A solenoid valve on a washing machine snaps closed in 50 milliseconds. A flush valve on a bathroom WC closes in 20 milliseconds. The 6 m/s of water moving through the riser doesn’t stop instantly — its kinetic energy converts to pressure. That pressure spike, compounded across pipe length, exceeds 25 bar in a typical residential riser and can split brass ferrules, blow check valve seats, or just sound like a gunshot at 3 AM. Water hammer arrestors absorb that surge.
This guide explains the PDI WH-201 sizing method, where to place arrestors, and the math that justifies a particular size to the AHJ.
What surge pressure actually equals
The Joukowsky equation gives the maximum pressure rise from instantaneous flow stop:
ΔP = ρ × c × Δv
Where:
- ρ = density of water = 1,000 kg/m³
- c = wave velocity in the pipe ≈ 1,300 m/s for copper, 600-800 m/s for plastic
- Δv = velocity change
For a 6 m/s flow in copper pipe stopped instantly:
ΔP = 1,000 × 1,300 × 6 = 7,800,000 Pa = 78 bar
That’s 78 bar above static pressure on top of, say, 4 bar static = 82 bar peak. This is theoretical (instantaneous valve closure); real solenoid valves close in 30-50 ms which limits the peak somewhat. But even at half — 30-40 bar — the spike exceeds the 16-bar working rating of typical plumbing fittings.
When you actually need an arrestor
Per PDI WH-201 and IS 1172, arrestors are required wherever:
1. Quick-closing valves are present — solenoid valves, magnetic stop valves, lever-operated ball valves, dishwasher and washing machine inlets.
2. Long runs > 30 m to a flush valve.
3. Lever-operated bathroom or kitchen appliances (sensor flush, lever-style mixer).
A traditional gravity-fed system with screw-down stop valves and slow-closing taps does not normally need arrestors. Modern systems with electronic flush valves, washing machine valves, and solenoid-controlled appliances need them at every appliance.
Where to place them
PDI WH-201 specifies arrestor placement:
- Within 1.5 m of the quick-closing valve. Further than that, the pipe length between the appliance and the arrestor itself becomes a hammer-generating section.
- On the supply side (between main and quick-close valve), not the discharge side.
- Vertical orientation with valve below (charge holds longer).
- Air-charged type (sealed, not air-gap) — air-gap models lose charge over time and require periodic recharging.
Sizing by fixture units (PDI WH-201)
The PDI method aggregates all quick-closing fixtures upstream of the arrestor by their hammer fixture units, then matches to arrestor letter sizes:
| Fixture | Quick-close fixture units |
|---|---|
| Lavatory faucet | 1 |
| Bathroom sink (single) | 1 |
| Kitchen faucet | 1 |
| Solenoid washing machine | 4 |
| Solenoid dishwasher | 2 |
| Flush valve WC | 8 |
| Flush valve urinal | 4 |
| Sensor flush WC | 6 |
| Sensor flush urinal | 4 |
| Bidet | 2 |
| Tub | 4 |
| Shower (single) | 2 |
| Bar / pantry sink | 1 |
Total upstream FUs determine the arrestor letter size:
| Arrestor letter | FU range | Typical capacity (L/s) |
|---|---|---|
| A | 1-11 | up to 0.34 |
| B | 12-32 | up to 0.95 |
| C | 33-60 | up to 1.77 |
| D | 61-113 | up to 3.34 |
| E | 114-154 | up to 4.55 |
| F | 155-330 | up to 9.74 |
For a typical hotel guest bathroom (1 lavatory + 1 sensor flush WC + 1 sensor flush urinal-not applicable + 1 shower + 1 tub) downstream of a single arrestor at the entry to the bathroom:
Total FU = 1 + 6 + 2 + 4 = 13 → Letter B arrestor
For 30 such bathrooms on a single floor riser at the riser inlet:
Total FU = 30 × 13 = 390 → Letter F arrestor
In practice, that single Letter F arrestor at the floor riser is often replaced by individual Letter A or B arrestors at each bathroom entry — the distributed approach has shorter unprotected pipe runs and is more reliable.
Distributed vs. central arrestor strategy
Distributed (one per fixture):
- Pros: shortest unprotected pipe runs; failure of one arrestor only affects one fixture
- Cons: more components to install + maintain
- Use for: hotels, hospitals, multi-flat residential
Central (one large arrestor at riser inlet):
- Pros: one component to install + maintain
- Cons: long pipe sections downstream remain unprotected; failure affects every downstream fixture
- Use for: warehouse / industrial / commercial with limited quick-close valves
PDI WH-201 recommends distributed for residential and hospitality. Central is acceptable only when fixture-distance from arrestor is documented as < 2 m on every quick-close branch.
Worked example: 4-storey office, single solenoid washing machine in pantry
Single solenoid washing machine in 4th-floor pantry. Pipe run = 3 m branch + 12 m vertical riser to ground.
Solenoid washing machine FU = 4 → Letter A arrestor.
Placement: vertical orientation, 0.5-1 m above pantry floor on the supply branch immediately above the cut-off valve to the washing machine.
Verification: surge pressure ΔP = ρ × c × Δv = 1,000 × 1,300 × 3.0 = 39 bar (assuming 3 m/s flow in 25 mm copper). Letter A arrestor rated for 8.5 bar working pressure with absorption capacity 0.5 L of water at design flow — adequate for one fixture FU=4.
Five mistakes that produce hammer at site
1. Sizing arrestor by pipe diameter instead of fixture units. The math is FU-based; pipe diameter alone says nothing about how much surge to absorb.
2. Placing arrestor 3-5 m from valve. The pipe between valve and arrestor still hammers — defeats the purpose. Stay within 1.5 m.
3. Using air-gap arrestor without recharge schedule. Air bleeds out within 12-24 months; pipe goes back to hammering.
4. Forgetting parallel-flow arrestor on dishwasher/washing machine combo. Each appliance gets its own arrestor — both quick-closing.
5. No isolation valve before the arrestor. Service replacement requires shutting the riser. Tedious. Always specify a 50 mm isolation valve immediately upstream of the arrestor.
Quick checklist
- [ ] All quick-close valves + appliances inventoried, FUs assigned
- [ ] Distributed strategy preferred (one arrestor per appliance or per bathroom entry)
- [ ] Arrestor letter size matches cumulative FU
- [ ] Arrestor within 1.5 m of valve, vertical orientation
- [ ] Air-charged sealed type (not air-gap)
- [ ] Isolation valve upstream of each arrestor for service
- [ ] Specify by PDI WH-201 letter code (A through F) on the BOQ
References: PDI WH-201:2010 Standard for Water Hammer Arrestors; IS 1172 Indian Code for Water Supply; ASPE Plumbing Engineering Design Handbook Vol 2; IPC 2018 §604.9 (Water Hammer).
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