Photograph of  this antiquated laundry sink with several unsanitary plumbing violations in view. Water Hammer Plumbing Noises
What causes water hammer or shock, what damage can it cause, how do I cure water hammer noises?

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Banging pipes & water hammer noise diagnosis, cure:

Water hammer noises in plumbing is also called hydrostatic shock. Our page top photo shows a water hammer noise suppression device produced by Oatey and available at building suppliers.

This particular water hammer noise suppressor is interesting because it's designed to be added to a hose bib or washing machine hose connection by a homeowner, avoiding having to cut and solder pipes.

The articles at this website will answer most questions about plumbing noise associated with water hammer or water surge, including the diagnosis and cure of water hammer noises as well as many other building plumbing system inspection or defect topics.

Green links show where you are. © Copyright 2017, All Rights Reserved.

Water Hammer Noises in buildings: Diagnosis & Cure

Water hammer correction (C) Carson Dunlop AssociatesWater Hammer & Banging Pipe Noise Article Series Contents

We divide water hammer or banging pipe noises in buildings into two categories: water hammer in the plumbing system and banging pipes in hot water or steam heating system.

In this article starting just below we describe water hammer and hydrostatic shock in house plumbing systems, typically water supply piping but also in irrigation systems, pools, spas.

[Click to enlarge any image]

Image of correcting water hammer noise (above) provided courtesy of Carson Dunlop Associates, a Toronto home inspection and education company.

Below we discuss the following: Definition of water hammer or hydrostatic shock noise in plumbing pipes. How to diagnose & cure water hammer noises & banging pipes. Curing water hammer by adding an air chamber or surge buffer. Where to buy water hammer noise suppressors, where & how to install water hammer or water shock noise suppression devices.

Curing water hammer by draining pipes to restore air charge in risers. Diagnose & cure other water supply & Drain Piping Noises. Sources of water hammer noise suppression devices. Plumbing supply and drain piping system inspection, testing, & repairs.

How to Diagnose & Fix Water Hammer Noises in House Plumbing Systems

Definition & Actual Cause of "Water Hammer" Noise

Water hammer (or hydrostatic shock) is a noisy pipe problem that occurs when valves are shut off quickly. You may hear banging water pipes, or clanging, rattling, or rumbling noises in the water piping when a plumbing fixture, sink, or clothes washer turns off.

Here is a more scholarly definition of water hammer that is rather clear:

Water hammer is a hydraulic phenomenon that is caused by a sudden change in the velocity of the water. This velocity change results in a large pressure fluctuation that is often accompanied by a loud and explosive noise. This release of energy is caused by a sudden change in momentum followed by an exchange between kinetic and pressure energy.

The pressure change associated with water hammer occurs as a [shock] wave, which is very rapidly transmitted through the entire hydraulic system. Severe or repeated water hammer events can lead to pipe failure.

The sudden change in velocity caused by the rapid closing of a valve can produce very high pressures in the piping system.

These pressures can be several times the normal operating pressure and result in burst pipes and severe damage to the irrigation system [Dr. Boman's interest, but also impacting any plumbing system - Ed.].

The high pressures resulting from the water hammer cannot be effectively relieved by a pressure relief valve because of the high velocity of the pressure wave (pressure waves can travel at more than 1000 ft per second in PVC pipe). - Boman (2014)

High Velocity Water Flow Rates Can Cause Water Hammer

Think of water screaming along at 100 mph: it's doing pretty nicely until it hits a curve (maybe a pipe elbow) or a wall (a valve that has suddenly shut). Then SLAM!

Watch out: if the velocity of water in your piping system is too fast, when faucets or other controls STOP that water flow you may hear a horrible hammering or banging in the piping system. Water hammer is more than a horrible noise, it can damage equipment, cause potentially dangerous leaks at temperature/pressure relief valves, and may even cause a divorce.

Keep in mind that where pipes are poorly supported, or where equpment such as pumps, valves, controls anywhere in the water supply system are causing variations in the water flow rate, you may hear a horrible banging water pipe noise even when the water valve itself is not being suddenly closed.

