How do you size water pipes for a commercial building in Australia?

Use the loading unit method in AS/NZS 3500.1. Assign a loading unit value to each fixture, sum them for each pipe section, then convert to a probable simultaneous flow rate using Table 3.3. Pick the pipe diameter that keeps velocity below 2.2 m/s while maintaining positive residual pressure at the furthest fixture.

What temperature must hot water be stored at in Australia?

AS/NZS 3500.4 requires hot water to be stored at 60 degrees C or above to prevent Legionella bacteria growth. The water must then be tempered down to a maximum of 50 degrees C at taps (or 45 degrees C in healthcare, aged care, childcare, and schools) using thermostatic mixing valves or tempering valves.

What is the maximum water velocity allowed in pipes under AS/NZS 3500.1?

AS/NZS 3500.1 sets a maximum water velocity of 3.0 m/s. However, best practice is to keep velocity at 2.2 m/s or less to reduce noise, water hammer, and pipe erosion.

What are the maximum water delivery temperatures at taps in Australian buildings?

Under NCC 2025 Volume Three (B2D5), heated water at personal hygiene fixtures must not exceed 50 degrees C generally. In healthcare facilities, aged care, childcare centres, and schools, the limit is 45 degrees C. Thermostatic mixing valves (TMVs) or tempering valves control delivery temperature.

How often do thermostatic mixing valves need to be tested in Australia?

AS 4032.3 requires TMVs to be field-tested at intervals of no more than 12 months. Thermostatic actuators must be replaced at intervals not exceeding 5 years, along with dynamic o-rings and seals.

Hot and Cold Water System Design

Section 1

What You Need to Know

Every building needs water that flows at the right pressure and the right temperature. AS/NZS 3500.1 (cold water) and AS/NZS 3500.4 (hot water) set the rules for pipe sizing, pressure limits, and safe delivery temperatures. Undersized pipes mean poor flow. Overheated water means scalds. Here are the rules and decisions that shape a water system design.

Section 2

The Rules

Pipes must be sized using the loading unit method to find the probable simultaneous flow rate for each section (AS/NZS 3500.1, Section 3)
Water velocity must not exceed 3.0 m/s in any pipe, though best practice is 2.2 m/s or less (AS/NZS 3500.1)
Pressure at outlets must be at least 50 kPa, and static pressure must not exceed 500 kPa (AS/NZS 3500.1)
Hot water must be stored at 60°C or above to prevent Legionella growth (AS/NZS 3500.4)
Delivery temperature at taps must not exceed 50°C generally, or 45°C in healthcare, aged care, childcare, and schools (NCC 2025 Vol. Three, B2D5)
Backflow prevention devices must match the hazard rating of the connected system: high, medium, or low (AS/NZS 3500.1, Section 4)

Section 3

What This Means in Practice

Take a 10-storey commercial office with bathrooms on each floor. Each level has 4 WCs, 4 basins, and 2 accessible basins. The designer adds up loading units for every fixture on every floor, then converts the total to a probable simultaneous flow rate. That flow rate sets the pipe size for each section of the riser and the branches.

The loading unit method accounts for the fact that not all taps run at once. A building with 40 basins does not need a pipe sized for 40 basins flowing together. The probable simultaneous demand is much lower. For cold water, Table 3.3 in AS/NZS 3500.1 converts loading units to flow rate in litres per second.

On the hot water side, the system must store water at 60°C to kill Legionella bacteria, then temper it down to 50°C (or 45°C for vulnerable users) at the tap. Thermostatic mixing valves (TMVs) or tempering valves handle this step. TMVs need annual testing and a full service every 5 years (AS 4032.3). Dead legs, where water sits without flowing, are a Legionella risk. Keep them as short as possible, and flush low-use outlets weekly.

Section 4

Key Design Decisions

1

Pipe Sizing: Loading Units to Flow Rate

Use the loading unit method in AS/NZS 3500.1 Table 3.1 and Table 3.3 for all projects. Assign a loading unit value to each fixture, sum them for each pipe section, then convert to probable simultaneous flow rate. Pick the pipe diameter that keeps velocity below 2.2 m/s while maintaining positive residual pressure at the furthest fixture.

Trade-off: Conservative pipe sizing (lower velocity) costs more in pipe material but reduces noise, water hammer, and long-term erosion.

2

Hot Water Storage and Tempering

Store at 60°C minimum. Install TMVs or tempering valves at each group of fixtures to bring delivery temperature down to 50°C (or 45°C for high-risk locations). Place valves as close to outlets as practical.

Trade-off: Storing at 60°C or above uses more energy than lower temperatures, but dropping below 60°C creates a Legionella risk that can shut down a building.

3

Circulating Hot Water vs. Dead Legs

Install a circulating pump and return line on any system where the furthest hot water outlet is more than about 12 metres from the heater. The return water must stay above 55°C. Without circulation, users wait a long time for hot water and the standing water breeds bacteria.

Trade-off: A circulating system adds a pump, return pipework, and insulation cost but eliminates long draw-off times and reduces Legionella risk.

4

Backflow Prevention Strategy

Identify every cross-connection point and assign a hazard rating (high, medium, or low) per AS/NZS 3500.1 Section 4. Match the device to the hazard: reduced pressure zone devices for high hazard, double check valves for medium, and air gaps or atmospheric vacuum breakers for low.

Trade-off: Higher-rated devices cost more and need annual testing, but using the wrong device for the hazard level means non-compliance and a health risk.

Section 5

Who Needs to Know What

Section 6

References

AS/NZS 3500.1:2021, Plumbing and drainage — Part 1: Water services
AS/NZS 3500.4:2021, Plumbing and drainage — Part 4: Heated water services
National Construction Code 2022, Volume Three, Part B1 — Cold water services
National Construction Code 2022, Volume Three, Part B2 — Heated water services
AS 4032.1-2005, Water supply — Valves for the control of heated water supply temperatures — Thermostatic mixing valves
AS 4032.3:2022, Water supply — Valves for the control of heated water supply temperatures — Part 3: Requirements for field-testing, maintenance or replacement of valves, taps and devices

Hot and Cold Water System Design

What You Need to Know

The Rules

What This Means in Practice

Key Design Decisions

Pipe Sizing: Loading Units to Flow Rate

Hot Water Storage and Tempering

Circulating Hot Water vs. Dead Legs

Backflow Prevention Strategy

Who Needs to Know What

Need this engineered for your project?

References

Related design memos

Domestic Hot Water Plant Sizing for Commercial Buildings

Thermostatic Mixing Valve Requirements

Legionella Risk in Hot Water Systems