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Eliminating the Water Hammer

Scott Gibson, Contributing Editor

We have a house with a gas forced-air heating system. The problem we have is our water pipes bang very loudly when we shut off a faucet or flush the toilet--basically, every time we use the water for anything. Advice?

What you are describing sounds like water hammer, which can be the bane of an older plumbing system. It doesn't have anything to do with the heat, especially a forced-air system, and everything to do with sudden changes in water pressure.

When you shut off a faucet, all the water that has been pouring into the sink or tub suddenly has nowhere to go. It just stops. That "hammer" creates a shock wave, causing a momentary spike in water pressure and making an unsettling bang that reverberates through your plumbing.

Along with driving you nuts, these repeated shocks can lead to leaks over time.

Plumbing in a "Riser"
Old school plumbers had an answer for this problem. They added something to the supply line called a riser, a length of pipe that was capped at the top. Inside the riser was a cushioning column of air that took the brunt of shock waves inside water pipes.

Risers were often hidden in the wall or otherwise kept out of view, and they apparently worked pretty well. The problem is that risers eventually get waterlogged, and when the air is gone so is its ability to absorb water hammer. They can also collect minerals and other glop from the water lines and become less effective.

If your older house has risers that are no longer doing their job, you can revive them by draining the water out of your plumbing system and refilling it. As water re-enters the system, it traps air inside the riser, just as it originally did, and the noise should go away.

If you want to try this, you need to shut off the water supply, turn off the water heater, and then drain everything from the lowest point in the house. Open some faucets at the top of the system--in an upstairs bathroom, for example--so water isn't held inside the plumbing. Be careful around the water heater. Then refill the system.

Arresters Also Can Do the Job
Getting your risers back in fighting trim may not be a one-time job. You may have to revisit this chore periodically, or you could install a device called a water hammer arrester.

An arrester accomplishes the same thing as a riser, except that it shouldn't get waterlogged. It's a sealed metal tube with a plunger inside that takes up any sudden changes in pressure, sort of a shock absorber for your plumbing.

A plumber could help you choose the right size and location for installing one or more arresters in the house. If you like to tackle these jobs by yourself, a good plumbing supply house should be able to give you all the advice you need. You can read up on the subject by visiting the Plumbing and Drainage Institute website ( and following the link to "Publications."

About the Author
An accomplished woodworker and carpenter, Scott Gibson is the former editor of Fine Woodworking magazine, and a former editor at Today's Homeowner and Fine Homebuilding magazines. He also is former managing editor of the Kennebec Journal, a daily newspaper in Maine.

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