[MEP Center Engineering]

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We are designing the domestic cold water system for a 10-storey residential block with 40 flats. Water is stored in a break tank in the basement and a booster set feeds the whole building. The highest outlet is about 30 m above the pumps.

What is the correct procedure to size the booster set?

• How do I get the design flow without simply adding up every fixture?
• How do I build up the pump head?
• How many pumps, and how is standby arranged?
• What do I do about high pressure on the lower floors?

Are there differences between EN 806 practice and IPC practice that matter here?

[MEP Center Engineering]

Size the set for the simultaneous (diversified) peak flow, not the sum of fixture flows, and build the head from static lift + required pressure at the most remote outlet + friction and component losses. Then check the lowest floors and add pressure reduction where the static pressure becomes excessive.

1. Design flow

• EN 806-3: add the loading units (LU) of all outlets and read the design flow from the standard’s LU-to-flow curve.
• IPC/UPC: add water supply fixture units (WSFU) and read the flow from the Hunter-based curve (flush-tank or flush-valve curve as applicable).
• Do not stack one diversity method on another: apply one method, once, to the whole building.

For the example below, assume the curve gives 4.0 l/s (14.4 m³/h, about 63 gpm). Use your own value; the procedure is what matters.

2. Pump head

1. Static lift from the pump to the highest outlet: 30 m.
2. Required flow pressure at the most remote outlet: 1.5 bar = 15 m (comfortable for thermostatic showers; the code minimum is lower, e.g. the 100 kPa assumed in EN 806-3, or the per-fixture minimum flow pressures in IPC 604.3).
3. Friction in pipes and fittings at peak flow plus meter, filter and check valves: 6 m (take it from your pipe sizing, not a guess).

Head = 30 + 15 + 6 = 51 m (≈ 5.0 bar, about 72 psi). With a flooded suction from the break tank you can ignore the small positive suction head and keep it as a margin.

3. Number of pumps and power

Use separate pumps with one standby, for example 3 × 50 %: two duty + one standby, each 2.0 l/s at 51 m. Do not use a twin-head pump as “duty + standby”; the standby must be a separate pump that can be isolated and removed while the others run.

Hydraulic power per pump = ρ·g·Q·H = 1000 × 9.81 × 0.002 × 51 ≈ 1.0 kW. With a pump efficiency of 0.65 the shaft power is ≈ 1.54 kW, so select the next motor size, 2.2 kW. Variable speed control on constant discharge pressure, a small membrane vessel to cover low draw-offs, dry-run protection on the tank and automatic duty rotation complete the set.

4. Pressure on the lower floors

With about 5 bar at the pump discharge, the floors close to the plant room see almost the full pressure. EN 806-2 aims to keep the pressure at outlets at or below about 5 bar, and IPC 604.8 requires a pressure-reducing valve where the static pressure exceeds 80 psi (552 kPa). In practice many designers keep outlets below 4 to 5 bar for noise and tap comfort: either split the riser into pressure zones or fit PRVs on the lower floors.

Common mistakes

• Sizing on the sum of fixture flows: the set is oversized and cycles at low demand.
• Forgetting the losses of the water meter, filter and check valves.
• No standby pump, or a twin-head pump treated as standby.
• Ignoring the pressure on the lowest floors.
• Undersizing the break tank so the pumps run dry at peak.

Related: Booster Pump Set Sizing for Buildings, Domestic Water Pipe Sizing with Fixture Units, Pressure Reducing Valve Stations.

Check the edition of the code that applies to your project.

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