Water Hammer
The banging or knocking in pipes that happens when fast-moving water is suddenly stopped by a closing valve, sending a shock wave back through the line.
Water hammer is the loud thud you hear when a washing machine or dishwasher valve snaps shut, or when you close a faucet quickly. Moving water has momentum; when a valve halts it in a fraction of a second, that energy slams into the pipe and reflects back as a pressure spike and an audible bang. Beyond the noise, the repeated shock loosens fittings, stresses solder joints, and over time can crack valves or rupture a supply hose.
The most common cause is the loss of an air cushion. Older homes used short vertical air chambers near fixtures to absorb the shock, but these waterlog over time and stop working. Modern fixes use a mechanical water hammer arrestor, a sealed device with a piston and air charge that does not waterlog. High household pressure makes hammer worse, so a system running at 90 PSI will hammer far harder than one held at 50, which ties the problem directly to the pressure reducing valve.
Diagnosis usually starts by matching the bang to an appliance. Quick-closing solenoid valves in washers, dishwashers and ice makers are the usual triggers, so hammer that appears right when the washer fills points straight to an arrestor at that connection. A whole-house knock on many fixtures points instead to excessive pressure or waterlogged chambers, a broader fix. Either way, ignoring it invites a leak, since the joint that finally gives way is rarely convenient.
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- Main Shutoff Valve : The valve that stops all water entering the house, typically located where the supply line comes through the wall near the meter or in the basement.
- Water Meter : The utility-owned device that measures how much water your home uses, usually located at the property line in a buried box or in the basement.
- GPM (Gallons Per Minute) : A measure of flow rate, the volume of water a fixture or system delivers each minute, distinct from pressure, which is the force behind it.