What is Water Hammer?

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  • Written By: R. Kayne
  • Edited By: L. S. Wynn
  • Last Modified Date: 30 October 2019
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Water hammer is a very loud banging, knocking or hammering noise in the pipes that occurs when the flow is suddenly turned off. It is caused by a pressure or shock wave that travels faster than the speed of sound through the pipes, brought on by a sudden stop in the velocity of the water, or a change in the direction. It's also been described as a rumbling, shaking vibration in the pipes.

You might hear this noise when the clothes washer stops filling, the sprinkler system shuts off or shifts to another zone, when the dishwasher changes wash cycles, or when a faucet is turned off suddenly. It exerts very great instantaneous pressures that can reach excesses of 1000 psi and over time can potentially damage the system by weakening joints and valves causing leaks or even ruptures in the pipes.

Several factors can contribute to water hammer such as:

  • Improperly sized piping in relation to water flow velocity
  • High water pressure with no pressure-reducing valve
  • Straight runs that are too long without bends;
  • Poor strapping of piping system to structure
  • No dampening system in place to reduce or absorb shockwaves

Older houses were often fitted with air-filled risers connected to the plumbing at various points to absorb the shockwave. These air chambers can become waterlogged over time. If you aren't sure if your house has air chambers there is a test you can perform to check by draining the plumbing to allow the risers to refill with air. To do this turn off water at the main, then fully open the lowest faucet on the system (probably the garden hose) to release water trapped in the pipes. Open a few of the highest faucets next. This will facilitate drainage and allow air in the pipes, refilling the risers. When the lowest faucet stops dripping, close all faucets. Turn the main back on. When you turn on the faucets for the first time there will be some vigorous spitting until the pipes are flushed of air.

If this procedure got rid of the air hammer, you have waterlogged risers but this fix is temporary as they will become waterlogged again. Moreover old air chambers can become full of muck, debris and bacterium that are unhealthy. New risers, called water hammer arresters do not allow water inside.

Due to the health risks of polluted air-chambers, the plumbing code in some areas does not allow new structures to be fitted with air chambers. If you are building a new house, using adequate diameter pipe and water hammer arresters at the valve and faucet junctions will ensure you will not have to deal with the issue in the future.

If eliminating the issue from an older house, locating the offending valve or faucet and installing a water arrester at that location is one solution, but the procedure might require breaking into a wall to reach the problem point. You can also try installing low-flow fixtures to reduce the velocity of the water traveling through the pipes. If your water pressure is more than 60 - 80 psi and your system does not have a pressure regulator, having a licensed plumber add one could also reduce water hammer. A setting of 40 psi will likely allow sufficient pressure while mitigating the likelihood of damaging water hammer.

Though it may be a little troublesome to address the problem, the potential cost of not addressing it is much greater. Fixing a plumbing system that has ruptured or even structural repairs as a result of water damage from leaking valves or joints is much costlier. Arresters run anywhere from $15 - $125 each, depending on capacity, and they come in different diameters. Consult with a knowledgeable salesperson before you buy.


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Post 27

I get water hammer sounds when my dishwasher runs or the kitchen sink water is turned off quickly. I just installed a water hammer arrestor and I am still getting the banging of the pipes. Any advice?

Post 26

The bang of the water hammer is often caused by the bouncing (= resonance) of the tap washer in the tap body, with the water pressure because the tap washer stem is too loose in the tap body. It should be snug not loose. Take out the tap washer and tap the brass stem with a hammer to make it a bit oval and not able to rattle/bounce in the tap body. Or else, replace the washer and/or tap body, making sure the tap washer is snug.

Post 25

I have my drip system on a manual timer. When the time shuts off, the noise and vibrations are horrible. Any suggestions?

Post 24

I get water hammer when turning on only the hot water in the two upstairs sinks in the same bathroom. This hammer occurs only while the hot water faucets are on and hot water is running and not when you turn off the hot water.

