Do new washing machines need water hammer arrestors? What codes and plumbers say

A homeowner on Reddit's r/Plumbing asked this week whether arrestors were worth adding after a washer swap, since the pipes made no noise and one of the units would not fit behind the machine. Under the model codes the answer is short. Section 604.9 of the International Plumbing Code calls for an arrestor listed to ASSE 1010 wherever quick-closing valves are used, installed to the manufacturer's instructions, according to a DataDrivenAEC summary of the 2024 edition. The International Residential Code, which governs single-family homes, has the same rule in Section P2903.5.

Quiet pipes don't earn an exemption. Timothy Allinson, a mechanical engineer writing in PHCP Pros, counts the solenoid valves on washers, dishwashers and ice makers as quick-closing under the code, although real hammer also depends on how fast the water moves. Flexible supply hoses can absorb much of the shock before it reaches the piping, he wrote, but a designer can't control which hoses an owner buys, so he specifies arrestors anyway. Minnesota took a different line: its amended version of the Uniform Plumbing Code requires arrestors only in systems where hammer occurs, mounted as near the quick-acting valve as they can go.

Placement is where the Reddit poster got stuck. The valve that snaps shut sits inside the washer, so an arrestor at the machine's inlet is closer to it than one at the wall box. Reuben Saltzman of Structure Tech, a Minnesota home inspection firm, points out that an arrestor at the wall valve does nothing for the hoses between that valve and the machine, and those hoses take the shock each time the machine stops the flow. He recommends stainless steel braided hoses over plain rubber and notes that washer makers, Maytag among them, tell owners to close the supply valves when the machine is idle.

Check pressure too. Section 604.8 of the IPC requires a pressure-reducing valve with a strainer whenever the building's static pressure runs above 80 psi, and the valve must hold it at 80 psi or below.