In a hotel in Mexico City we heard an incredible vibrating banging noise in water pipes in the wall near our bathroom whenever the sink faucets were opened. Opening to run water either faster or slower can often stop that banging/vibration problem.

Leaks from water hammer traced to high water velocity are also discussed at CLOGGED SUPPLY PIPES, REPAIR .

Watch out: When Increasing Water Flow Rate or Velocity or GPM, such as when adding a more powerful water pump or when adjusting a water pressure regulator to give faster water flow at fixtures, watch out for both water hammer & water scrubbing damage.

High Velocity Water Flow Rates Can Cause Scrubbing Damage & Water Piping Leaks

A second problem with very high water velocity rates through building piping is scrubbing or wearing away of the pipe interior: a problem that occurs at very high water speeds (measured in feet per second) in piping systems.

Generally the maximum safe water velocity or speed in residential water piping systems is 7 feet per second, and for 2-inch or larger pipes you should not exceed 5 feet per second. Some piping types such as PEX (CTS SDR-9) at smaller diameters such should have flow rates limited still further. 3/8" PEX should be limited to 3 gpm at 10 ft3/s. [CTS = copper tube size and SDR = standard dimension ration of pipe wall thickness to pipe diameter].

Scrubbing is more of a worry in metal piping than in the smoother (less friction) plastic piping systems. Scrubbing and corrosion are common sources of pinhole leaks in water supply piping.

Convert Flow Rates in GPM to Flow Rate in Feet per Second

Note: to convert between cubic feet per second of water flow into gallons per minute we need to know just two magic numbers:

1 minute = 60 seconds

1 cubic foot = 7.5 U.S. Gallons

A flow rate of one cubic foot per second, then will give us 7.5 gallons per second or in a minute, 60 x 7.5 = 450 gallons per minute. That's a lot of water.

Table of Water Flow Rates in GPM Converted to Water Feet per Second for Typical Plumbing Fixtures

Flow Rate in GPM at the Fixture for US Gallons Cubic Feet of Water per Second Cubic Feet of Water per Minute
1.0 gpm 0.002228 ft3/s 0.13 ft3/m
1.5 gpm 0.00334 ft3/s 0.2 ft3/m
2.0 gpm 0.00446 ft3/s 0.27 ft3/m
3.0 gpm 0.00668 ft3/s 0.4 ft3/m
4.0 gpm 0.00891 ft3/s 0.53 ft3/m
5.0 gpm 0.01114 ft3/s

0.668 ft3/m

18.0 gpm (example irrigation system) 0.04010 ft3/s 2.406 ft3/m

That's not so fast, right. But water hammer can still occur. At these flow rates, water pipe scouring problems are unlikely. Higher water flow rates and velocities are found in some residential sprinkler systems and possibly in other special equipment where scouring could be a concern.

Note that for a fixed pipe diameter, changes in water pressure will produce a corresponding change in flow rates.

To convert any of our GPM numbers to litres per minute (LPM) multiply GPM x 3.78.

Resource: Stryker, Jess, "Increasing Minimum Available GPM", - retrieved 5/13/15, original source: Mr Stryker is a landscape architect practicing in California.

Stryker provides a table for irrigation system designers who need to know maximum recommended water flow rates in GPM for various types of pipe and pipe diameters. That data will not pertain to most residential interior plumbing systems.

Water Hammer Can Also Occur even at Low Water Velocity Rates

Really? Water hammer noises can occur in a building piping system even when the water supply is turned off: the main water shut-off is closed. What on earth could explain that?

A combination of residual water pressure in the building piping system and anything that turns water flow on and off such as a timer-operated watering irrigation system can be enough to cause water hammer.

While water hammer is associated with both high water velocity and often a rapidly-closing water valve, the operation of a water valve can cause a hammering noise even when water velocity is low and water pressure is modest.

Reader Anonymous (by private email) described this case to us:

I had a water hammer going in my house even after shutting off the mains. Yes!!. It was a water hammer and it really drove me crazy. I ultimately diagnosed it to a faulty sprinkler timer on the faucet outside my house which was controlling drip irrigation.

