How to Correct Water Hammer
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How to Correct Water Hammer

How to correct water hammer problems in your home's water supply lines.

Most people have heard the annoying knocking or banging sound inside the walls of your home when you run your washing machine or someone flushes a toilet. This is called hydraulic shock, commonly referred to as “water hammer.” Water hammer can be damaging to water heater tanks, quick connectors, and other plumbing fixtures.

When trying to determine the source of hydraulic shock the first thing you look for is a location where there is a quick change in velocity. It is often assumed that water hammer occurs when a valve or fixture closes instantly. While this does cause a hydraulic shock wave, any time the velocity is lowered will also result in water hammer.

As an example, if water is flowing in a pipe at 14 feet per second, and suddenly slows to

4 feet per second, you have the same shock wave as if water was flowing at 10 feet per second and instantly stops. The shock wave intensity is based on the change in velocity, not the stopping of flow. It would be like driving in your car at 20 mph and being hit from behind by a car going 30 mph, the net effect would be like being hit by a car going 10 mph.

A shock wave occurs anytime you change the velocity; it is simply a matter of intensity which is based on initial pressure, material modulus of elasticity (Measure of the inherent rigidity or stiffness of a material), amount of change in velocity and time to change the velocity.

The key to eliminating the hammering sound is to lower the intensity. This can be accomplished by slowing the rate of change in velocity, lowering the initial velocity, which is accomplished by increasing pipe size, changing the piping material or installing a water hammer arrestor. However, in order to install a water hammer arrestor, you need to find the source of the intensity. The water hammer arrestor is only effective when installed within 20 feet of the source of the intensity.

Most plumbers eliminate water hammer by installed an air chamber which consist of a straight length of copper pipe coming out of a tee fitting inside the wall next to the fixture that is 8 to 12 inches long and capped. Water hammer can occur when they are made too short or eventually fill up with water and eliminate this shock absorber. If you have a water filled air chamber it can be fixed by draining and recharging the plumbing system. This will eliminate most problems, but will have to be done again when the water hammer returns. Recharging may last a year or more.

Many people think that anchoring the piping to the framing with clamps will correct the problem. This may reduce the amount of noise generated, but the hydraulic shock will still be present in the system, it just may travel to another point farther down the line and possibly damage a fitting or fixture over time.

Draining and Recharging your Water System


• Shut off the main water valve located near the water meter.

• Starting at the top floor, open all the sink faucets.

• Go to next lowest level in your home and open the faucets in your kitchen, powder room, and laundry tub and let all the water from the above floors drain out.

• If you have any fixtures in the basement, open them now.

• Go back to the highest level in the home and open the tub/shower faucets.

• Next flush all the toilets.

• Leave the faucets in an open position.

• There should be no water coming out of any faucets save a few drips from the lowest faucet.


• Close the basement faucet or lowest level faucet in the house.

• Now close all the upper faucets to allow air to remain in the pipes that will recharge the air chambers you may have in your home.

• Open the main water valve to let the water back into your pipes.

• Starting with the highest faucets, turn on the faucets and let the air and water out until it is clear. (Remove the aerators from your faucets to prevent debris from clogging them.)

• Open the shower faucets.

• Flush the toilets.

• Once the water is clean, turn off the faucets starting at the highest floor and work your way down through the house. (There may be some air still in the piping the next time you use your sink or shower, this is normal.)

Other Solutions

A more modern approach is to install a water hammer arrestor. As previously stated, they should be installed as close to the problem as possible, if it is in a bathroom installing it under the lavatory sink is the best bet. In some cases you may need to install a small air tank on top of your hot water heater to help reduce the pressure created by the heated water.

If your water pressure is very high, you may want to install a pressure reducing valve after your water meter. Contact you local water company if you have extremely high pressures, since they may be required to reduce the pressure and it will save you a few hundred dollars in material and labor. Typically the pressure should be between 50 and 70 psi, but if you have a fire sprinkler system, you may need to maintain a certain pressure to ensure that the sprinkler heads have the correct flow. Homes located at the ends of water mains usually have higher than average water pressure and your local water company should correct this for you. You can tell if you have high pressure if your pressure and temperature relief valve on your water heater opens from time to time, or you can purchase a water gauge at a home center that threads onto an exterior hose bib.

The website  has a useful table to help calculate placement and number or arrestors that are required.

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Comments (2)
pete sure

Interesting topic on water hammer, but I have a unique case. I only get the hammering when a tap is opened not closed as you state. I have a solar water system which vibrates when any of the house taps are opened.

Pete, for that situation it sounds like you may need an expansion tank, or check the operation of the exp. tank already installed on the system. As the water is heated it expands and in a closed system it can create a pressure imbalance. It the system has a heat exchanger, a separate water loop from the solar collector into the storage tank, you may have a leak in the loop or the circulator pump can be causing the problem. You should check the water pressure to make sure it is below 60 psi, 50 to 55 psi is normal.