What Is Water Hammer? How to Fix It in Pipes

Prevent Plumbing Damage With a Water Hammer Arrester

Fixed pipes under sink with water hammer issues

The Spruce / Kevin Norris

Project Overview
  • Working Time: 20 - 30 mins
  • Total Time: 20 - 30 mins
  • Skill Level: Intermediate
  • Estimated Cost: $20 to $40

Water hammer is a common plumbing problem. It creates an annoying sound, a loud knock, or repetitive banging that can be quite startling. And while you may want to wish it away as simply one of those harmless everyday sounds, it can damage your plumbing system. Learning the basics of water hammer and fixing it can save you thousands of dollars in repairs.

What Is Water Hammer?

Water hammer is the sudden stoppage of water flow that causes water supply pipes to bang against studs, joists, beams, or even against other water supply or drain pipes. Also called hydraulic shock, it causes an alarming sound and a sensation. It can sound like a repetitive hammering sound from the walls, and you can often hear vibrations from the pipes or house supports near it.

It's often strong enough to feel it under your hand as you hold the faucet or the handle. You may even see the faucet slightly jump. The shock waves from the water hammer can even cause dishes and glasses in cabinets to vibrate.

What Causes Water Hammer

Water hammer is caused by water being shut off quickly in plumbing systems with blocked or non-functioning air chambers. In properly running plumbing pipes, these air chambers prevent the water from stopping suddenly and banging against the studs, beams, or other pipes. When these pipes hit together, they can loosen the pipes from their fittings, causing them to burst or come undone suddenly.

While sink, tub, and shower faucets can cause water hammer, clothes washers and dishwashers can more often cause water hammer because they can shut off water faster than by hand. These appliances use solenoid valves, which can close off as quickly as 30 milliseconds.

Tip

Although you don't hear it as loudly as metal pipes, water hammer can also be an issue with plastic water supply pipes, such as CPCV and PEX. Pipe and water damage is still a potential danger. If you have open areas where plastic plumbing pipes are exposed, watch the pipes for signs of movement and flexing when someone abruptly turns off faucets. Water hammer in plastic pipes also calls for remedy.

How Water Hammer Can Damage Plumbing

Left untreated, water hammer can cause significant damage by destroying pipework, the home's plumbing system, and appliances connected to the pipes. This damage has a ripple effect that can result in water damaging the home, mold issues, and structural integrity issues like rot.

  • Water leaks
  • Ruptured pipes
  • Plumbing joint failure
  • Damage to pumps and valves
  • Appliance problems
  • Exterior and structural damage

Methods for Fixing Water Hammer

Air Chambers

Air chambers can quiet water hammer by acting as a shock absorber for the water that comes to a sudden halt. Air chambers look like capped pipes extending from the supply pipes. The air chamber acts as a cushion for the water, compressing the air in the chamber and not putting pressure on the rest of the pipework.

This simple solution requires advanced plumbing skills for installation. Plus, air chambers can fill with water over time, so the pipes may occasionally need to be drained.

Tip

Water supply pipes must be secured appropriately. It's always a great idea to replace broken or loose straps as a part of a proper house maintenance schedule. When possible, replace with cushioned pipe clamps made of steel and soft EPDM rubber.

Water Hammer Arresters

A water hammer arrester is a simple device installed on a specific fixture or appliance. Arresters absorb the shock wave created by a sudden stoppage of the water flow. It is similar to an air chamber; the main difference is it has a chamber filled with gas or air that's sealed with a piston or diaphragm. The diaphragm separates the water and the gas or air.

What Is a Diaphragm?

In plumbing, a diaphragm is a flexible disc that responds to pressure to form a seal, stopping water flow. Diaphragms are often made of various kinds of rubber and other materials.

A water hammer arrester is often spliced into the point where the water supply hose meets the shutoff valve. No pipes need to be cut or sweated back together. Look for water arresters that are combined with a valve.

