How to Size a Booster Pump for a Commercial Building in the Philippines

Most pump room complaints in Philippine commercial buildings trace back to a duty point that was never calculated. The two failure modes are mirror images of each other. Oversizing is the common one: a pump selected on peak theoretical demand runs far left of its best efficiency point, cycles on and off against the pressure switch, hammers the check valves, and burns energy that the building pays for every month for the life of the equipment. Undersizing is the visible one: pressure collapses on the top two floors during the morning and evening peaks, and the usual field response is to raise the cut-in pressure, which stresses the piping without adding any flow.

Both come from skipping the fixture unit count and going straight to a horsepower guess based on storey count. The correct sequence is fixed and it is the same sequence used by the Philippine National Standard Plumbing Code, which is based on the ICC International Plumbing Code and the Hunter's Curves method: count water supply fixture units, convert those units to a probable peak flow, compute total dynamic head from static lift plus residual pressure plus friction, choose a pump configuration that keeps each pump inside its efficient band, and only then convert the duty point into a motor rating. Every step below follows that order, in both litres per second and gallons per minute, because Philippine drawings routinely mix the two.

Start with the fixture schedule, not the floor area. Every plumbing fixture on the drawing carries a water supply fixture unit (WSFU) value that expresses its probable demand, and a flush valve water closet carries several times the load of a flush tank unit because it draws a large volume over a few seconds. Total the WSFU for the whole building, then split the total into flush valve dominant and flush tank dominant groups, because the two follow different Hunter's Curve branches and a mixed building sized on the wrong branch can be off by thirty percent or more.

Convert the WSFU total to probable peak flow using Hunter's Curve rather than by adding the fixture flow rates together. This is the single most misunderstood step in the whole calculation. The curve exists precisely because fixtures do not all run at once, and simple addition produces a demand figure that no building will ever see, which is where oversizing begins. Add anything that runs continuously rather than intermittently, such as cooling tower makeup, kitchen equipment with a dedicated feed, or a laundry line, as a separate constant flow on top of the Hunter's Curve result, because those loads are not probabilistic and the curve does not account for them.

The output of this step is one number: peak flow in gallons per minute and cubic metres per hour. Multiply GPM by 0.227 to get cubic metres per hour, or by 0.063 to get litres per second.

Total dynamic head (TDH) is the pressure the pump must generate, and it is the sum of three components measured in metres of water column. First is static lift: the vertical distance from the suction water level, usually a ground level cistern or an underground reserve tank, to the highest fixture served. Use the actual floor to floor dimension from the drawings rather than a rule of thumb, since Philippine commercial floor heights vary widely between office, retail podium, and back of house levels.

Second is residual pressure at that highest fixture. Code sets a minimum flow pressure of around 8 psi (0.55 bar, or roughly 5.6 metres) at ordinary fixtures, but flush valve water closets and commercial kitchen equipment need substantially more, commonly in the 15 to 25 psi range, so size the residual to the most demanding fixture on the top floor and not to a lavatory faucet. Third is friction loss through the riser, horizontal mains, fittings, valves, strainers, and any backflow preventer, calculated at the peak flow from Step 1. Sum the three and convert units consistently: 1 bar equals 10.2 metres of head, and 1 psi equals 0.703 metres. The result is your required head. Pair it with the peak flow and you have a duty point, expressed as flow at head, which is the only form in which a pump can actually be selected.

Configuration is a redundancy and turndown decision, not just a capacity decision. JOHOB sizes commercial booster sets across three bands.

• Simplex, a single pump, covers duty points up to about 125 GPM (28 cubic metres per hour) and suits small commercial buildings where a brief outage during service is tolerable.
• Duplex, two pumps, covers up to about 350 GPM (80 cubic metres per hour) and is the default for occupied commercial buildings, since one pump can be isolated for maintenance while the building stays in water.
• Triplex, three pumps, covers up to about 750 GPM (170 cubic metres per hour) and is used where the flow range between night minimum and morning peak is wide enough that a two pump set would spend most of the day running one pump far off its curve.

