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Nailing the Source of Water Hammer in Pipes

Do-It-Yourselfer's Answer to Knocking: Add Air Chambers


Besides disturbing with its bumps and thumps, water hammer in home pipes can damage the system and cause leaks. Knocking can burst a pipe or loosen a fitting behind the wall.

There is something the do-it-yourselfer can do. Installing air chambers at kitchen and laundry sinks and bathroom basins and tubs usually does the trick. In fact, many modern plumbing codes require anti-hammer air chambers everywhere except toilets and outside sill cocks. Air chambers are inexpensive and easy to install.

Water hammer is the audible result of tremendous over-pressures produced inside a water supply system when fast-flowing, pressurized water comes to a stop as you close the valve quickly. Air chambers at the faucets provide soft, air-filled "pillows" to bring the rushing column of water in the pipe to a gentle stop.

Changing faucets can produce water hammer where there was none before in older plumbing systems. Older, slower-acting two-handle faucets often don't shut off quickly enough to cause it. newer single-handle faucets stop the stream much more suddenly and can cause water hammer.

A typical house air chamber is 12 inches long and is made of half-inch pipe capped at the top and installed vertically in the supply line just before it reaches the shut-off valves. You can make the chambers from either plastic or copper pipe and fittings available in standard sizes.


Fixtures supplied by pipes coming up through the floor are least apt to have air chambers. These are among the easiest to make up yourself. Here there is plenty of room to have chambers 18 inches long and, if the problem is severe, they can be made up with three-quarter-inch--rather than one-half-inch--pipe. Because of height limitations, wall-supplied fixtures must have shorter air chambers, and these should be made from three-quarters-inch tubing. Both hot and cold sides of the water supply need air chambers.

Making up air chambers using plastic pipe and fittings, where local plumbing codes permit, is easy and long-lasting. Solvent cement, rather than pipe fitting to sweat soldering, is used to join pipes and fittings.

You can face the air chambers in any direction when you install them simply by lining up their tees. However, once they're installed, you can't change their direction without altering the lengths of the riser tubes. You can install an air chamber without having to open the wall or floor simply by assembling it to fit under the sink or fixture. Always include a shut-off valve for emergencies.

All air chambers need to be recharged with air occasionally or whenever water hammer comes back. You can do this by turning off the water and draining the pipes to admit air.

If you install a line shut-off valve with a built-in waste drain, you can recharge the system simply by opening them and catching the small amount of water that's contained in the line to the faucet.

If you have standard shut-off valves with no waste drains, shut off the water supply at the main house valve and slip out the riser tubes.

Check to make sure the toilet tank is full, and turn off its shut-off valve. This keeps tank water from back-siphoning into the water supply system.

After draining the air chambers, retighten the hand nuts when you've reinserted the risers in their adapters.

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