Q: We live in a duplex that was converted from a 1912 single-family house. The upstairs flat had a kitchen added, and doors were installed for each unit. This house is built right up against houses on either side, a typical but stripped-down Victorian.

The plumbing works OK — good water pressure, good hot water temperature. But whenever the toilet in the upstairs unit is flushed, there is a loud banging or clanging noise when the water shuts off. This also happens when the bathroom faucet is used. If you have water running other than very slowly, and turn off the faucet, other than very gently, you get the same bang or clang.

I am concerned this may be symptomatic of a bigger problem, and, of course, dread calling in a plumber.

A: This is a textbook description of a water hammer, a malady common to old homes that comes in many flavors, from a severe "bang" to an imitation of a machine gun to a wheeze. One reader even described it as "harmonic noise."

Water hammer is hydraulic shock. It’s the sudden increase in water pressure in the pipes when there is a change in the direction or velocity in the water. Water is not moving in the pipes when all the valves in your water system are closed. The pressure is constant. When a faucet is turned on or the toilet fills, the water flows. Depending on the water pressure in the municipal system, it probably moves at a pretty good clip.

When the toilet valve closes or you turn off the faucet, the water flowing in the pipeline suddenly stops. This results in the transfer of energy created by the flowing water to the pipe walls, making the pipes shudder and vibrate. The pressure wave bounces back and forth, hitting the sides of the pipe until it dissipates because of friction. The pressure wave travels at the speed of sound, hence the banging you hear.

When you gently close the lavatory faucet, you’re essentially allowing the pressure to gradually increase. That’s why there is no bang. The other faucets may also have this dampening effect.

There are three ways to go about eliminating water hammer, but each requires that you expose the pipes. Unfortunately, unless you’re confident in your plumbing skills, the latter two will require you to enlist the services of a licensed plumber.

First, if you can get at the pipes, make sure they are firmly affixed to the framing of the house with clips or plumber’s tape. The goal here is make sure the pipes don’t move with the pressure buildup of the on-again, off-again of the fixtures.

A second, somewhat temporary fix is to expose the pipes where they exit the walls to the sink and the toilet. You’ll probably see plumbing fittings called 90-degree elbows. Replace these elbows with tees and install a capped 12-inch piece of pipe at the top of the tee. This air chamber acts as a shock absorber, dissipating the pressure waves. In time, the air may dissipate and the hammer will return.

The best 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.

Severe water hammers may damage pipes at the joints. If a water hammer is consistent and loud, a licensed plumber should be contacted to determine if the line should be fitted with a surge arrester or to determine if another solution might be more appropriate.


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