Here's How: Stop Loud Water Hammer

James Dulley on Jul 30, 2026

Dear James: I often hear a loud knocking sound when we close a faucet quickly or the clothes washer shuts off. What is causing this sound, is it dangerous and how can I fix it? -- David R.

Dear David: The loud knocking sound that you are hearing is the water suddenly stopping inside the water pipes when the flow stops. It is commonly called water hammer and is often caused by excessively high water pressure in the supply line.

The water pressure in a house is determined by the elevation difference between the house and the water tower. As the elevation difference between the two increases, the water pressure increases. An ideal water pressure for a home is typically about 60 psi (pounds per square inch).

With higher water pressure, the water flows through the pipes at a more rapid pace. When it is quickly shut off at a valve that stops the flow instantly, it creates a shock wave you hear as the knocking noise. It can actually get loud enough to awaken a sleeping child.

To stop the wave and loud noise from occurring, you need to install a pressure-reducing valve and an expansion tank. A pressure-reducing valve has a screw that allows you to adjust the water pressure. It is usually installed where the water main enters your house.

If you want to have the high-pressure water still going to an outdoor faucet to wash your car or perform other tasks, install the valve after that branch of piping. Be sure to check your local plumbing codes before you begin any plumbing work for this project.

If you like to know the exact pressure of the water, you can install a pressure gauge along with the pressure-reducing valve. To save money and not install a gauge, you can determine the proper adjustment needed as you go. If the water hammer still occurs, just adjust the screw a little further.

The downfall of a pressure-reducing valves is it can block the backflow of water into the main water inlet from your house. As the water temperature in your water heater varies throughout the day, the water expands and contracts.

If your valve blocks the water flow back into the main inlet, the expanding water will have no place to go. The buildup of water pressure can cause leaks from faucets and the water heater pressure and temperature safety valve.

To make sure this won't happen, install an expansion tank in the cold water inlet pipe running to your water heater. This is a small tank with a flexible bladder inside of it. When the water heats up and expands inside the water heater, the excess water volume goes into this tank.

The cost to install a pressure-reducing valve depends upon where you have to locate it and the difficulty to access the area. To install a pressure-reducing valve, first shut off the main water supply to your house.

Next, remove a section of piping long enough for the pressure-reducing valve and an isolation valve on either side of it. Solder the valves into the line and adjust the pressure. The expansion tank will need to be mounted on a tee in the cold line by the water heater.

========

Send your questions to Here's How, 6906 Royalgreen Dr., Cincinnati, Ohio, 45244 or visit www.dulley.com. To find out more about James Dulley and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.