The controlling question is turndown. A building with a large occupied peak and a near zero overnight demand needs staged pumps so that low demand is met by one small pump rather than by cycling a large one. Where the building cannot tolerate any interruption, specify a duty and standby arrangement with each pump rated for the full duty point, and alternate lead pump on every start so the running hours stay even. JOHOB supplies these sets built around vertical multistage pumps from Grundfos, DAB, Speroni, and CDL.

With flow and head fixed, motor power falls out of a single formula. Hydraulic power in kilowatts equals flow in cubic metres per hour multiplied by head in metres, divided by 367 multiplied by pump efficiency. Use the efficiency from the manufacturer's curve at your actual duty point, not the headline best efficiency figure, since a pump running away from its best efficiency point can lose ten to fifteen points of efficiency and the motor has to cover that loss. As a planning value before curve selection, 0.65 to 0.75 is a reasonable band for a vertical multistage pump on a building water service.

Round the calculated figure up to the next standard motor frame rather than sizing exactly, and confirm the motor has margin across the full operating range, not only at the design point, because a pump run out to the right of its curve at low system pressure draws more power than it does at the duty point. Then verify the selection against the pump curve itself: the duty point should sit near the middle of the curve, the shutoff head should exceed the system's static requirement, and NPSH available at the suction should comfortably exceed NPSH required. Check the pressure rating of the pump casing, the piping, and the hydropneumatic pressure tank against the shutoff head rather than the duty head, because shutoff is the pressure the system actually sees when every fixture closes.

A fixed speed set holds pressure between a cut-in and a cut-out setting, so the building sees a pressure band rather than a constant pressure, and the pump starts at full torque every cycle. A variable frequency drive holds a constant discharge pressure by modulating pump speed against a transducer signal, which removes the pressure swing at the fixtures, softens the start so the motor and the piping stop taking inrush and hammer, and cuts energy draw at part load because power falls roughly with the cube of speed.

Specify a VFD where any of four conditions apply: the building has a wide daily demand swing such as a hotel, a hospital, or a mixed use podium; the occupants are sensitive to pressure variation, which covers commercial kitchens, laundries, and medical fixtures; the pump room is close enough to occupied space that start and stop noise matters; or the operator is optimising running cost over the equipment life rather than first cost. Stay with fixed speed where demand is flat and predictable and first cost is the binding constraint. JOHOB supplies VFD duplex control panels built by Chutian using Inovance drives, alongside star delta, soft starter, and MCC panels where those suit the application better. Panels are supplied with CE marking on the electrical scope. Pair the panel with a correctly sized hydropneumatic pressure tank in either case, since the tank absorbs small demand and keeps the pumps from cycling on minor draws. The full comparison is in our VFD versus fixed speed pumps guide.

The following is an illustrative calculation, not a project reference. Take a ten storey office building with a ground level cistern, flush valve water closets, and a fixture schedule totalling 480 WSFU. Hunter's Curve on the flush valve branch returns a probable peak of roughly 175 GPM, which is 40 cubic metres per hour or 11 litres per second. For head: static lift from cistern water level to the tenth floor fixtures at 3.4 metres per storey gives 34 metres; residual pressure at a top floor flush valve at 25 psi gives 18 metres; friction through the riser, mains, fittings, and strainer at peak flow gives 12 metres.

To get this checked, send JOHOB the fixture schedule with counts and flush type, the number of storeys with floor to floor heights, the water source and cistern level, the riser layout with pipe sizes and material, and any continuous loads. JOHOB returns a written sizing recommendation and configuration proposal within 3 to 5 business days, and responds to RFQs within 1 business day. To convert your own GPM figures to litres per second, see our GPM to litres per second conversion guide.

• Guide: 15 Storey Booster Pump Sizing
• Commercial Booster Pumps for Philippine Buildings
• Vertical Multistage Pumps
• VFD Duplex Control Panels
• Hydropneumatic Pressure Tank Sizing
• Guide: VFD vs Fixed Speed Pumps
• Guide: Convert GPM to Litres Per Second
• Condominium Booster Pump Replacement, Manila

Reviewed: August 2026