Short definition
A water hammer arrestor is a sealed bladder device that absorbs the pressure spike from a fast-closing valve. Unlike legacy air chambers, arrestors don’t waterlog — captive air sits behind a permanent bladder. PDI Size A handles a typical residential washing machine; install within 6 feet of the quick-acting valve.
What it is
A bladder arrestor is a small cylindrical pressure vessel — usually 4-6 inches long — with an internal rubber bladder separating water from a permanent air pocket. When a valve closes fast, water pressure spikes; the bladder compresses the air pocket; the spike rebounds gently rather than as a bang.
Selection (PDI sizing):
- AA, A, B, C, D, E, F — Plumbing & Drainage Institute standard sizes, smallest to largest.
- Size A typical for a residential washing-machine connection.
- Size B-C for kitchen branches with multiple fixtures.
- Match the size to the fixture-unit count of the branch served.
Placement:
- Within 6 feet of the quick-acting valve. Closer is better.
- One per ~6-meter (20-foot) branch.
- Many washing-machine outlet boxes have built-in arrestor ports — use them.
Installation: tee into the supply line within reach of the valve. Most arrestors thread into a standard ½-inch or ¾-inch tee fitting. WA code (UPC §609.10 as amended) requires arrestors at quick-acting valves on washing machines, dishwashers, and ice makers in modern construction.
Why it matters to a homeowner
If your washing machine bangs the wall every fill cycle, a $25-$40 arrestor solves it in 30 minutes — once you have access to the connection. The arrestor is the standard, durable answer; legacy air chambers waterlog over years and stop working, but arrestors are sealed for life (typically 10-15 years before bladder fatigue).
When a plumber says “I’ll install arrestors at the laundry hookups,” that’s a typical $100-$200 line item on a remodel. On a finished install, the cost is mostly access — opening the wall to reach the connection.
Common failure modes
- Bladder failure after 10-15 years. Arrestor becomes inert; replace.
- Improper placement (too far from valve). Reduces effectiveness.
- Undersized for the fixture-unit count. Doesn’t fully absorb the spike.
- “Air chamber” mistakenly called an arrestor. Capped-stub air chambers waterlog; true arrestors don’t.
Common variants
- Captive-air bladder arrestor (modern, sealed) vs. air chamber (legacy capped stub). Different products.
- Mini-expansion vessel — sometimes used interchangeably with “arrestor” but technically refers to thermal-expansion vessels.
- PDI-certified arrestor vs. generic “shock absorber.” Look for the PDI marking.
Washington note
WA-amended UPC §609.10 requires water-hammer arrestors at quick-acting valves on washing machines, dishwashers, and ice makers in new construction and remodels. Pre-1995 homes often don’t have them; if you’re seeing water hammer in an older home, the install is a homeowner-friendly upgrade that meets current code.
For hilly Seattle homes with high street pressure (100+ psi on Queen Anne, Magnolia, West Seattle), arrestors are necessary but not sufficient. A PRV at the meter does the system-wide work; arrestors handle the individual quick-acting events. Both, in that order.