It had failed and it was trying to switch on the value and turn it off intermittently and the quantity of water in the lines was sufficient to result in shock even though the mains were shut. And since there were drippers at the other end, it took a while for the water to drain out sufficiently.

I thought I would let you know so that you can update your site with the hope that it will help others. Your site was one of the first ones that I visited to diagnose the issue.

Bottom Line :

Anything mechanical or electrical which can turn water supply on & off on can cause a water hammer, especially when it is failing. It is hard to diagnose this as the appliance can be outside your house or even batter operated. - 2017/10/25 anonymous by private email


With the water main off I'm surprised that anything would produce water hammer - at least as I know it, since the sound requires a certain velocity of water running in pipes followed by a sudden closing of a valve.

Are you saying that the water supply system remained pressurized enough to cause water hammer until the sprinkler system drained down?

Reader follow-up:

Yes. I too was surprised. I too thought that enough water velocity is required. And it seems there is sufficient pressure in the lines. One thing that I must mention that there are only a few 1/2 GPH drippers on the sprinkler/irrigation line and there is a pressure controller too on the line.

I wouldn't have believed this myself but I can "reproduce" the issue.

The moment I turn on the faucet connected to the battery operated irrigation controller, the hammer comes back and keeps going for a while even after the mains are turned off.

If I leave the mains on, the hammer never goes away. It is as-if there is a ghost in the pipes.

How Water Hammer Can Damage Water Pipe or Pipe Connections and Result in Leakage

In more layman-like terms, water hammer works like this: water passing through a pipe has momentum or velocity. When the valve is shut quickly, the momentum of the water carries it into the valve with considerable force.

Since water is essentially incompressible, a large pressure is built up against the valve, and there is low pressure upstream in the pipe. The high-pressure water wants to flow to the low-pressure area.

This happens so quickly that a small vacuum is created against the valve as the water moves away from it. This can result in cavitation as the water is pulled back against the valve a second time. This continues back and forth in slowly diminishing shock waves.

Pressures up to 600 psi (some sources say 1000 psi) can result from water traveling up to 3,000 miles per hour, for very short periods.

Water hammer can result in loud noises in supply plumbing pipes. Water hammer only occurs as valves are closed. If a valve is closed slowly, and the noise does not occur, one can be sure that water hammer is the problem.

Water hammer is common with quick-closing electrically operated valves on appliances such as washing machines and dishwashers. Air chambers can be installed to control water hammer, as Carson Dunlop's illustration (above) demonstrates.

Causes of Water Hammer / Hydrostatic Shock Include

Diagnostic Tips for Determining that Banging or Rattling Pipe Noise is Water Hammer

If you hear a rattling, banging, or clanging that seems to come from building pipes AND if the noise stops when no water is being run in the building most likely you're hearing water hammer.

Try this: when you hear the water hammer noise, turn water on to increase the flow rate at one or more fixtures, OR turn the water flow rate off slightly to slow the water flow. Often changing the water velocity in the piping system will cause the water hammer to stop - a diagnostic clue.

If you hear banging pipes when no water is running (check to make certain that all water is off, including filling toilets, outdoor sprinklers etc.) then the banging noise may be a heating pipe problem that we discuss at BANGING HEATING PIPES RADIATORS

Cures for Water Hammer or Air Hammer - Hydrostatic Shock Noises in buildings

Again quoting our admired hydraulics expert Brian Boman,

The best prevention of water hammer is the installation of valves that cannot be rapidly closed and the selection of air vents with the appropriate orifice that do not release air too rapidly. Pipelines are usually designed so that velocities remain below 5 fps in order to avoid high surge pressures from occurring. - Op. Cit.

- The Home Reference Book

Also see SOUND CONTROL for PLUMBING for an extensive list of causes and cures of building plumbing noises.

Also see NOISES COMING FROM WATER HEATER for the diagnosis and cure of clanking or thumping noises that may be coming from your water heater or heating boiler.

