October, 2019
Inspection News and Views from the American Society of Home Inspectors

Water Hammer Arrestors


Water is heavy and it takes energy to move it around. Think about a commercial hydroelectric generator: Water moving downward from a lake to the lower discharge generates a lot of electricity.

Pressure moves water through plumbing 

That same energy equation applies in our homes. When a valve opens, pressure in the system pushes water through the fixture. Close the fixture and the water stops; the energy of the moving water is then absorbed by the piping and the remaining water. An abrupt halt can trigger a loud “hammer” and, over the long term, it can even damage piping. 

Water stops too quickly—BAM! 

Plumbers have known for hundreds of years that when fast-moving water stops suddenly, this puts stress on piping and causes pounding—also known as hammering. In the 1850s, municipal water systems dealt with this by using a large open standpipe to absorb surges in volume and energy as a piston pump moved water into the piping.

In homes, many types of water hammer arrestors have been used over the years (Photos 1 and 2). A water hammer arrestor contains an air chamber to absorb energy when the water suddenly stops moving. In older plumbing systems, you might see curled copper tubes, copper cans, vertical pipes and other strange devices at the water main.

Water hammer arrestors evolved 

Water hammer arrestors evolved from there. Simple manufactured arrestors appeared on the market. In addition, plumbers installed arrestors made from standard plumbing parts (Illustration P015C). These worked great as long as there was an air cushion in the upright tube. Over time, the air cushion was lost and the system had to be drained to re-establish the air cushion.

Today, manufactured water hammer arrestors are placed wherever there is an electrically operated solenoid valve. The valve uses electric current to generate a magnetic field, which operates a mechanism that regulates the fluid flow. Examples include washing machines and dishwashers; the solenoid valve can abruptly halt the water flow. 

In modern water hammer arrestors (Illustration P032C), a disc separates water from the air cushion. Moving in response to changes in water pressure, the disc prevents the air cushion from being absorbed into the water.

What to watch for 

Every home should have water hammer arrestors, but they may not be visible. If the owner of an older home reports problems with water hammer, this means the old arrestor has lost its air cushion. For washing machine piping, the solution may be as simple as adding a threaded arrestor directly on the supply hose fittings. 

To learn more, attend Tom’s technical presentations at educational sessions for ASHI chapters. Tom will present “Basement Inspections, Reporting and Identification of Defects” at ASHI InspectionWorld® 2020 in New Orleans. Tom can also provide his knowledge for your educational event; contact him at Tom@HTOYH.com.

Tom Feiza has been a professional home inspector since 1992 and has a degree in engineering. Through HowToOperateYourHome.com, he provides high-quality marketing materials that help professional home inspectors educate their customers. Copyright © 2019 by Tom Feiza, Mr. Fix-It, Inc. Reproduced with permission.