IPC Hunter's Curve demand calculation (WSFU aggregation), cold/hot branch sizing, service pipe sizing (IPC 604.4), static pressure budget, elevation loss (0.433 psi/ft), and booster pump check. Build fixture counts floor-by-floor to see total demand and available pressure at the highest fixture.
| Fixture Type | Qty | Cold WSFU ea | Hot WSFU ea | Total WSFU | |
|---|---|---|---|---|---|
4 | 2.5 | 0 | 10.0 | ||
4 | 0.5 | 0.5 | 4.0 | ||
4 | 1 | 1 | 8.0 | ||
2 | 1.5 | 1.5 | 6.0 | ||
2 | 2.5 | 0 | 5.0 | ||
| TOTALS | 24.0 | 9.0 | 33.0 | ||
| Pipe | ID (in) | Velocity (fps) | Status |
|---|---|---|---|
| ¾" | 0.811 | 17.41 | Too fast |
| 1" | 1.049 | 10.41 | Too fast |
| 1¼" ✓ | 1.38 | 6.01 | OK |
| 1½" | 1.61 | 4.42 | OK |
| 2" | 2.067 | 2.68 | OK |
| 2½" | 2.469 | 1.88 | OK |
| 3" | 3.068 | 1.22 | OK |
| 4" | 4.026 | 0.71 | OK |
This simulator builds a fixture schedule floor-by-floor and applies IPC Hunter's Curve to calculate peak simultaneous demand GPM, size the service pipe, and compute a detailed pressure budget to determine residual pressure at the highest fixture and whether a booster pump is required. Plumbing engineers use it for complete water service design from the utility meter to the top-floor fixture.
The simulation aggregates water supply fixture units (WSFU) from all fixture groups in the schedule, computing separate cold, hot, and total WSFU totals using IPC 2021 Table 604.1 values. Hunter's curve — a piecewise approximation of IPC Table 604.1 — converts total WSFU to peak demand GPM. The curve has diminishing slope at high WSFU counts, reflecting the probability that large numbers of fixtures cannot all operate simultaneously.
The pressure budget starts with available street pressure, subtracts service pipe friction loss (Hazen-Williams), elevation head loss (0.433 psi per foot of rise), and fixture, meter, and backflow preventer losses. The residual pressure at the highest fixture is compared against the IPC minimum: 15 psi for standard fixtures and 25 psi for flush valve closets and urinals. If residual falls below minimum, the required booster pump pressure boost is calculated and displayed.
IPC Section 604 (Water Supply Sizing) governs the fixture unit method and Hunter's curve application. IPC Table 604.1 provides WSFU values. IPC Section 604.6 sets minimum residual pressure requirements (15 psi standard, 25 psi flush valve). IPC Section 604.4 limits service pipe velocity to 8 fps. ASHRAE 188 (Legionellosis: Risk Management for Building Water Systems) requires that recirculating hot water systems maintain water above 122°F and sets requirements for stagnant water management that affect service pipe sizing decisions. Local water utility requirements may specify minimum service pipe size or materials.
For tall buildings (above 8–10 stories), the street pressure is usually insufficient to serve upper floors and a booster pump system is required. Mid-rise buildings may use a single booster package; high-rise buildings use multiple pressure zones with pressure-reducing valves (PRVs) at each zone break to limit the maximum pressure to IPC Section 604.8 limit of 80 psi at any fixture. Size PRVs for the full service pipe flow rate with 5–10 psi allowance for PRV pressure drop.
The service pipe run length to the building has a major effect on friction loss, particularly at high flow rates. A 1-inch service pipe carrying 20 GPM over 100 ft at Hazen-Williams C=130 loses about 18 psi — which may be acceptable for a single-story building but would eliminate all working pressure margin for a multi-story structure.
Build the fixture schedule by selecting fixture types and quantities for each row. Add rows for each group of fixtures in the project (restrooms, kitchens, hose bibbs, etc.). Adjust the building and site parameters: street pressure from the utility flow test, service run length from the meter to the building main, meter elevation, and highest fixture elevation. The Hunter's Curve chart plots your demand point in real time. The pressure budget panel shows every deduction from street pressure to fixture residual. The service pipe sizing table identifies the minimum pipe size that keeps velocity below 8 fps at peak demand. If a booster is indicated, the boost pressure required is shown.
Without a flow test, use the minimum static pressure reported by the water utility — typically 40–60 psi in residential areas and 50–80 psi in commercial districts. For design purposes, always use the minimum guaranteed pressure, not the typical operating pressure. Utilities state minimum guaranteed pressure in their service agreements, and the system must work at that minimum. If minimum pressure is unavailable, assume 40 psi and plan for a booster pump system.
Reduced pressure zone (RPZ) backflow preventers have pressure drops of 5–12 psi at design flow depending on size and manufacturer. Double check valve assemblies (DCVA) lose 1–3 psi. Always obtain the pressure loss vs. flow curve from the selected device manufacturer at the design flow rate. The simulator uses a simplified 5 psi allowance for combined meter and backflow losses — verify this against the actual device specifications and meter head loss table from the water utility.
Gravity tanks (roof tanks) were standard in high-rise buildings before reliable electric pumps became available. Today, they are rarely used for domestic water in new construction but remain common in certain jurisdictions (New York City, for example) and in buildings where pump failure cannot be tolerated. A gravity tank at roof level provides reliable pressure at upper floors equal to the static head of the tank above the fixture — typically 15–25 psi from a 2-3 story tank height. Booster pump-to-tank systems combine electric pumps with gravity tank reliability.
Flush valves (Sloan-type, Zurn-type) discharge a large volume (typically 1.6–3.5 gallons) very rapidly — within 3–5 seconds — and require a minimum 25 psi residual pressure at the valve to operate properly. If multiple flush valves operate simultaneously, the instantaneous demand far exceeds the Hunter's curve steady-state peak. IPC Table 604.1 assigns higher WSFU to flush valve closets (4.0 WSFU) than tank-type (2.5 WSFU) to account for this, and requires a higher minimum residual pressure to ensure adequate flow during the flush cycle.
Multi-zone buildings require a separate pressure analysis for each zone. The high zone is typically the critical case for booster pump design. The low zone must be checked to ensure that street pressure plus booster pump head does not produce pressures exceeding 80 psi at low-floor fixtures — PRVs are installed to reduce pressure in low zones when the pump head is sized for upper floors. Size PRVs for full service flow with the valve in the partially throttled position, and verify that PRV outlet pressure holds stable across the full flow range from zero demand to peak demand.
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