Hydraulic Riser Design for Multi-Storey Buildings

What You Need to Know

Hydraulic risers carry water up through a building. They include cold water, hot water, fire hydrant, and fire sprinkler pipes that run vertically through dedicated shafts. AS/NZS 3500.1 and AS 2419.1 set the rules for pipe sizing, pressure limits, and fire hydrant supply. Get the riser shaft size, pressure zones, and fire rating wrong early, and you face costly rework once construction starts.

• Static pressure at any outlet (except fire services) must not exceed 500 kPa. Install pressure reducing valves where the system would otherwise exceed this limit (AS/NZS 3500.1, Clause 3.3.4)
• Minimum pressure at any fixture outlet must be at least 50 kPa (AS/NZS 3500.1, Clause 3.3)
• Pipe sizing must use the loading unit method. Assign loading units to each fixture, sum them for each pipe section, and convert to probable simultaneous flow rate (AS/NZS 3500.1, Section 3, Tables 3.1 and 3.3)
• Water velocity must not exceed 3.0 m/s in any pipe. Best practice is 2.2 m/s or less (AS/NZS 3500.1)
• Fire hydrant systems must deliver a minimum of 10 L/s at 150–200 kPa residual pressure per outlet, depending on the state. Buildings over 25 m high need two fire pumps (AS 2419.1:2021, Clause 2.2.8)
• Service shaft openings in Type A construction must be protected with fire-rated construction to prevent fire spread between floors (NCC 2025, C4D14)
• All pipe penetrations through fire-rated riser shaft walls must be sealed with systems tested to AS 1530.4 and AS 4072.1

Take a 15-storey commercial office. The water authority delivers mains pressure at around 350 kPa at ground level. Each storey adds roughly 30 kPa of static head. By the ground floor of a gravity-fed system, the bottom fixtures could see over 800 kPa if served from a roof tank. That exceeds the 500 kPa limit, so the designer must split the building into pressure zones.

A typical zone covers 5 to 8 floors. Each zone gets its own pressure reducing valve (PRV) station on the riser. The PRV station needs a Y-strainer, isolation valves, and a drain point for servicing. These stations sit inside the riser shaft or an adjacent services cupboard, and they need clear access for maintenance.

On the fire side, a DN100 or DN150 fire hydrant riser runs the full height of the building inside a fire-isolated stair or dedicated fire shaft. The booster assembly at ground level connects to the fire brigade, and internal landing valves serve each floor. The riser, supports, and all penetrations through fire-rated walls must meet the fire resistance level set by the NCC for that building class.

The hydraulic riser shaft itself carries multiple services: cold water riser, hot water riser and return, fire hydrant riser, fire sprinkler riser, and sometimes gas. Typical shaft widths range from 400 mm for a simple residential building to 800 mm or more for a commercial tower with full fire services. Every pipe penetration through the shaft wall at each floor needs a fire-rated seal or collar.