Domestic Water System Sizing Notes

Domestic Water System Sizing

  • Domestic water piping is sized based on:
    • Flow rate (quantity) in gallons per minute (gpm).
    • Water pressure in pounds per square inch (psi).

Definitions

  • Fixture Supply: Water supply pipe connected to an individual fixture.
  • Branch: Piping system part, excluding riser/main/stack; combines two or more fixture supply pipes.
  • Main: Principal pipe artery to which branches connect.

Fixture Supply

  • Minimum flow rate/pressure for fixtures in Table 604.3.
  • Maximum flow rate for fixture types in Table 604.4.
  • Minimum supply pipe sizes for fixtures in Table 604.5.

Sizing of Water Piping System

  • Code lacks specific domestic water system sizing method.
  • Appendix E offers two methods; this class covers the uniform friction loss method.

Estimate Water Supply Demand

  • Determine water supply fixture units (WSFU) for each plumbing fixture from Table E103.3(2); fixture supply pipe does not need assigned a WSFU value for sizing.
  • Add WSFU’s for each branch, main, and service pipe segment toward building service.
  • HOT + COLD \neq TOTAL
  • Convert WSFU's to GPM using Table E103.3(3), interpolating if needed.
  • Use flush valves if they account for 10% or more of total fixtures.
  • Some engineers only use flush tank column for sizing hot water pipes.
  • Minimum water service size = ¾”.

Water Pressure

  • Plumbing fixtures need minimum water pressure to operate (psi).
  • 1 FT of water (feet of head) = 0.433 psig = pressure (psig) = height (feet).

Pressure Analysis

  • Water pressure changes from public main to building points due to:
    • Elevation change (static head).
    • Pressure loss through fittings/valves/components.
    • Friction loss from pipe flow (diameter, length, GPM).

Ensuring Adequate Pressure

  • Determine WSFU and GPM for each pipe segment.
  • Available water pressure in municipal main (contact supplier, hydrant flow test).
  • Account for pressure fluctuations; use low pressure for design.

Pressure at Worst Case Fixture

  • Identify fixture needing greatest pressure (Table 604.3).

Pressure Change Due to Elevation

  • Measure height from pressure source to highest fixture outlet (static head loss).
    • Subtract pressure if outlet is higher.
    • Add pressure if outlet is lower
    • Example: 52’ x 0.433 = 22.5 psi

Pressure Loss at Tap to Main

  • IPC TABLE E103.3(4).
  • Based on size and flow through tap.

Pressure Loss Through Components

  • Meters: Loss based on size and flow.
  • Backflow Preventers: Check cut sheets for pressure loss curves.

Pipe Friction Loss

  • Pressure loss per 100’ of piping, based on:
    • Type, diameter, and length of pipe.
    • Flow (GPM).

Maximum Velocity

  • High velocity can damage piping and cause noise.
  • Hot water: 11fps, Cold water:
    • 2” = 1-1/2” = 6.5fps

Equivalent Length of Pipe

  • Fittings/valves cause pressure loss; accounted for by equivalent pipe length, found in Table E103.3(6).
  • Multiplier (1.2 to 1.5) often used for fittings/valves.
  • Measure pipe length to furthest fixture.

Uniform Friction Loss Sizing Method

  1. Determine max pressure drop allowed by pipe friction loss.
  2. Determine equivalent pipe length from pressure point to remote fixture (Measured length x 1.2).
  3. Determine max pressure drop allowed per 100’ of pipe.
  4. Add “do not exceed” lines to pipe pressure loss chart (velocity, max psi/100’).
  5. Follow GPM line until hitting a “do not exceed” line, then use pipe size to the left.

Water Pressure Concerns

  • Maximum water pressure within building is 80 psi.
  • Pressure reducing valve (PRV) needed if service entry pressure is higher.
  • Exception: PRV not required if pressure reduces by the time water reaches fixture outlet.
  • If available water pressure is too low, a booster pump may be required.