped air chamber some older homes still have: that design can slowly fill with water over time and stop cushioning anything at all.
Plumbing code calls for a water hammer arrestor anywhere a quick-closing valve is installed. In a typical home, that means the washing machine hookup and any dishwasher or ice maker with a solenoid-controlled supply valve. The arrestor has to sit close to the valve it protects and stay reachable, since it is the part most likely to need replacement someday. Manufactured units also have to meet ASSE 1010, the industry performance standard for engineered water hammer arresters. That standard sets rules for how much surge pressure the device must absorb without leaking or losing its gas charge.
A missing or failed arrestor does more than make noise. Repeated pressure spikes stress pipe joints, valve seats, and fittings throughout the system. Over years, that stress can loosen connections or crack fittings never built for repeated hydraulic shock. Installing an arrestor at the one fixture that bangs is usually a quick fix. But a whole-house pattern of banging after several fixtures shut off points to a different problem, like water pressure running too high, and an arrestor alone will not fix that.