What causes water hammer and how to fix it

April 20, 2017

Water hammer is a specific plumbing sound, as opposed to just noisy pipes. Water hammer often makes a sound that can be recognized by its pounding, banging or thumping noises in the pipes. It is not only annoying, but over time, it can be detrimental to your plumbing, fittings and heating units. [Photo credit: istock.com/rkafoto]

What causes water hammer and how to fix it

Read on for an overview of what causes water hammer and what you can do to fix it.

1: What’s that noise?

Although water may seem to flow smoothly through the pipes, it actually churns and tumbles as it moves its way through. If noises start when you open a valve or a faucet, it's likely caused by air in the pipes, rather than water hammer. Water hammer usually occurs when a valve is closed.

  • Closing a valve suddenly interrupts the flow of water and sends a shockwave through the water that makes the pipes vibrate violently and create noise.
  • An air bubble in the pipe can also cause these vibrations but, because there's no shock involved, the water swirls around the air bubble and the pipes knock against the walls repeatedly to create a "knocking" sound.
  • A noise when a pump starts could be water hammer, air in the pipes, or both.
  • If water hammer is heard when a dishwasher or washing machine valve closes, it's likely that the appliance is demanding more water than one or more of the pipes supplying it can safely handle.
  • Water hammer is often caused by high water pressure. The faster the water travels through the pipes, the noisier the sound of the water hammer.

Good to know!

If the noise starts just after you've had some plumbing work done, or you've recently installed a new appliance, start by checking those areas first.

2: Check your pipes

  • Make sure all your pipes are securely clipped to the studs to stop any normal vibrations.
  • If possible, have someone turn off the valves while you inspect the pipes to make sure that the water hammer noise isn't just a noise from loose pipes.
  • If you find any valves that are sized smaller than the lines, replace them.

3: Check your water pressure

  • The average residential water pressure is approximately 60psi.
  • You can test your own water pressure with a water pressure gauge, however, many utility companies will run the test for free.
  • Water pressure over 80psi can lead to water hammer and can damage fixtures and appliances.
  • If you find high water pressure, consider installing a pressure-reducing valve (PRV).
  • Many plumbing codes now require the installation of a PRV to keep water at a safe pressure. If one is already installed, it may need to be adjusted. Consult with a plumber if you're unsure on how to do this.
  • Fixing the high water pressure in the house may solve the problem.

4: Does your plumbing have air chambers?

Many older homes have air chambers in their plumbing system.

  • The air chamber is a vertical length of pipe with a cap at the end that is installed on the supply lines.
  • These chambers were originally filled with air that could absorb the shockwaves in water and avoid water hammer.
  • However, over time these air chambers can become full of water. To fix this situation, drain your plumbing system and reintroduce air into the chambers.
  • Inspect the chambers at this point before turning the water back on to make sure they are not clogged with scale or residue.

Good to know!

Most plumbing codes now consider air chambers to be "primitive." If you don't have air chambers, you probably won't need any or be allowed to install them. Check with a plumber if in doubt.

5: Install a water hammer arrestor

If adjusting the water pressure or refilling the air chambers hasn't solved the water hammer problem, you might want to consider purchasing a water hammer arrestor.

  • A water hammer arrestor provides a cushion of air that absorbs the shockwaves passing through the water.
  • There are several types of water hammer arrestors, but most employ a diaphragm, bellows, or piston mechanism.
  • When a valve closes suddenly, the arrestor absorbs the water shockwaves by compressing the air charge until the momentum of the moving water has safely dissipated.
  • Water hammer arrestors are usually installed within six feet of whichever valve is rapidly closing – washing machines and dishwashers are the usual culprits.
  • They can be installed on both hot and cold lines.

Good to know!

Water arrestors are small and relatively inexpensive and can be purchased at a plumbing or hardware store.

The next time you hear violent banging or thumping noises coming from your plumbing pipes, don’t panic. Let your new-found knowledge about water hammer noise give you peace of mind and possibly save you some expensive repairs down the road.

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