Water Hammer After a Hookup: Banging Pipes When a Valve Shuts

What it
usually means.

Water moving through a pipe carries momentum, and when a valve slams shut, that moving column stops all at once and sends a pressure spike back through the plumbing. The sound is a single bang or a rattle in the wall. Modern appliances make it more likely than the ones they replace, because their inlet valves are electric solenoids that close in a fraction of a second, and a washer or a dish care machine opens and closes them many times per cycle. A new appliance in an old house is where it shows up: pipes that were never strapped to the framing, a supply line run straight to the valve with no slack, very high street pressure, or old air chambers in the wall that have filled with water over the years and no longer cushion anything. Refrigerators and ice makers can knock too, with each short fill. Water hammer is not an emergency, but it stresses the new connections, the valve on the appliance and the joints in the walls with every cycle. The fix is at the installation: water hammer arrestors fitted at the connection, supply lines with a gentle loop instead of a tight pull, and a check of the house pressure. Strapping pipes inside the walls is a plumber's job.