What causes the loud banging sounds in my home's pipes?

The issue you're describing is commonly known as water hammer. While it can occur in a regular faucet, it’s more frequently found in pipes connected to appliances like a washing machine or dishwasher. These appliances use an electrically operated valve known as a solenoid valve (see How Electromagnets Work for more on solenoids). These valves shut off rapidly.

Picture this: The washing machine is filling with water, and the valve remains open. Water rushes through the pipe toward the machine. Once the washing machine is full, the valve suddenly closes. Inside the pipe, there may be 5 to 10 pounds of water, moving quickly, that comes to an abrupt stop. The momentum of the water forces the entire piping system to shift, creating the noise you hear. If the pipe is long and straight, the noise can be especially intense due to the significant motion.

Clearly, this isn't ideal for the pipes – every time you hear that banging sound, it means your pipes are being stressed and flexed. The typical solution is to install a "shock absorber" near the valve. A simple shock absorber might consist of a foot-long piece of vertical pipe attached near the valve. This pipe is filled with air. When the valve closes, the rushing water moves into the vertical pipe, compressing the air inside, which slows the flow gradually. However, a simple vertical pipe may cause the air bubble to dissolve into the water, so a sealed system with a rubber bladder is often used instead (this solution also works in other orientations besides vertical).

Another method is to use a coil of copper pipe. If this issue is common throughout the plumbing system, there are additional solutions. You might consider installing a pressure-reducing valve on the house’s water supply line if the pressure is too high (which can happen in lower areas). Sometimes, an expansion tank is necessary on the cold water side of the water heater. Using ¾-inch pipes throughout the house is another potential fix, as larger pipes can help reduce water speed.