How to Install a Water Hammer Arrestor

Learning how to install a water hammer arrestor is one of the best weekend projects for a homeowner tired of loud banging pipes. Water hammer is the shock wave created when a fast-closing valve, like the solenoid in a washing machine or dishwasher, suddenly stops a column of moving water. The pressure spike slams through the pipes, making them bang, shudder, and rattle inside the walls. Left alone, that repeated pounding can loosen pipe joints, crack fittings, and shorten the life of your appliances. A water hammer arrestor absorbs the shock before it reaches the rest of your plumbing, and most installations take less than an hour with basic tools.

What Is Water Hammer and What Does an Arrestor Do?

Water hammer happens because water is heavy and nearly impossible to compress. When water flows through a pipe at 40 to 80 psi and a valve snaps shut in a fraction of a second, all that moving water has nowhere to go. Its momentum converts into a pressure spike that can briefly reach several hundred psi, and that spike bounces back and forth through the system until it fades. You hear it as a single loud bang or a series of knocks right after a washing machine fills, a dishwasher cycles, or a toilet stops filling.

Older homes sometimes relied on vertical air chambers, which are capped lengths of pipe that trap a pocket of air. The problem is that the air slowly dissolves into the water over weeks and months, so the chamber fills with water and stops working. That is why the banging often returns after a home sits for a while, and why draining the system sometimes seems to fix it temporarily. A modern water hammer arrestor solves this properly. It contains a sealed air chamber separated from the water by a piston or diaphragm, so the air cushion never gets absorbed. When the pressure spike hits, the piston compresses, absorbs the energy, and the pipes stay quiet.

It is worth confirming the noise is actually water hammer before you buy parts. A single sharp bang when a valve closes is classic water hammer. A steady rattling while water runs is usually a loose pipe strap. A high-pitched whistle points to a worn valve or excessive pressure. If you are not sure, read our guide on how to fix water hammer to pin down the cause before you start.

Types of Water Hammer Arrestors

There are three common types, and the right choice depends on where the noise is coming from and how comfortable you are with basic plumbing connections. Screw-on arrestors thread directly onto the hose bib or valve outlet and are the easiest option for washing machines. They come in pairs, one for hot and one for cold, and install in minutes with no tools beyond a pair of pliers. Expect to pay $15 to $30 for a pair.

Tee-style arrestors install into the supply line itself and are the most versatile option. They connect with sweat (soldered), push-fit, or compression fittings, and they work for dishwashers, ice makers, and any fixture with a fast-closing valve. A single tee arrestor costs $10 to $25, and push-fit versions from brands like SharkBite run $20 to $35 each. Push-fit is the best choice for a first-time DIYer because it needs no torch, no solder, and no special skill.

Whole-house arrestors are larger units installed near the main shutoff or the water meter. They help when several fixtures cause banging, or when the problem valve is buried inside a wall. These cost $40 to $100 and are best installed where the main line enters the home, ideally on a vertical section of pipe. If your home has a pressure regulator, check the static water pressure first: anything above 80 psi is too high under most US plumbing codes and will make water hammer worse. A pressure reading is free with a $10 gauge from a hardware store.

Tools, Materials, and Costs

For a screw-on washing machine arrestor, you need almost nothing: the arrestor pair, a pair of channel-lock pliers, a towel, and a bucket. For a tee-style installation you will need a pipe cutter, a deburring tool, emery cloth, and the fitting type you chose. Push-fit needs only the cutter and deburring tool. Sweat connections need a propane torch, lead-free solder, flux, and a flame shield, so most beginners should skip soldering and use push-fit instead.

Before you buy, check your pipe size and material. Most US homes use 1/2-inch copper or PEX for fixture supply lines and 3/4-inch for the main. Arrestors are sold by pipe size, and a 1/2-inch arrestor on a 3/4-inch line will not seal properly. Check the label on the valve or measure the outside diameter of the pipe. Also pick up Teflon tape and a spare supply hose if your washing machine hoses are more than five years old, since you will already have them disconnected.

Where to Install a Water Hammer Arrestor

Placement matters more than the brand you buy. An arrestor works best when it sits close to the valve that causes the shock, ideally within 6 feet and on the supply line feeding that fixture. For a washing machine, screw the arrestors onto the hot and cold hose bibs behind the machine, then attach the hoses to the arrestors. For a dishwasher, install a tee arrestor on the hot water supply line under the sink, as close to the dishwasher shutoff valve as you can get. For an ice maker or refrigerator, a small arrestor on the 1/4-inch supply line near the saddle valve or shutoff does the job.