- Adapted with permission from The Home Reference Book

Eight Steps to Stop Water Hammer Noise & Water-Hammer-Caused Plumbing Leaks or System Damage in Buildings

Jumper valves or sink faucets in a New York home (C) Daniel FriedmanThe sink faucets shown above were installed by the author; in some countries these simple valves are referred to as "jumper" valves.

The following list of water hammer elimination advice is an expansion of material from multiple sources including those cited throughout this article and at REFERENCES.

  1. Install a pressure regulator to keep building water supply pressure below accepted limits (500kPa in Australia)
  2. Install a water hammer eliminator between the building water supply piping source and the highest or most-distant plumbing fixture or point of use.
  3. Install water hammer suppression devices at each automatically-operated appliance water flow control solenoid or valve such as at washing machines and dishwashers.
  4. Use ball valves in place gate valves or "jumper valves" where possible. Ball valves close more smoothly and less abruptly than some other designs. Also install ball valves to control water flow at shutoffs or at points of use where the fixture valve is normally open (such as at toilets)
  5. Use sound isolating piping supports, mounts or clips when routing water piping. When penetrating a stud, if using a sealant for sound isolation, be sure it is distributed uniformly around the pipe. Also see Kwolkoski (2006)
  6. Install soft-closing water valves at fixtures or other points of use.
  7. Where typical point-of-use fixture water control valves (jumper valves in Au) are in use, install spring-loaded washers in the valve body.
  8. Clip all pipes as per AS NZS 3500.


Reader Reports Saga of Searching for an End to Water Hammer at the Water Heater (Cylinder, Geyser)

9/12/2014 Bruce Reid said:

I have a brand new water heater which i rent from my utility company. Ever since they installed it, i've had water hammer noises on all fixtures in my house be they hot or cold. At first I thought the noise was just a loose pipe, so I called the utility to come and re strap the pipes tightly to the joists like they were before. The new heater is in a slightly different position than the previous one so they had to pull the pipes down a few inches.

They also used Shark Bite connectors this time around. That's the only difference I can see. So now I have this hammer noise on any faucet or toilet in the house, and even the outside garden hose. The noise is loudest RIGHT NEXT to the water heater. If i turn off the water supply to the water heater all the noises go away. so that means it's the the heater's fault, right? My questions are these:

1) Why would a new water heater cause WH noises even on cold lines?

2) Should the utility have to pay for this? 3) The instructions for the arresters are to put them near the fixture that is causing the noise. Well ALL fixtures can trigger it, but i'm pretty sure I'd blame the heater. Would it be wise to install an arrestor right at the cold water intake of the water heater?



Your diagnosis is important and helpful but maybe not the last word. The water hammer may be due to the velocity of water entering or leaving the heater - not the fault of the heater itself.

Try slightly closing the cold inlet to the heater to see if that fixes the trouble. If it doesn't you'll want to install one or more water hammer prevention devices ahead of or after that appliance.

Reader follow-up:

9/13/14 Bruce Reid said:

Hi Dan, thanks for the tip.

I turned it halfway off this morning and am still hearing the banging, and i'm not sure if it's my imagination or not, but there seems to be a related "echo bang" now. Instead of one loud bang, i hear 2 small ones.

I'm going to experiment with this a bit more at different levels of closure of the cold water inlet.

Still. Old water heater- no noise. New water heater - lots of noise. One can only assume that it's the water heater's fault right? As i was saying, the only difference in my house is one new water heater, two pipes (in and out) that were slightly stretched to reach the heater and the use of shark bite connectors to the WH instead of soldering the pipes


On track to debug, one can but ask, so what changed: just the heater, or perhaps also nearby piping; possibly the new heater replaced one whose inlet was partly clogged, slowing water flow. Or a valve was changed, or pipe routing.

Ultimately understanding water hammer (velocity and sudden stoppage) leads to either modifying flow rate or installing anti-hammer devices.

Reader follow-up:

thanks again for your continuing advice. This is really useful.

My most recent tests to help debug. I used my garden hose from outside and brought it downstairs next to my WH. I brought a 5 gallon bucket down there to fill with the hose. I submerged the hose to keep the noise down.

