Booster pump sizing for residential and commercial buildings
How to calculate design flow, total head and the number of pumps for a variable-speed booster set, with the mistakes that cause undersized systems.
Gabtic engineering team · · 6 min read
1. Design flow from fixture units, not apartment count
Add up the fixture units (WC, basin, shower, sink, washing machine) for the building and convert them to a peak probable flow with the method in your plumbing code. Multiplying the number of apartments by a guessed flow per apartment is the most common cause of oversized pumps in small buildings and undersized ones in towers.
2. Total head
Total head = static height from the pump to the highest outlet + friction losses in pipes and fittings + the residual pressure required at that outlet (often 1.5–2 bar for modern mixers and flush valves) − any positive suction pressure from the tank.
3. Number of pumps and standby
Split the duty across two or three pumps with one standby. Variable-speed drives on each pump hold a constant pressure as demand changes, cut energy at night and reduce water hammer.
- Up to ~40 apartments: 1 duty + 1 standby is usually enough.
- Towers and hotels: 2–3 duty + 1 standby with pressure zoning.
4. Check NPSH and the tank connection
Pumps drawing from a ground tank with a long suction line can cavitate. Keep suction pipes short and one size larger than the pump inlet, and confirm the available NPSH is above what the pump needs at the end of its curve.