Estimating Peak Water Demand for a Multi-Storey Building

Peak demand is the number every other part of the water system depends on — tank volume, pump capacity, riser diameter, and meter size all flow from it.

Total the fixture units

Build the fixture schedule for the whole building, separating flush-valve and flush-tank water closets — they sit on different Hunter curves. Sum WSFU for the entire supply system.

Convert with Hunter’s curve

Hunter’s curve (or the equivalent code table) converts total WSFU to probable peak flow. It is deliberately non-linear: at 1000 WSFU the peak flow per fixture unit is a fraction of what it is at 50 WSFU, because simultaneous use becomes statistically unlikely at scale.

Add continuous demands (cooling tower make-up, irrigation, process water) after the curve — they are not probabilistic, so they add directly.

What the peak flow sizes

• Water meter and service pipe — sized for peak flow at acceptable velocity and pressure loss.
• Storage: typically one day of average demand (average ≈ peak × a diversity factor, or from litres/person/day × population). Split between a ground cistern and a roof tank.
• Transfer pump: fills the roof tank in 2–4 hours → capacity = tank volume / fill time.
• Booster / pressure pump: sized for peak flow at the head needed to serve the top floor at minimum residual pressure.
• Riser: carries peak flow to the upper zones at 1.5–2.4 m/s.

Zoning tall buildings

Static pressure above about 45 m over-pressurises low-floor fixtures. Split the building into vertical pressure zones of 8–12 storeys, each fed from its own tank level or through pressure-reducing valves, and estimate the peak flow per zone.