Water Hammer Test conditions:

  1. Turn water inlet to the WH completely open. (this is the normal configuration). Fill the water bucket in steady 2-3 second bursts from the hose. The result is the expected Bang followed by 1-2 smaller echoing bangs about a second later.
  2. Turn water inlet to the WH completely CLOSED. Fill the water bucket in steady 2-3 second bursts from the hose. The result is SILENCE. Not a sound
  3. Turn water inlet to the WH to half open. Fill the water bucket in steady 2-3 second bursts from the hose. The result is the expected Bang followed by 1-2 smaller echoing bangs about a second later.
  4. Turn water inlet to the WH to 1/10th of a turn open (practically closed). Fill the water bucket in steady 2-3 second bursts from the hose. The result is the expected Bang followed by 1-2 smaller echoing bangs about a second later.

My intent here was to find that sweet spot in the water inlet valve that would reduce the sound to zero. Apparently that doesn't exist. Any opening means noise.

So there you have it. In a cold water only test, we get bangs from the area around the WH on any situation where the water inlet is open. Even just a crack.

On hot water tests, using another fixture, for example, the kitchen sink, with hot water tap turned on the bang is a bit bigger.

So if hammer arresters are the solution, where would you suggest putting them. on the cold water supply line to the WH? Note that's a bigger diameter pipe than the rest as you'd expect.

(I've run out of available text space here so I'll put more info in a later post.) thanks again!

Reply: where to install the water shock / water hammer arrestor

Nice going Bruce.

I confess that I've tried fixing water hammer by adjusting building water pressure or by controlling flow rate into an appliance like a water heater with only mixed success.

Have you tried buying a water hammer arrestor device and installing it on the water supply line near the heater? I'll summarize again the standard things to try when stopping water shock or water hammer noise in a building.

Heating Pipe Noises: Banging Pipes on Steam or Hot Water Heating Systems

Please see  our complete articles on heating system noise diagnosis and cure at HEATING SYSTEM NOISE DIAGNOSIS

and at BANGING HEATING PIPES RADIATORS. Excerpts are below.

If it sounds as if someone is down in your BASEMENT or cellar banging on the heating pipes with a hammer, and particularly if your building is heated with steam radiators, the noise you hear may be due to water hammer in the steam piping system.


In both one-pipe and two-pipe steam heat systems steam rises into the building's heating radiators, forcing air out of the radiator's steam vent , then making the radiator hot. Inside the hot radiator steam condenses back to water as heat is radiated (by the "radiator") into the room.


This steam condensate must drain back into the steam boiler where it is subsequently re-heated to steam to continue the heating cycle. But if the condensate is having trouble returning to the steam boiler your heating pipes may become waterlogged.

This happens because when the steam boiler water level drops and is not replenished by returning condensate, the automatic water feeder will just send more water into the boiler.

Condensate accumulating in the steam piping (when it should be returning to the boiler) not only water-logs the system, it also means that cooler condensate (water) comes into contact with hotter rising steam in the piping. This contact can cause rapid expansion/contraction in the heating pipes and produces the loud "pipe banging" noise we are discussing.

Your heating service technician should be someone familiar with steam heating systems and the proper layout and function of condensate return lines in your home. The tech will look for a problem that is blocking condensate return to the heating boiler, such as a clogged strainer in the system piping, a steam trap clogged with rust, minerals, or sediment, or a similar problem.

A separate problem: failure of individual steam radiators to get hot, could also be due to blocked condensate return.

If a radiator's steam vent is not working, or if a one-pipe steam system's radiator has settled so that it is no longer properly tipped to send condensate back into the steam pipe (and back to the boiler), that radiator will stop working. But individual radiator troubles do not usually explain banging heating pipes.

Watch out: If your heating boiler does not have an automatic water feeder and you've been putting makeup water into the boiler manually, a blocked condensate line and low water in the boiler will eventually lead to total loss of heat when the low water cutoff switch , a key boiler safety device, simply shuts down the boiler.


