Why do commercial buildings get water hammer, and what is a water hammer arrestor?

Because they use fast-closing valves in volume. The code's own wording is "quick-closing valves," and a commercial building has plenty:

• Flushometer valves on toilets and urinals, which open for a few seconds and then shut hard. The flushometer page explains how they work.
• Sensor faucets and electronic valves.
• Solenoid valves on dishwashers, ice machines, coffee brewers, steamers and other equipment.
• Long branch lines and high pressure. PDI's sizing method accounts for "simultaneous usage, pipe size, length, flow pressure and velocity," which are exactly the things that grow in a larger building.

When a valve shuts, the moving water has to stop, and the sudden stop sends a pressure surge through the pipe. That surge is the knock you hear. For the household version, see why pipes bang when you shut off the water.

IPC Section 604.9 is short: "The flow velocity of the water distribution system shall be controlled to reduce the possibility of water hammer. A water-hammer arrestor shall be installed where quick-closing valves are utilized." It adds that arrestors "shall be installed in accordance with the manufacturer's instructions" and "shall conform to ASSE 1010." Phoenix's published amendments to the 2024 IPC do not change Section 604.9, so the base text applies to IPC-based plans there. A UPC-based design follows that code's own water hammer section.

A sealed, pre-charged shock absorber on the water line. Zurn describes its Model 1260XL as designed to protect "residential, commercial and industrial water lines during pressure surges following quick valve closure," with the surge absorbed "by the pre-charged cushion of air permanently sealed within the water hammer arrester." Inside is a piston that moves against that cushion. The sheet lists the unit as PDI certified to WH-201 and ASSE listed 1010.

Zurn says its Model 1260XL may be installed within a stud bay "without the need for an access panel," though access rules for your building come from the plans. Older systems used plain air chambers; the household page on water hammer covers those.

By fixture units, using the Plumbing and Drainage Institute's standard PDI-WH201. The method works like this:

1. 1Add the water supply fixture units on each branch line, hot and cold separately. In the fixture unit table printed with Zurn's sizing information, a public flush valve water closet counts 8 units, a public flush tank water closet 5, and a flush valve urinal 4.
2. 2Look up the total in the PDI sizing table to get an arrestor size, marked AA, A, B, C, D, E or F.
3. 3Adjust for pressure. The PDI table "is based on a maximum water pressure in the branch of 55 psi. When the pressure in the branch exceeds 55 psi, the water hammer arrester should be increased by one size."
4. 4Place it. The recommended location is "at the end of the branch line between the last two fixtures served," for branches that do not exceed 20 feet. Beyond that, "an additional water hammer arrester must be installed."
5. 5For a long run to a single piece of equipment, put it "as close as possible to the point of quick closure."

Zurn's sheet adds that for best performance static line pressure should not exceed 80 psi. If the building pressure is higher, a pressure reducing valve is part of the fix; see where the pressure reducing valve goes relative to the meter.

Start with the basics from the code and the sheet above. Check the static pressure with a gauge, since high pressure makes the surge worse. Look at whether the flush valves, solenoids and equipment branches have arrestors at all, and whether any that exist are sized and placed for the branch they serve. A plumber can then add or move devices near the quick-closing valves.

HQ is licensed and insured and gives free estimates on commercial plumbing, so a noisy building can be checked and quoted before any work begins.