Orientation is simple: most modern arrestors can be installed in any position, vertical or horizontal, because the sealed piston works regardless of gravity. Older air-chamber style units had to be vertical, but sealed piston arrestors do not. Still, a vertical installation with the arrestor pointing up is the most common and keeps the unit out of the way. Avoid installing an arrestor on the drain side of a fixture or downstream of the valve, where it cannot intercept the pressure wave. It must sit on the supply side, between the water source and the fast-closing valve.

How to Install a Water Hammer Arrestor: Step by Step

Start by shutting off the water to the fixture you are working on. For a washing machine, turn both hose bibs clockwise until they stop. For a dishwasher, close the hot-water shutoff under the sink. If there is no fixture shutoff, use the main shutoff for the house. Open a faucet at the lowest point in the home to drain pressure from the lines, and keep a bucket and towels handy. If you are unsure where your main shutoff is, see our guide on how to shut off the main water supply.

For a screw-on washing machine installation, unscrew the supply hoses from the hose bibs and let any water drain into the bucket. Thread an arrestor onto each bib by hand, then tighten about a quarter turn with pliers. Do not over-tighten: snug is enough, and cracked plastic threads are a common beginner error. Reattach the hoses to the arrestors, making sure the rubber washers inside the hose ends are seated flat and not twisted. Turn the water back on slowly and check for drips.

For a push-fit tee installation, mark the supply line where the arrestor will go and cut out a section slightly shorter than the fitting, following the length printed on the arrestor packaging. Deburr the cut ends inside and out, then clean them with emery cloth until they shine. Push the arrestor onto the pipe ends until each side clicks or reaches the depth mark, then tug firmly to confirm the grip. Turn the water back on slowly and watch every joint for five minutes. Push-fit fittings hold instantly, but a shallow insertion will leak under pressure.

Testing Your Work and Fixing Leaks

Once the water is back on, run the fixture that used to cause the banging and listen. Start a washing machine fill cycle or run the dishwasher and stand near the pipes. The bang should be gone or reduced to a soft thud. If you still hear a sharp knock, the arrestor may be too far from the valve, undersized for the line, or installed on the wrong branch. Check the placement first, then consider a larger arrestor or a second unit closer to the valve.

Check every connection for leaks twice: once right after turning the water on, and again after ten minutes of normal use. A slow drip at a threaded joint usually means the connection needs another quarter turn or a fresh wrap of Teflon tape. A drip at a push-fit joint means the pipe was not inserted to full depth or was not deburred properly; shut the water off, remove the fitting with a disconnect clip, and redo it. A soldered joint that weeps needs to be taken apart, cleaned, and re-soldered, since there is no shortcut for a bad solder joint.

Common Mistakes to Avoid

The most common mistake is installing the arrestor too far from the problem valve. An arrestor at the water heater will not protect a washing machine on the other side of the house, because the shock wave dissipates into the long run of pipe before it reaches the cushion. Keep the unit within 6 feet of the fast-closing valve whenever possible.

Another frequent error is using only one arrestor for a washing machine. Washers have two supply valves, hot and cold, and either one can slam shut. Always install a pair, one on each line. Similarly, do not rely on a single arrestor to fix banging from three different fixtures. Each fast-closing valve deserves its own unit if it makes noise.

When to Call a Plumber

Most arrestor installations are safe DIY work, but some situations call for a professional. If the banging continues after arrestors are installed at every noisy fixture, the cause may be excessive municipal pressure, a failed pressure regulator, or a main line issue that needs a whole-house solution. If you find corroded, galvanized, or lead supply lines while working, stop and get a plumber to assess the pipe condition before cutting into it. A plumber can also help if the noisy valve is inside a finished wall and there is no access panel, since opening drywall and repairing it is a bigger job than the arrestor itself.

Professional installation typically costs $150 to $350 including parts, depending on access and the number of arrestors. That is a fair price if soldering is involved or if the valve is hard to reach. If you are unsure at any point, call a licensed plumber at (877) 762-1902 for help diagnosing the noise and installing the right arrestor the first time. For more DIY guidance, browse the DIY plumbing hub for step-by-step guides on common homeowner repairs.