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How to stop loud knocking in the water pipes

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Q: I live in a first-floor unit in a 344-unit building that opened in 2006. There is a loud knocking when someone turns off the bathtub/shower, dishwasher or washing machine and when the ice maker turns off. The kitchen sink, bathroom sink and toilet do not knock. My neighbors don't have this problem.

The issue recently became critical after I got a new washer. My old top-loader filled with water, agitated and drained. The new front-loader adds water a little at a time, and the constant on and off is seriously shaking the pipes. I've consulted with plumbers who say I should install a pressure-regulating valve at each water outlet or that I should rip through walls to anchor the pipes better. What's best?


A: The knocking sounds are what’s known as water hammer, caused when water flowing in pipes suddenly shuts off and vibrates with enough force to cause the pipes to knock against wood framing. To prevent this, plumbing is supposed to be installed with air chambers to absorb the shocks and with enough strapping to keep pipes securely tied to the framing.

Washing machines, dishwashers and ice makers are especially prone to water hammer, because they have solenoid switches that instantly turn off the water instead of the gradual closing that happens with a faucet. Manufacturers now make inexpensive, easy-to-install water hammer arresters where these appliances connect to the water system. The arresters come in various configurations to connect to different types of pipes or appliances.

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For a washer, get ones with hose threads. Depending on the space available in your home, you can screw them on to the hot- and cold-water faucets or to the inlets on the back of the machine. If there is enough room where the supply lines connect to the faucets, just turn off the water, disconnect the supply hoses, screw on the arresters and reconnect the hoses. To install the arresters directly on the washer, turn off the water, pull out the machine to access the back and make the connections there. This might be marginally safer, because the shock arresters will be closer to where the vibrations begin. With some shock arresters, the air chamber, which is in the closed pipe on one half of the T-shape fitting, has to be pointed upward. This isn’t critical with the Sioux Chief MiniRester ($14.99 at Ace Hardware).

For a dishwasher, the hammer arrester is usually installed between the wall valve or valves and the supply line or lines, depending on whether the dishwasher has only a cold-water connection or connects to both hot and cold water. Most dishwashers require hammer arresters with ⅜ -inch male and female compression fittings.

Most ice makers need ¼ -inch compression fittings.

Once you’ve fixed the problems at the washer, dishwasher and ice maker, if water hammer problems continue elsewhere, you have a few options. One would be to install water hammer arresters into piping close to where the noises occur. Or if pipes are visible, perhaps in a basement, you may be able to tighten existing pipe straps or add more. Copper pipe should be supported every six feet, and galvanized or black steel pipes every 12 feet. Plastic pipe needs supports every 32 to 48 inches, depending on the type of plastic and the pipe diameter. Adding pipe insulation, which comes in foam tubes with a slit so that they press in place, can also help when sufficiently supported pipes are still knocking against framing.

When pipes aren’t exposed, though, there is no way to add hammer arresters or even pipe clamps or insulation without ripping into walls. Although it’s simple enough for a plumber to cut out a short section of copper pipe and sweat-solder a water hammer arrester into the space, cutting through walls causes a lot of follow-up work. One tip: Don’t depend on a plumber to cut through walls in a way that makes them easy to repair. Instead, line up the repair person first, and ask that person to cut the access holes.

Avoid most of the mess if you can. After adding water hammer arresters in key spots, Steve Givens, the master plumber at Excellence Plumbing in Washington (202-271-4405;, pinpoints where remaining noises are happening. Then he drills a ¼ -inch-diameter hole through the wall, inserts the tube for a can of spray foam and presses the trigger. “When the foam solids up, it will hold that pipe in place,” he said.

Givens charges a service fee of $150, which covers quick, simple repairs that he can do on the same visit. When customers aren’t sure where to start in addressing water hammer problems, he talks to them first and asks them to send pictures, such as the setup under the sink where the dishwasher connects. Then, when customers want him to, he can order the appropriate shock absorbers and bring them with him, so he can install them and address any remaining issues. This is generally just the cost of the devices plus the service fee or a price that he has estimated beforehand.

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