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Calcimator

Water Hammer Arrestor Calculator

Size water hammer arrestors per PDI WH-201 standard based on fixture units and water pressure.

Inputs

Results

Total fixture units

16

PDI size (0=AA to 6=F)

1

Connection size (in.)0.5
Arrestors needed2
PRV needed?0
Expansion tank needed?0
Peak pressure without (PSI)130
Peak pressure with (PSI)65
Arrestor parts cost$30.00
Total installed cost$130.00
Pdi SizeA
How to Use This Calculator
  1. Count each type of plumbing fixture: bathroom sinks, kitchen sinks, toilets, dishwashers, and washing machines.
  2. Enter your static water pressure in PSI — if you don't know it, measure at a hose bib with a gauge.
  3. Review the PDI Size and Arrestors Needed — install one arrestor per solenoid valve (washer, dishwasher).
  4. Check if a PRV (Pressure Reducing Valve) is flagged — pressures above 80 PSI require one.
  5. Review Total Installed Cost to budget for the full water hammer solution including PRV and expansion tank.

What each input means

Bathroom sinks
Bathroom lavatory faucets (1 FU each).
Kitchen sinks
Kitchen sink faucets (2 FU each).
Toilets
Tank-type toilets (3 FU each). Flushometer types use more.
Dishwashers
Dishwashers have solenoid valves that cause water hammer (2 FU each).
Washing machines
Washing machines are the #1 cause of residential water hammer (4 FU each).
Water pressure (PSI)
Static water pressure. Above 80 PSI requires a pressure reducing valve.

What each result means

Total fixture units
Combined supply fixture units for PDI sizing.
PDI size (0=AA to 6=F)
Arrestor size class: 0=AA, 1=A, 2=B, 3=C, 4=D, 5=E, 6=F.
Connection size (in.)
Arrestor pipe connection diameter.
Arrestors needed
Recommended number of arrestors (one per solenoid valve).
PRV needed?
1 = pressure reducing valve needed (pressure > 80 PSI).
Expansion tank needed?
1 = thermal expansion tank needed (closed system with PRV).
Peak pressure without (PSI)
Estimated pressure spike during water hammer without arrestor.
Peak pressure with (PSI)
Estimated pressure with arrestor installed.
Arrestor parts cost
Cost of arrestor devices.
Total installed cost
Parts + labor for arrestors, PRV, and expansion tank if needed.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Bathroom sinks = 2, Kitchen sinks = 1, Toilets = 2, Dishwashers = 1 = 6 input(s) provided
  2. Calculate Total fixture units
    Total fixture units
    16 = 16
  3. Calculate PDI size
    1 = 1
  4. Calculate Connection size
    Connection size
    0.5 = 0.5
  5. Calculate Arrestors needed
    Arrestors needed
    2 = 2

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators.

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