Banging is heard in the piping when a washing machine valve closes. What is this?

Water hammer is a pressure transient. Water moving through a pipe carries momentum, and when a valve closes quickly that momentum has nowhere to go — the moving column slams to a stop and the kinetic energy converts into a pressure spike that travels back along the pipe as a shock wave, striking fittings and any point where the pipe is unsupported. The characteristic signature is in the timing: the noise occurs at the instant of closure, not during flow. Appliance solenoid valves are the classic trigger because they close far faster than a hand-operated tap. The consequences run from noise to loosened joints and, over time, damaged fixtures, and the remedies are arrestors near the fast-closing valve, adequate pipe support, and addressing high static pressure where present.

A fast-closing valve stops a moving column of water abruptly, and the momentum converts into a pressure shock that travels back through the piping. The bang coinciding exactly with valve closure is the signature of water hammer.

Water hammer occurs when flow stops abruptly and the fluid's momentum generates a pressure surge. Quick-closing solenoid valves in washing machines and dishwashers are frequent causes, and remedies include water hammer arrestors, secure pipe support and pressure regulation.

Bang at the Stop is hammer. Noise during flow is air or pump; noise at the instant of closing is momentum hitting a closed door.

Note whether the piping is loosely supported. Unsecured pipe amplifies the noise and lets the surge move the pipe against framing, which is a separate reportable condition.

• •Confusing hammer with trapped air because both are noises in pipes
• •Ignoring the timing of the noise relative to valve operation
• •Reporting the noise without checking pipe support