3.1 Water Service & Distribution Sizing: WSFU, Demand, Pressure & Friction

Why this section matters

Sizing and Design is the second-largest area on the exam, with 9 questions. Water sizing problems combine several tables. Appendix E of the 2021 IPC lays out a method, and section 604.1 says any approved, accepted engineering method may be used. Exam problems normally follow Appendix E, so tab Tables 604.3, 604.4, 604.5, E103.3(2), E103.3(3) and the friction charts.

Pressure and size limits in Chapter 6

Table 604.3: flow and pressure at the fixture

Table 604.5: minimum fixture supply pipe

A fixture supply must end within 30 inches of the fixture connection (604.5).

Water supply fixture units (Appendix E)

A water supply fixture unit (wsfu) weights each fixture by how much water it uses, for how long and how often. Public fixtures get higher values because they are used more often. Table E103.3(2) lists separate cold, hot and total loads. For a fixture with both, each side is about three-quarters of the total. Size cold-only or hot-only piping with the cold or hot value, and size piping that carries both with the total.

Demand: Table E103.3(3)

Fixture units are converted to probable peak flow. Use the flushometer-valve column whenever the system is mainly flushometer valves.

The two columns become equal at 1,000 wsfu (208 gpm). Above that, flush valves no longer dominate the demand.

The friction-loss method

1. Static pressure at the main or meter, using the lowest expected pressure (604.6).
2. Elevation loss: vertical rise to the highest fixture × 0.433 psi per foot.
3. Losses through the meter and devices, such as the meter, backflow assembly and PRV, from manufacturer data.
4. Required flow pressure at the critical fixture, from Table 604.3.
5. Pressure left for friction = static − elevation − meter and devices − fixture pressure.
6. Developed length = measured length + equivalent length of fittings and valves. Appendix E's equivalent-length table gives values. For example, a 90-degree elbow counts as 2.0 feet in 1/2-inch pipe, 3.0 feet in 1-inch and 7.0 feet in 2-inch. A tee through the branch counts as 3.0, 5.0 and 10.0 feet in those sizes.
7. Friction allowance per 100 feet = pressure left for friction ÷ developed length × 100.
8. Pick the smallest pipe whose friction loss at the design flow stays within the allowance. Then check the velocity.

Velocity. The IPC sets no fixed velocity number. Section 604.9 only requires velocity to be controlled, and Appendix E tells you to choose a design velocity. The Copper Development Association recommends no more than about 8 ft/s for cold water and 5 ft/s for hot water up to 140°F in copper. Treat those as design guidance, not code.

Worked example: two-story medical office (flushometers)

• Static pressure: 68 psi. Highest fixture: 22 feet above the meter.
• Meter loss 4.5 psi, RPZ assembly loss 9.5 psi.
• Critical fixture: siphonic flushometer water closet, which needs 35 psi (Table 604.3).
• Piping: 110 feet measured plus 40 feet of fitting allowance, so 150 feet developed.
• Load: 65 wsfu, mainly flush valves.

1. Demand: halfway between 60 wsfu (54 gpm) and 70 wsfu (58 gpm) in the flushometer column gives about 56 gpm.
2. Elevation loss: $22 \times 0.433 = 9.5$ psi.
3. Pressure left for friction: $68 - 9.5 - 4.5 - 9.5 - 35 = 9.5$ psi.
4. Allowance: $9.5 \div 150 \times 100 \approx 6.3$ psi per 100 feet.
5. Type L copper at 56 gpm (Hazen-Williams, C = 150):

The building main must be 2 inches. If the design had wrongly used 15 or 20 psi for the flush valve, the allowance would have looked large enough for 1-1/2-inch pipe. The flush valve would then starve at peak demand.

Exam traps

• Wrong demand column: 100 wsfu is 43.5 gpm on flush tanks but 67.5 gpm on flush valves.
• Wrong fixture pressure: a flush valve needs 35 or 45 psi, not 8 psi.
• PRV trigger: static pressure over 80 psi, not 100 psi.
• Hot and cold loads: each is about 3/4 of the total for a fixture with both. Do not simply add the hot and cold values together.