Water Hammer Arrestor Calculator
Determine the size and number of water hammer arrestors needed based on fixture types, pipe size, and system pressure.
Results
Visualization
How It Works
Water hammer occurs when fast-closing valves suddenly stop water flow, creating a pressure shockwave. Arrestors absorb this shock with an internal piston or bellows chamber. PDI sizing (AA through F) is based on fixture units and pipe size.
The Formula
Fixture Units = Quick-Close Valves x 2
Size = Based on PDI WH-201 chart (pipe size vs fixture units)
Size = Based on PDI WH-201 chart (pipe size vs fixture units)
Variables
- FU — Fixture Units — standardized load measure per PDI
- PDI — Plumbing and Drainage Institute — sets arrestor standards
Worked Example
3 quick-close valves on 3/4" pipe: 6 FU. Size A arrestor. Install 2 — one at washer, one at dishwasher.
Practical Tips
- Install arrestors as close to the offending valve as possible
- High pressure (>80 PSI) worsens water hammer — add a PRV first
- Piston-type arrestors are permanent; bellows types may need replacement
- Air chambers lose their air over time — mechanical arrestors are better