What is a water hammer arrestor?

A water hammer arrestor is a compact vertical or horizontal chamber filled with air and a spring mechanism that absorbs sudden pressure waves when water valves shut quickly. When a faucet, toilet, or washing machine valve closes abruptly, flowing water has nowhere to go and creates a shock wave that travels through the pipes. Without absorption, this surge creates the loud banging or knocking sound known as water hammer, which stresses fittings, joints, and appliances over time.

The arrestor sits near the problem point, typically close to quick-closing fixtures or appliance supply lines. As the pressure spike hits the device, the air chamber compresses and the internal spring gives way, dampening the shock. The mechanism then re-expands, returning to standby. This cycle repeats each time a valve closes, preventing the pipe banging that would otherwise accumulate stress on your plumbing system.

Water hammer is more common in older homes or systems with high water pressure. It not only creates annoying noise but can lead to pipe joint separation, loose connections, or accelerated wear on washing machine fill valves and dishwasher supply lines. Installing an arrestor at the source of the problem is a direct, low-cost fix. Many plumbers recommend them as a preventative measure during renovations or when replacing fixtures. The device requires no moving parts on the water side itself, making it reliable and maintenance-free once installed. If you're hearing persistent knocking in your walls or near appliances, a plumber can diagnose water hammer and install an arrestor to match your pipe size and configuration.