What is a water hammer?

• A. A loud bang in the piping caused by trapped air
• B. A device used to hammer water pipes
• C. Happens when water coming through the hose is shut off too quickly. Jams the engine
• D. A technique for cleaning hoses

Answer: (C)

Water hammer is a pressure surge that travels through a water-filled plumbing system when the flow is interrupted suddenly. When water is moving and you shut a hose or nozzle off quickly, the moving water can’t stop instantly because of its inertia. The momentum of that water creates a shock wave that travels back through the hose and piping, producing a loud banging sound and putting stress on hoses, fittings, and the pump. In firefighting practice, this is why a gradual shut-off or controlled pressure release is used rather than snapping valves closed.

Other options don’t describe this phenomenon: it isn’t caused by trapped air, isn’t a device, and isn’t a cleaning technique.