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Posted by Philip Schmidt | G+ | Aug 29, 2010

How to Stop Water Hammer

A couple of easy fixes for this very common problem.

Do you hear a banging sound coming from inside the wall or the depths of your basement whenever your washing machine stops filling? That’s water hammer. It’s caused by valves that close quickly, such as solenoid-type valves in washing machines and dishwashers, but it can also happen with toilets, faucets and other fixtures. When the valve closes suddenly, all that flowing water in the pipe stops abruptly and backs up into itself, creating a lot of pressure (more than your system is designed to handle, in truth) and kinetic energy that makes the pipe jump, banging it against the framing or other neighboring structures.

In many cases, the problem of water hammer is compounded by loosely secured pipes, which move more than they should, allowing them to get a nice, full “swing” before striking against the framing, etc. This movement and hammering is not only annoying to you, it’s also bad for your pipes. It stresses the pipe joints and can ultimately lead to leaks and other failures. 

How to Stop Water Hammer


Eliminating (or effectively reducing) water hammer usually involves two solutions: securing the pipes and installing one or more water hammer arrestors. A water hammer arrestor is a capsule-shaped device that installs into a water supply pipe. The capsule contains an air- or gas-filled sealed chamber that acts as a shock absorber to suppress the pressure surge inside the water line. Some arrestors are made for easy DIY installation, while others require soldering pipe, a job you should leave to a plumber unless you really know what you’re doing.

Securing loose pipes is easy to do and is the first step you should take to minimize the hammer noise and undo wear and tear on your pipes. First, you need to identify the offending appliances and/or fixtures: Have a helper turn the water on and off at each suspected appliance while you locate the pipe that’s hammering (usually in your basement or crawlspace).

If you see the pipe moving—not just vibrating with the jolt of the water backup—it needs more anchors to keep it in place. Add appropriately sized pipe clamps or straps as needed to secure the pipe to the floor, wall or other wood framing members. Pipe clamps may be metal or plastic and may be screwed or nailed in place. With metal, make sure the clamp is the same material as the pipe, to prevent corrosion from galvanic action. With some straps, you can also add a piece of pipe insulation to help keep the pipe snug and add some shock absorption. Be careful not to alter the pipe’s original resting position when securing it.

If the water hammer was minor to begin with, securing the pipes might effectively solve your problem. In other cases, an arrestor is called for.

Installing Water Hammer Arrestors


Water hammer arrestors designed for easy installation are single-fixture units that use compression fittings for splicing into the supply line of individual fixtures and appliances. For toilets, faucets and dishwashers, you can install these arrestors either between the water supply line and the fixture’s shutoff valve or between the shutoff valve and the supply riser or tubing leading to the fixture. Arrestors for clothes washers screw right in between the water hoses and the shutoff valves. Install an arrestor at each offending fixture or appliance, following the manufacturer’s directions. Fixtures that use both hot and cold water need an arrestor on each water line. 

Arrestors designed for branch lines can serve multiple fixtures or appliances, if desired. These come in solder-only, threaded and compression versions and must be sized and positioned correctly for proper performance. Non-solder types allow for easier replacement if the arrestor ever fails (which can happen sooner with hard water), but threaded arrestors still need a soldered fitting to screw into. Since branch water lines are usually concealed (where leaks may not be noticed or stopped easily), standard arrestors should be installed by a licensed plumber.

In many homes, securing the pipes and adding arrestors on specific fixtures is enough to stop most water hammer problems. If not, it’s time to call a plumber.

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