What is water hammer?

Water hammer is the loud banging or knocking noise that travels through your plumbing when a valve closes suddenly, halting the flow of water and creating a pressure surge in the pipes. Common triggers include washing machine fill valves shutting off, dishwasher solenoids slamming closed, or any quick-acting shutoff valve in the system. The shock wave reverberates through the supply lines, often producing a startling bang or series of bangs.

This isn't just annoying. Repeated water hammer can stress pipe joints, loosen fittings, and damage fixtures over time. In severe cases, it can lead to pinhole leaks or crack solder joints, making it a legitimate concern for homeowners rather than a minor inconvenience.

The standard fix is installing water hammer arrestors (also called shock arrestors) at strategic points in the supply line, typically near the appliance or fixture causing the problem. These devices contain a sealed air chamber that absorbs the pressure spike before it can rattle through your system. Plumbers in Ontario commonly install 1/2-inch or 3/4-inch arrestors depending on the line size, and they're inexpensive to add during new construction or repairs. If you hear banging in your walls when appliances shut off, a plumber can diagnose the source and recommend arrestor placement to stop it.