What is a water hammer arrestor?

What this means for your home project

1. Look at the washer’s hot and cold supply connections and note whether the proposed mounting point is at the shutoffs or the back of the washer. Source: Oatey, read 2026-11-05.
2. Request a written arrestor schedule that gives the fixture-unit total for each hot or cold branch and the corresponding Table V selection. Source: Plumbing and Drainage Institute, read 2026-11-05.
3. Ask for a plan for access during servicing in the written scope so the reply identifies how the device can be reached for replacement. Source: Jay R. Smith Mfg. Co., read 2026-11-05.
4. For work in New York City, obtain the chosen arrestor’s installation instructions and ask the installer to identify how the proposed installation follows them. (source)

Where should I install Oatey Quiet pipes?

Oatey (read 2026-11-05) identifies the washer connections to discuss when choosing where its Quiet Pipes arrestors will sit. “They can be mounted directly to the hot and cold connections on the back of washer or to the hot and cold water shut offs for the washer.” (source) “We recommend they be mounted to the hot and cold shut offs for more stability.” (source)

Can hammer arrestors be installed upside down?

Oatey (read 2026-11-05) explains the orientation allowance for its own gas-filled arrestors when planning a washer installation. “Oatey hammer arrestors are gas filled chambers which absorb shock no matter what angle they are installed.” (source)

Will the hammer arrestors ever fill with water?

Oatey (read 2026-11-05) explains what water inside its sealed arrestor would mean for the repair-or-replacement decision. “The arrestors are gas filled and sealed with internal O-Rings.” (source) “The only way that the arrestor will fill with water, is if one of the internal O-Rings fail.” (source) “The O-Rings cannot be replaced and the arrestor would have to be replaced.” (source)

Where are water hammer arresters required?

New York City section 604.9 (read 2026-11-05) gives this jurisdiction-specific installation rule to discuss with your installer. “A water-hammer arrestor shall be installed where quick-closing valves are utilized and when otherwise required by this code.” (source)

Do water hammer arresters require access panels?

Smith (read 2026-11-05) offers a servicing recommendation to discuss before deciding how the installation will remain accessible. “It is always wise to provide access to any mechanical device that may need servicing or replacing during the life of the building.” (source)

Will water hammer arresters control the movement in piping mains?

Smith (read 2026-11-05) separates causes of pipe movement so the proposed scope can address the cause of the noise. “The greatest movement is caused by shock which can be controlled by the installation of water hammer arresters.” (source) “The movement caused by water flow can be controlled by the proper placement of pipe hangers and supports.” (source) “A pressure reducing valve installed on the discharge side of the water meter can be most helpful in protecting the residential piping system.” (source)

How do you size a water hammer arrestor?

Use PDI’s fixture-load scale to choose capacity; New York State section 604.9 names the product standard (read 2026-11-05). “The following symbol listing has been devised to denote the range in sizes for water hammer arresters, “AA” is the smallest sized unit - “F” represents the largest unit.” (source) “The National Plumbing Code offers this definition: “A fixture-unit is a quantity in terms of which the load producing effects on the plumbing system of different kinds of plumbing fixtures are expressed on some arbitrarily chosen scale.”” (source) “When the weight in “fixture-units” for cold and hot water branch lines serving a group of fixtures has been determined, this data can be applied to Table V.” (source) “Water-hammer arrestors shall conform to ASSE 1010.” (source)

What drives the cost of installing a water hammer arrestor?

PDI (read 2026-11-05) identifies when a multiple-fixture branch needs additional equipment; include that branch length in the installation scope. “The location of the water hammer arresters shown in Fig. 18 applies to branch lines that do not exceed 20’ in length, from the start of the horizontal branch line to the last fixture supply on this branch line.” (source) “When the branch line exceeds the 20’ length, an additional water hammer arrester should be used.” (source)

Why is this water hammer arrestor used instead of a pipe cap?

PDI (read 2026-11-05) explains the air-retention difference to consider when comparing an engineered arrestor with a capped-pipe air chamber. “The unit consists of a capped piece of pipe, the same diameter as the line it serves, and its length ranges between 12” and 24”.” (source) “During this process, the water absorbs the air, and as the unit becomes waterlogged, it loses its ability to absorb shock.” (source) “Engineered devices also use gas or air to control water hammer.” (source) “The gas or air, however, is permanently sealed in the unit.” (source)

Sources

• Water Hammer Arrestors — Oatey; read 2026-11-05.
• Water Hammer Arresters FAQ — Jay R. Smith Mfg. Co.; read 2026-11-05.
• 2022 Plumbing Code, chapter 6, section 604.9 — New York City Department of Buildings; published 2022; read 2026-11-05.
• 2025 Plumbing Code of New York State, section 604.9 — New York State Department of State; published 2025; read 2026-11-05.
• Standard PDI-WH 201, revised 2017, pages 7, 9, 11, 19–22 and 24 — Plumbing and Drainage Institute; published 2017; read 2026-11-05.