How to Install a Water Hammer Arrestor on a Dishwasher

What is water hammer

Water hammer is a thumping or banging sound that occurs when a fast-moving column of water is abruptly stopped by a valve. In a dishwasher, the inlet valve snaps shut at the end of a fill cycle, creating a pressure surge that travels back through the pipe and causes the pipe to vibrate against framing or straps. The noise is often mistaken for a mechanical problem inside the appliance, but it actually originates in the plumbing.

The severity of water hammer depends on the length and diameter of the supply pipe, the flow velocity, and the speed at which the valve closes. Dishwashers typically use an electrically operated inlet valve that closes in a fraction of a second, which makes them a common source of water hammer in a kitchen.

Do you need an arrestor

If you hear a single loud bang right after the dishwasher starts filling or shortly after it stops, you likely have water hammer. Not every installation will produce this noise; the water pressure in your home and the length of the supply line play a role. If the banging is persistent and not caused by loose pipes, a water hammer arrestor is the simplest permanent fix.

An arrestor contains a sealed air chamber or a spring-loaded piston that compresses when the pressure spike occurs, absorbing the shock. It does not restrict flow and does not require maintenance. Adding one to the dishwasher supply line is a low-effort improvement that can also protect washing machine valves and other appliances connected to the same branch.

Some dishwashers, particularly those from manufacturers like Bosch and Fisher & Paykel, are designed to minimize vibration, but no dishwasher can eliminate water hammer created by the plumbing. An arrestor works externally to address the source.

Tools and parts

You will need a water hammer arrestor that is compatible with your supply line connection. Most dishwashers use a 3/4-inch female inlet connection, but check your manufacturer's manual. The arrestor typically has a threaded inlet and outlet, allowing you to insert it directly in the line.

Gather the following tools before starting: an adjustable wrench, a pipe wrench, thread seal tape (often called plumber's tape), a bucket, and a towel. You may also need a second wrench to hold the existing fitting steady while you loosen the connection.

• Water hammer arrestor (sized for your pipe diameter)
• Adjustable wrench (or two)
• Thread seal tape for metal or plastic threads
• Bucket and towel to catch drips
• Flashlight to see under the sink or cabinet

Installation steps

Start by shutting off the water supply to the dishwasher. The shutoff valve is usually located under the sink or in the cabinet next to the appliance. It may be a small quarter-turn valve with a red or blue handle. Turn it clockwise until it stops. Place a bucket under the supply line to catch any remaining water.

Disconnect the existing supply line from the shutoff valve and from the dishwasher inlet. If the line is flexible braided hose, loosen the nuts with an adjustable wrench. If it is a rigid copper tube, you may need to cut the line, but most installations use a flexible connection.

Wrap the threads of the shutoff valve outlet with thread seal tape in a clockwise direction (as you look at the outlet). Screw the arrestor onto the shutoff valve first. Then attach the dishwasher supply line to the other side of the arrestor. If the arrestor has arrows indicating flow direction, align it so water flows toward the dishwasher.

Reconnect the supply line to the dishwasher inlet valve. Use thread seal tape on the inlet threads if the connection is not a flared fitting. Tighten all fittings with two wrenches: one to hold the arrestor or valve, the other to turn the nut. Do not overtighten, as that can crack plastic or deform metal threads.

Once everything is hand-tight, give each nut a quarter turn with a wrench. Turn the water supply valve back on slowly. Listen for any hissing or leaking. Check every connection with a dry towel. Run a short rinse cycle and then a full wash cycle to confirm the water hammer is gone.

Placement tips

The arrestor should be installed as close to the dishwasher's inlet valve as possible, but it must also be accessible in case you need to replace it later. In most under-sink installations, you can install it directly between the angle stop (shutoff valve) and the braided supply line. That placement is easy to reach and works for most dishwashers.

If your dishwasher has a connection under the appliance rather than in an adjacent cabinet, you can install the arrestor on the line that runs to the floor, as long as you can still reach it. Avoid placing the arrestor behind a wall or in a location where it would be buried behind drywall, because it may need service at some point.

Keep the arrestor oriented in the direction specified by the manufacturer. Most are installed vertically or horizontally without issue, but some piston-type units have a preferred orientation to ensure the air chamber or spring stays in contact with the water column. Check the label or instruction sheet that came with the product.

Testing and troubleshooting

After installation, run a complete dishwasher cycle. The banging should be absent. If the noise persists, the arrestor may not be sized correctly for the water pressure in your home, or it may be faulty. Also check that the arrestor is not installed backwards. Some models have a one-way internal check valve, so flow direction matters.

If the banging continues only when the dishwasher stops filling, you may need a second arrestor on the cold water line if the dishwasher uses a dual inlet. Some dishwashers have a single hot water connection, but if yours has a separate cold water line, install a second arrestor there.

Water hammer can also be caused by loose pipes that are not strapped properly. After installing the arrestor, inspect the supply lines and the dishwasher drain hose to ensure they are secured with brackets. Vibration from a dishwasher motor can amplify hammer noise if pipes are free to move.

If you are installing a new dishwasher or upgrading an existing one, consult our dishwasher supply line guide and inlet valve explanation to understand the full connection. For a check on your home water pressure, see the inlet water pressure article.

After the installation, always verify for leaks at the fittings. A slow drip can lead to water damage inside the cabinet. Use the plumbing connection leak check guide as a checklist.

What to pick for your use

Questions

What does a water hammer arrestor do?

A water hammer arrestor absorbs the pressure surge that occurs when the dishwasher's inlet valve closes quickly. It contains a cushion of air or a spring-loaded piston that compresses, preventing the shock wave from traveling through the pipes and causing a banging noise.

Where is the water hammer arrestor installed?

It is installed on the hot water supply line that feeds the dishwasher, typically between the shutoff valve under the sink and the flexible supply line that connects to the appliance. It should be placed close to the dishwasher but remain accessible for future servicing.

Can I install a water hammer arrestor without a plumber?

Yes, this is a DIY-friendly task for most homeowners. You only need a couple of wrenches and thread seal tape. The main risk is over-tightening or failing to shut off the water, so follow the steps carefully and always test for leaks after turning the water back on.

Will a water hammer arrestor fix all banging noises?

It fixes noises caused by water pressure surges. If the banging persists after installing the arrestor, the pipes themselves may be loose or strapped incorrectly, or you may need a second arrestor on a separate supply line. It will not address noises from a failing wash pump or spray arm.