How to Size a Booster Pump for a Commercial Building (Philippines)

Undersized pumps cycle constantly, wear out fast, and fail to deliver adequate pressure at the top floor. Oversized pumps waste energy, create water hammer, and stress plumbing. Here is the five-step sizing process we use on every project.

Step 1: Calculate Your Water Supply Fixture Units (WSFU)

The Philippines National Standard Plumbing Code, based on the ICC IPC and Hunter's Curves methodology, assigns each plumbing fixture a WSFU value based on how much water it draws and how often it is used. Count every fixture in your building, multiply each by its WSFU value, and sum the total.

Step 2: Convert WSFU to Flow Demand Using Hunter's Curve

The relationship between WSFU and simultaneous flow demand is not linear: it follows Hunter's Curve, because not every fixture is used at the same time. At high WSFU counts, the diversity factor takes over.

For our 594 WSFU hotel: peak demand is roughly 195 GPM, or 12.3 L/s, or 44 m³/hr.

Step 3: Calculate Required Head Pressure

Head pressure required at the pump discharge equals static lift plus friction losses plus residual pressure at the highest fixture. Static lift is the height from the pump centerline to the highest fixture: for a 10-storey building with the pump in the basement, roughly 10 x 3.5 m = 35 m of static head. Friction losses run typically 0.1 to 0.3 m per meter of pipe at design flow: for 50 m of vertical plus 100 m of branch piping at 0.15 m/m, friction is about 22 m. Residual pressure at the highest fixture should be minimum 3 to 5 bar (30 to 50 m head) to satisfy fixture manufacturer minimums; use 40 m to be safe.

Step 4: Choose Configuration (Simplex vs Duplex vs Triplex)

• Simplex: up to 125 GPM (28 m³/hr). Small commercial: boutique hotel, small clinic, small office building. Cost-effective and simple.
• Duplex: up to 350 GPM (80 m³/hr). Mid-to-large commercial. Two pumps in lead-lag rotation, with redundancy if one fails. Our 44 m³/hr hotel example falls here.
• Triplex: up to 750 GPM (170 m³/hr). Large commercial and industrial: high-rise, large hospital, industrial process water. Three pumps in lead-lag-standby.

For the 44 m³/hr hotel: duplex configuration, each pump rated 25 m³/hr at 100 m head. With one pump running at full load handling 100% of peak, the second pump alternates. On high demand, both run at 50% each for lower wear.

Step 5: Choose Motor Size (kW / HP)

Motor size = (Flow in m³/hr x Head in m) / (367 x pump efficiency). For a single pump at 25 m³/hr and 100 m head with 65% efficiency: (25 x 100) / (367 x 0.65) = 10.5 kW, rounded to 11 kW (15 HP). This lands the hotel on a duplex 2x 11 kW CDL 20-5 booster pump system, VFD-controlled, with a 500 to 1,000 L hydropneumatic bladder tank.

When to Add a VFD

Any booster pump 5.5 kW or larger that will run intermittently or under variable demand should be VFD-controlled. A VFD (variable-frequency drive) modulates motor speed to match real-time demand instead of cycling the pump on and off at full power. Typical energy savings run 25 to 40%, and typical payback on a duplex 5.5 kW+ system is 12 to 24 months. Fixed-speed booster pumps make sense only for very small (2.2 kW or less) low-cycling applications where the VFD cost is not justified.

Common Sizing Mistakes We See

• Sizing pumps to total WSFU instead of Hunter's Curve peak demand: oversizes the system by 2 to 3 times.
• Ignoring residual pressure at the highest fixture: undersizes head, and top-floor showers dribble.
• Skipping the hydropneumatic bladder tank: the pump cycles on and off constantly, killing its lifespan.
• Using cast-iron pumps for potable water: they corrode within 2 to 3 years and contaminate the water.
• Undersized VFD panel: overheats and trips under sustained peak demand. Always size the VFD 20% above the rated motor.

What to Send Us for a Sizing Recommendation

• Building type and floor count
• Fixture count by type, or the as-built plumbing plan
• Utility supply (Maynilad, Manila Water, deep well, cistern) rated flow and pressure at your service connection
• Any special demand: laundry on-site, commercial kitchen, pool, irrigation
• Timeline

Related Products

• Vertical Multistage Pumps
• VFD Duplex Control Panels
• Hydropneumatic Bladder Tanks
• Complete Booster Pump Systems