Post 21

hot water faucet in laundry room has a 'suction' noise when turned off; a finger held to the faucet is sucked in unless faucet is closed extremely tightly. What is the cause?

Post 20

To fix this, turn of water main. Go inside and open all of your faucets and flush every toilet in house until it doesn't fill. Turn of all faucets when they are "bled" out. With faucets off, turn on water main -- slowly -- until fully on and back off a 1/8-1/4 turn.

The pipes will now have the proper amount of air and pressurization. No need for funky gadgets, widgets, etc. It works, its free and you don't need a plumber!

Post 19

I had a similar problem. It was due to an increase in water pressure.

After the city replaced the ancient water lines, the hammering began. Most often when the toilet was flushed. I went in the basement after flushing. I was able to locate the source. Improperly hung or too close to each other water pipes. After fixing the hung issue I added water pipe insulation to place a silent barrier between pipes that were too close to each other or the rafters. Problem solved.

Post 18

I just had a cartridge replaced in my shower to stop a leak. now after you flush a toilet or turn off a water faucet, the pipes bang loudly. They never banged before. Anyone?

Post 17

Had a bad water hammer problem for about six months. Found the problem to be a faulty china made shower faucet cartridge I had replaced. Put in a new OEM part, and problem solved. Don't buy cheap parts.

Post 16

What causes a knocking noise of the sprinkler system between the first and second cycle?

Post 15

When I flush the toilet upstairs, there is a loud banging in the garage. The main shut off valve and the pressure reducing valve is located there.

I just put in a new Wilkins BR4 Pressure Reduce Value and this problem has come up.

It only happens when toilet is flushed and not when the sink or the shower are used. Any advice would be appreciated.

Post 14

My house just got our pipes replaced with copper ones. Then suddenly these banging noise start to arise. It bangs everywhere. I feel our water running much more powerful than before. I don't know what the problem could be? Any suggestion?

Post 13

I get water hammer only when running my bathroom sink. if i turn the valve under the sink to lower the amount of water (pressure) making it to the sink, will that eliminate the hammer?

Post 12

Is it possible for a water hammer to split a hot water cylinder?

Post 11

I have a house built in 1996 anytime my neighbor runs their sprinkler or uses their hose I can hear water hammer really bad in the pipe closest to our water meter in that corner of the house.

We moved in only a few months ago and can't really figure out what to do or whats causing it?

Post 10

Dear anon7603,

Which washer was defective?

Post 9

i hear loud tapping noise after turning off the water in my kitchen sink. what could be causing it?

Post 8

Can water hammering affect only 1 cylinder of 16 cylinder diesel engine or it will effect all of them? The single bearing of an engine will worn out due to water hammering effect or will damage more bearings? Please info me.

Post 6

When I flush the toilet upstairs, there is a loud banging noise, which I believe is a water hammer. My home was built in 1997, but I have lived there since 2001. It started doing it shortly after I moved in and I had a plumber install a connection/pipe to the pipe going into the wall from the toilet. It did not fix the problem. It still does it while the water is running on the sink next to it, and I can somewhat hear the rumble of pipes in the floor when it happens.

Also, I have had the water pressure checked by the city and it is within range according to them but I do have exceptionally good water pressure. None of my neighbors have any problems though. Any suggestions?

Post 5

excellent insight and explanation. Thank you!!!!

Post 4

depending on your house, the rumble might be the water simply flowing though the pipes. i noticed this when a second floor toilet was flushed and the sound went from the ceiling to the floor (you can't hear this sound if its under the house.)

Post 3

Whenever I flush the toilet in the master bathroom, I can hear the pipes rumble 30 (or so) feet away. This only happens when the master's toilet is flushed. Would this most likely be water hammer or some other issue?

Post 2

We get water hammer, not when we turn the water off, but when we turn the water on first thing in the morning. What could be causing this?

Post 1

I just repaired a water hammer problem. It was only hot water. replaced washer. At that time I noticed the screw that holds the washer, backed out. After replacing washer, the problem was gone. Try this first before getting too involved.

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