Water-Pressure Regulators

A water-pressure regulator ensures that the overall water pressure from the main line entering the home remains at a safe level. Normal water pressure runs between 40 and 60 psi (pounds per square inch); above this, high water pressure can cause water hammer. Water pressure above 80 psi can also damage appliances.

Install the water-pressure regulator where the main water supply enters the home. Putting the device at the main water supply protects dishwashers, toilets, and other appliances and fixtures against high water pressure. If you suspect you're having water pressure issues, test the water pressure. You can get an inexpensive water pressure gauge from most hardware stores.

What You'll Need

Equipment / Tools

  • Bucket or towel
  • Tongue-and-groove pliers or adjustable wrench
  • Old towels

Materials

  • Water hammer arrester
  • Plumber's thread seal tape

Instructions

Materials and tools to fix water hammer issues in pipes

The Spruce / Kevin Norris

How to Install a Water Hammer Arrester

  1. Turn off the Water

    Shut off the water to the clothes washer, dishwasher, or other areas that experience water hammer. The device usually has a shutoff valve that can be turned clockwise to shut off the water. Sometimes, you may find an intermediary shutoff valve farther up the line. You may want to shut off the home's entire water supply for installing multiple water hammer arresters.

    Tip

    The washing machine will have two shutoff valves, one for hot and another for cold. The dishwasher will have one shutoff valve for hot.

    Water pipe turned off with shutoff valve

    The Spruce / Kevin Norris

  2. Disconnect the Hoses

    • Use tongue-and-groove pliers or an adjustable wrench to turn the hose fittings loose.
    • Use a basin to catch spilled water from disconnecting the water hoses. Have a few old towels nearby, too.
    Water hoses disconnected from valves with wrench

    The Spruce / Kevin Norris

  3. Install the Water Hammer Arrester

    The instructions are slightly different depending on the type of arrester you use:

    • Threaded: For threaded water hammer arresters, thread the device onto each valve or inlet. Turn the female fitting clockwise by hand, then tighten it with the pliers.
    • Compression: For compression fittings, slide the compression ring against the valve, then thread on the compression nut and tighten.
    • Push-Fit: Push-fit water hammer arresters need no installation tools (though removal requires a small plastic tool). Clean copper tubes of burrs. Make sure that all cuts are square. Push the fitting firmly onto the pipe.

    Tip

    If using a threaded water hammer arrester, for a tighter fit, you can wrap the male end of each shutoff valve or water inlet with thread seal tape or Teflon tape before threading the arrester on. Wrap the tape tightly three or four times clockwise. Wrap any male threads on the water hammer arrester with tape, too.

    Water hammer arrestor threaded unto valve

    The Spruce / Kevin Norris

  4. Connect the Supply Tube or Hoses

    • Use the pliers or the wrench to connect the water supply tube or hose to the water hammer arrester.
    • Tighten the connection.
    Water supply hoses connected to water hammer arrestor

    The Spruce / Kevin Norris

  5. Turn on the Water

    • Slowly turn on the water supply by turning the shutoff valve counterclockwise. If you shut off the water to the entire house, faucets may sputter before water begins running freely again.
    • Use a flashlight to inspect the connections for leaks visually.
    • Run every appliance on a water hammer arrester on one test cycle.
    Water supply turned on by moving shutoff valve counterclockwise

    The Spruce / Kevin Norris

FAQ
  • How serious is water hammer?

    A lot of the time, the severity of the water hammer depends on the frequency. If it only occurs once and doesn't repeat, it's less cause for concern. However, if you feel rattling when it happens, along with loud, repetitive banging noises, heed your pipe's warnings and look into it.

  • Why did I suddenly get a water hammer?

    An air disturbance in your water pipes causes water hammer. It could be a surge or drop in water pressure, a clogged air chamber, or a sudden turning off of water at the shut-off valves.

  • Can a plumber fix water hammer?

    Plumbers are your best experts for fixing water hammer, although a handyperson, subcontractor, or DIYer can do it too.

The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Residential Plumbing Overview. National Association of Certified Home Inspectors.

  2. Water Pressure. Padre Dam Municipal Water District.