What is a Water Hammer?


A water hammer is a disruptive shock caused by water suddenly stopping when the flow is stopped by a valve closing. Water hammer has a distinct sound, which you can hear in this video:

This happens because the moving water has momentum, but suddenly has no space to move forward. So, it pushes against the closed valve for a short time until its energy dissipates through vibrations, or “hammer”.

They are especially common when you turn on or off a water faucet or flush the toilet. Water hammer can also cause pipes to vibrate, knock against framing members, and move partitions.

What causes a water hammer?

Water hammers happen when you shut off water quickly. It usually happens when someone shuts the water off too fast, like when they turn it off while taking a shower or running the sink full blast to wash dishes. Sometimes it’s due to faulty appliances, or because of high usage at certain times — say in early morning and late afternoon every day.

Are water hammers dangerous?

Yes, water hammers can damage faucets and valves. It’s also bad for the pipes themselves and the home. The pressure on the pipes may even force them to burst or separate, which can cause flooding and big messes in your bathroom or kitchen.

How to prevent water hammer?

– Decrease the use of hot water. In general, cold water causes less damage than hot water because it doesn’t have as much force pushing against any shut-off valves. You can decrease the temperature on your thermostat or install a mixing valve to reduce the temperature of hot water going into fixtures.

– Install a spring device on your toilet’s flush valve. These will close more slowly, preventing water from moving as quickly and slowing the rate of flow in your home’s pipes. You can also use a ball cock for this purpose (if it is still working). Sometimes, old flapper valves leak and allow air to enter the system. This lower pressure allows water hammer to happen more easily.

-If you’re doing plumbing repairs, use carpenter’s tape on the threads of your shut-off valves to prevent damage.

– If water hammer is happening in your pipes, put a check valve on them so it won’t happen again. These are installed under toilets and sinks near the shut-off valves, stopping water from moving in the wrong direction.

– Put a small shock absorber device on your sink sprayer or faucet to reduce damage to them if they are being slammed by high water pressure.

– If you experience persistent water hammer problems, contact a professional plumber for help. They can install air chambers and other devices which will balance water pressure throughout your home’s plumbing system.

Should you call a plumber for water hammer?

Yes. If the water hammer is causing damage to your plumbing, sinks, and appliances — and especially if you’re having frequent leaks — call a plumber for help.

What is a water hammer arrestor?

A water hammer arrestor is a mechanical cushion that helps prevent banging noises. It’s usually installed between the meter and building to absorb pressure surges when shutting off the main valve at the street.

Are water hammer arrestors required by code?

Water hammer arrestors are required by code in most areas. However, you can usually get away without them if you have very short pipes between the meter and your home, or if you have an air chamber or backwater valve installed.

What’s the difference between water hammer & resonance?

Water hammer is a plumbing noise that happens when water stops moving quickly. Resonance is a different kind of noise which occurs in pipes carrying both hot and cold water, like those between the meter and your home. For resonance to occur, the pipes need to be very long (at least 20 feet) and they must carry both hot and cold water.

How common are water hammer & resonance issues?

Either one is occasional. They happen more likely in homes with long runs of pipe or that get a lot of use, like office buildings and schools. However, they’re usually not serious problems unless the pipes carrying both hot and cold water break from wear and tear.

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