Q: What is the answer for a hammering sound in pipes? Our house was built in 1917, so we know the pipes are old. We have two toilets, but neither one leaks. We also have a dishwasher. Sometimes, after a shower or after the dishwasher has run through its on-and-off cycles, the pipes will hammer. If we run a faucet and turn it off gradually, this will often stop the hammer.

What’s really strange, however, is around 4 p.m., the pipes often start a racket when there have been no faucets operating anywhere. It’s almost a daily ritual.

A: What you describe is a version of a water hammer or a hydraulic shock. It’s the sudden increase in water pressure in the water system when there is a change in the direction or velocity of the water.

This can manifest itself in a number of ways. One type occurs when a fixture is shut off suddenly, and water rushing through the pipe is brought to a quick halt. The sudden stop creates a shock wave and the "boom." Another type can occur when water flowing through pipes under fairly high pressure is partially restricted. You’ll hear a staccato "bang, bang, bang."

Frankly, we’re puzzled about your afternoon "visitor." But we’re willing to bet that one way or another, pressure is abruptly changing in your supply pipe. It could be from a toilet flushing, or the washing machine cycling, or perhaps a timed sprinkler system — in short, anything causing water to move through the pipes.

There are a number of ways to deal with water hammer. Try the simplest first.

Drain the system. Shut off the water to the house at the main valve. There is most likely a gate valve where the water main enters the house, or the shut-off could be at the water meter. Next, open all of the faucets (sinks, showers and tubs) in the house. Make sure you go to the lowest water outlet. It will usually be a hose bib, but could be a laundry sink in a basement.

When all of the water is out of the system, close all of the faucets and turn the water back on to the house. With luck, it will eliminate the water hammer. Be careful when turning the faucets back on, as they will spit air until the water refills the pipes. This should work, but the process may have to be repeated regularly.

If draining the system isn’t effective, there are two other ways to go about eliminating water hammer. The first you can do yourself, but the latter may require you to enlist the services of a licensed plumber.

If you can get at the pipes (in the basement or crawl space), make sure they are firmly affixed to the framing of the house with clips or plumber’s tape. Every early 20th-century house we’ve worked on has water lines running every which way in the crawl space, often not secured to the structure. Installing clips and plumber’s tape reduces the chance that pipes will move with the change in pressure and help restrict water hammer.

Our second solution is to have a licensed plumber install an in-line surge arrester. Surge arresters are cut into the water line and act as shock absorbers reducing the change in water pressure that is the ultimate cause of water hammer.

Finally, we have to tell you that any plumbing system installed in 1917 is well past its useful life. We bet you have galvanized pipes that are badly corroded, restricting the flow of water and playing havoc with the pressure.

The final and best solution, if you can afford thousands of dollars, is to hire a licensed plumber or repiping specialist to install new copper supplies.

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