Other causes of heating system noises are discussed at HEATING SYSTEM NOISE DIAGNOSIS.

How to Choose & Install a Water Hammer Arrestor

Water hammer arrestor with hose fittings for washing machine (C) Sioux Chief Typically the installation instructions provided with water hammer arrestors discuss two considerations:

  1. Choose the size of water hammer arrestor depending on the number of fixtures that are likely to be running (and shutting off) at once in the building.

    Tables provided by the manufacturer will consider the number of plumbing fixtures and will recommend a water hammer arrestor size. Generally it won't hurt to install a larger capacity water hammer arresting device than you thought you needed.
  2. Install the water shock arrestor or water hammer arresting device near (but not at) the end of a water supply branch between the last two plumbing fixtures on that branch - for plumbing branch lines that are 20 feet or less in total length.

    Watch out: do not install the water hammer arrestor at the very end or uppermost end of supply piping in a building. A water shock arrestor installed in this location is unlikely to be effective.

    For longer plumbing supply pipe runs (longer than 20 ft) you may require at least one additional water hammer arrestor. In this case place the second water hammer arrestor at the mid point of the total length of the plumbing pipe run.
  3. Install a water hammer arrestor on the supply side of particular shock-causing clothes washers or dishwashers or other appliances that are causing the water shock noise.

    For example you may need to install a water hammer arrestor on the supply side of a dishwasher or clothes washer. This improvement is likely to be much less costly than replacing the solenoid valve in those appliances.

    Tip: special water hammer arrestor devices are available that include hose connectors to permit them to be installed right at the hot and cold water hook-ups of most washing machines.

    Other water hammer arrestors are available using compression or press fit connectors for installers who are not handy at soldering copper pipe. Shown above, the Sioux Chief water hammer arrestor with hose thread fittings suitable for installation at a clothes washing machine.
  4. Install a water hammer arrestor on both hot and cold water piping systems. If you observe particular water hammer noise when running hot water don't forget to install a water hammer arrestor on the hot water piping system.
  5. If installing a water hammer arrestor is not enough to stop water hammer, ask an experienced plumber to inspect the plumbing system: there may be improper piping, routing, securing, or other errors that must be corrected.

Summary of steps to stop water shock or water hammer in a building

  1. Install a water hammer arrestor on the piping system to absorb the shock of sudden stoppage of water flow
  2. Water pressure reduction: Try reducing the building water pressure right at the pressure regulator
  3. Water flow rate reduction: Try reducing the water velocity in the piping by using flow-restrictor outlets at faucets, shower heads, tubs.

    You can approximate this by partly closing the supply valve to a fixture. If you see that one plumbing fixture is particularly guilty of causing that banging slamming water hammer noise start there. You might try installing a flow restrictor ahead of the offending appliance or fixture.
  4. Use slow-closing water supply valves: slow-closing valves are available for toilets and possibly clothes washers
  5. Change water supply piping diameter (smaller) or re-route piping to use shorter pipe runs before there is a turn or bend in the piping (this may be an expensive option so it's my last choice)

Sioux Chief water hammer arrester - sold world wideWater Hammer Arrestor Size & Location Specifications

Where to install the water hammer arrestor: this information is now found at WATER HAMMER ARRESTOR LOCATION & SIZE

Water Hammer Arrestor Plumbing Codes, Installation Guides & Instruction Manuals

This information is now found at WATER HAMMER DEVICE CODES & MANUALS

Where to Buy Water Hammer Arrestors - water hammer suppression products

This data is now found at WATER HAMMER DEVICE SOURCES


Continue reading at WATER HAMMER ARRESTOR LOCATION & SIZE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see WATER HAMMER NOISE FAQs questions & answers posted originally at this article.

Or see this

Water Hammer & Banging Pipe Noise Article Series


Or see NOISE / SOUND DIAGNOSIS & CURE - complete catalog.

Or see PLUMBING DRAIN NOISE DIAGNOSIS may indicate defective or clogged plumbing: how to diagnose and cure drain sounds




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WATER HAMMER NOISE DIAGNOSE & CURE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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