Why Hammer Arrestors Matter: Protect Your Dishwasher, Washing Machine, and Plumbing System
Why Do Pipes Make a Loud Banging Noise?
Have you ever heard a loud banging...
Have you ever heard a loud banging or knocking sound inside your walls after your dishwasher or washing machine finishes filling with water?
Many homeowners assume something is seriously wrong with the plumbing. In many cases, however, the problem is caused by water hammer.
From my experience working on residential homes and condominium plumbing systems, water hammer is one of the most common plumbing issues that homeowners overlook until the noise becomes frequent.
Fortunately, a small plumbing device called a hammer arrestor can often help reduce these pressure shocks and protect your plumbing system.
Watch the Step-by-Step Video
Watch this short demonstration to learn what a hammer arrestor does, where it is commonly installed, and why it helps reduce water hammer in residential plumbing systems.
Why This Matters
Water hammer is more than an annoying noise.
When water flow stops suddenly, the moving water has nowhere to go. The pressure wave travels through the plumbing pipes and creates a sudden shock.
Over time, repeated pressure shocks may contribute to:
- Loud banging or knocking noises
- Pipe vibration
- Stress on shut-off valves
- Stress on plumbing fittings
- Wear on appliance supply hoses
- Increased strain on plumbing components
Although one occurrence may not cause immediate damage, repeated water hammer should not be ignored.
How Does a Mechanical Water Hammer Arrestor Work?
A modern mechanical water hammer arrestor typically contains a sealed, gas-charged chamber separated from the plumbing water by a piston or diaphragm.
When a fast-closing valve suddenly stops moving water, a pressure surge travels through the supply piping.
The arrestor helps absorb the surge by allowing the piston or diaphragm to move and temporarily compress the gas inside its sealed chamber.
As the pressure returns toward normal, the compressed gas pushes the piston or diaphragm back toward its original position.
This helps reduce the intensity of pressure shocks transmitted through the plumbing system.
Unlike a traditional capped air chamber, a properly designed mechanical arrestor separates its gas cushion from the water, helping maintain its effectiveness over time.
Why Dishwashers and Washing Machines Need Hammer Arrestors
Modern dishwashers and washing machines use electrically controlled valves.
These valves open and close very quickly.
When the valve closes instantly, water moving through the pipe suddenly stops.
This rapid stop creates a pressure surge known as water hammer.
Because these appliances cycle on and off many times during normal operation, they are among the most common locations where hammer arrestors are installed.
Dishwasher Water Hammer Arrestor Location
A dishwasher water hammer arrestor is installed on the pressurized water-supply line serving the dishwasher.
Depending on the plumbing layout, it may be installed under the kitchen sink or at another accessible location.
The arrestor should be positioned and sized according to the manufacturer's installation instructions and applicable plumbing requirements, generally near the quick-closing valve responsible for the pressure surge.
Installing an arrestor near the shut-off valve does not automatically guarantee that it is positioned correctly for every dishwasher installation.
Washing Machine
Laundry hammer arrestors are commonly installed:
- At the laundry outlet box
- On both the hot and cold water supply lines
- Behind the washing machine
Many newer laundry outlet boxes already include integrated hammer arrestors.
Signs You May Have Water Hammer
You may have water hammer if you notice:
- Banging pipes after a dishwasher fills
- Knocking sounds when the washing machine stops filling
- Pipe vibration inside the walls
- Repeated clicking or thumping after water shuts off
These symptoms do not always mean the hammer arrestor has failed, but they indicate the plumbing system should be inspected.
Can a Hammer Arrestor Wear Out?
Yes.
Although hammer arrestors are designed to last many years, they are mechanical devices.
Depending on the design and plumbing conditions, they may eventually lose effectiveness.
If banging noises develop where none previously existed, a plumber may recommend inspecting or replacing the hammer arrestor after confirming that no other plumbing issue is causing the noise.
Other Possible Causes of Banging Pipes
Not every banging noise is caused by water hammer.
Other possible causes include:
Loose or improperly supported water-supply pipes
Excessive water pressure
A malfunctioning pressure-reducing valve
Thermal expansion and movement of hot-water piping
Pipes contacting framing or other building materials
Appliance-related vibration or mechanical noise
The timing, location, and characteristics of the sound are important clues.
A qualified plumber can help determine whether the noise is caused by a pressure surge, pipe movement, or another mechanical condition.
When Should You Call a Plumber?
Consider contacting a qualified plumber if:
- The banging becomes louder over time.
- Pipes visibly shake during appliance operation.
- Water supply hoses move excessively.
- Water leaks are present.
- You are unsure whether a hammer arrestor is installed.
- The noise continues after replacing the hammer arrestor.
Ignoring repeated water hammer can place unnecessary stress on plumbing components over time.
Frequently Asked Questions
Is water hammer dangerous?
Usually not immediately. However, repeated pressure shocks may increase wear on plumbing components over time.
Can I install a hammer arrestor myself?
Some hammer arrestors are simple threaded fittings, while others require plumbing modifications. If you are unfamiliar with plumbing work, consult a qualified plumber.
Does every dishwasher need a hammer arrestor?
Not always. Building codes, plumbing design, water pressure, and appliance installation methods all influence whether one is required or recommended.
About the Author
This article was prepared and reviewed by Charles J., a Certified Red Seal plumber with more than 10 years of hands-on experience in residential and commercial plumbing in British Columbia, Canada.
CJ BC Plumbing Tips provides practical plumbing education to help homeowners understand water-supply systems and recognize common plumbing problems.
Visual Disclosure
The accompanying video uses AI-assisted 3D animation to explain how water hammer arrestors work and where they are commonly installed.
The technical plumbing guidance was prepared and reviewed by CJ BC Plumbing.
Related Plumbing Guides
Learn more about appliance water-supply connections, pipe vibration, and practical plumbing maintenance with CJ BC Plumbing Tips.
Final Thoughts
A hammer arrestor is a small plumbing component, but it plays an important role in protecting your plumbing system from repeated pressure shocks.
If your dishwasher or washing machine regularly causes banging noises inside the walls, do not ignore the warning signs.
Identifying the cause early may help reduce unnecessary wear on pipes, valves, fittings, and appliance connections.
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