What is the maximum pipe pressure drop for chilled water under NCC 2025?

NCC 2025 Table J6D8a limits chilled water pipework pressure drop to 400 Pa/m for straight sections in constant-speed systems. Variable-speed systems must stay below 300 Pa/m for larger pipes (Table J6D8b).

How do you size chilled water pipes?

For pipes 50 mm and smaller, keep velocity below 1.2 m/s. For pipes larger than 50 mm, size by friction loss and stay under 400 Pa/m. Minimum velocity is 0.6 m/s to carry air to vent points. Most designers target 1.5 to 3.0 m/s for mains.

What is the typical pump head for a commercial chilled water system?

A typical commercial building needs 150 to 300 kPa of total pump head. This covers pipe friction, fittings, the chiller evaporator (40-80 kPa), AHU coils (30-60 kPa), and control valves (30-60 kPa), plus a 10-20% safety margin.

Chilled Water Design: Pipe Sizing + Pump Head Guide

Q: What is the standard chilled water supply and return temperature in Australia?

Standard Australian chilled water design uses 6°C supply and 12°C return, giving a 6°C delta T. The international convention is 5.5°C (6.7°C supply, 12.2°C return). High delta T designs push to 8°C or more, which reduces flow rates and allows smaller pipes and pumps.

Q: What is the difference between primary-secondary and variable primary flow?

Primary-secondary uses two loops: constant-flow primary through chillers and variable-flow secondary through the building. Variable primary flow (VPF) uses one loop with variable-speed drives, saving about 15% pump energy. VPF requires a minimum flow bypass at 40-50% of chiller design flow.

Section 1

What You Need to Know

Chilled water systems cool large buildings. A chiller makes cold water at about 6°C. Pumps push that water through pipes to cooling coils in each room. The water absorbs heat, warms to about 12°C, and returns to the chiller. Getting the pipe sizes, pump heads, and temperatures right keeps the system efficient and the building comfortable.

Section 2

The Rules

Chilled water pipework pressure drop must not exceed 400 Pa/m for straight sections in constant-speed systems (NCC 2025, Table J6D8a)
Variable-speed pump systems must keep pressure drop below 300 Pa/m for larger pipes (NCC 2025, Table J6D8b)
Chilled water circuits must have automatic variable temperature control to improve chiller efficiency (NCC 2025, J6D3(1)(k))
Circulating pumps must meet an energy efficiency index of 0.27 or less (NCC 2025, J6D8(2), per EU Regulation 622/2012)
All chilled water pipe insulation must include a vapour barrier on the outside to prevent condensation (NCC 2025, Section J)
Time switches are required for air-conditioning systems above 2 kW capacity (NCC 2025, J6D3(3))

Section 3

What This Means in Practice

Take a 5,000 m² office building with a 500 kW cooling load. At a standard 6°C delta T (6°C supply, 12°C return), the flow rate works out to about 20 L/s. That needs a chiller, primary and secondary pumps, a pipe network, and cooling coils in each AHU (air handling unit) or FCU (fan coil unit).

Pipe sizing follows two rules. For pipes 50 mm and smaller, keep velocity below 1.2 m/s. For pipes larger than 50 mm, size by friction loss and stay under 400 Pa/m (ASHRAE Fundamentals, Chapter 33). The minimum velocity is 0.6 m/s. Below that, air bubbles will not travel to the vent points and the system fills with trapped air. Most designers target 1.5 to 3.0 m/s for mains.

Pump head covers every pressure drop in the longest pipe loop: pipe friction, fittings, the chiller evaporator (typically 40 to 80 kPa), AHU coils (30 to 60 kPa), and control valves (30 to 60 kPa). A typical commercial building needs 150 to 300 kPa of total pump head. Add a 10 to 20% safety margin, then match the flow and head to a pump curve from the manufacturer.

Section 4

Key Design Decisions

1

Air-Cooled vs. Water-Cooled Chiller

Water-cooled chillers are more energy efficient but need a cooling tower on the roof. Air-cooled chillers sit on the roof or at ground level and need no tower. Pick water-cooled for buildings above 500 kW cooling load where roof space and water supply allow it.

Trade-off: Water-cooled chillers cost more to install (tower, condenser water pumps, pipework, water treatment). But they are more energy efficient over the building life because water rejects heat better than air. The 30-year life-cycle cost of a water-cooled system is typically lower than air-cooled.

2

Primary-Secondary vs. Variable Primary Flow

Primary-secondary pumping uses two loops: a constant-flow primary loop through the chillers and a variable-flow secondary loop through the building. Variable primary flow (VPF) uses one loop with variable-speed drives on all pumps. VPF saves about 15% pump energy but needs a bypass line and tighter BMS (building management system) control.

Trade-off: VPF eliminates the secondary pump set, saving capital cost and plant room space. But the controls are more complex, and you must maintain minimum flow through each chiller (typically 40 to 50% of design flow).

3

Standard Delta T vs. High Delta T

Standard Australian design uses a 6°C delta T (6°C supply, 12°C return). The international convention is 5.5°C (6.7°C supply, 12.2°C return). High delta T pushes to 8°C or more. Higher delta T means lower flow for the same cooling load, which means smaller pipes and smaller pumps.

Trade-off: High delta T needs larger coils with more rows and denser fins to transfer the same heat at lower flow. Fan energy at the AHU may increase. Check that the coil surface temperature still drops below 14°C for proper dehumidification.

4

Pipe Insulation and Vapour Barriers

All chilled water pipes must be insulated with a vapour barrier on the outside (NCC 2025, Section J). Without the vapour barrier, moisture in the air condenses on the cold pipe surface. This causes dripping, ceiling damage, and mould.

Trade-off: Closed-cell elastomeric insulation with integral vapour barrier costs more than standard fibrous insulation but is faster to install and less prone to failure at joints.

Section 5

Who Needs to Know What

Section 6

References

National Construction Code 2022, Volume One, Part J6 — Air-conditioning and ventilation
National Construction Code 2022, Volume One, Part J5 — Air-conditioning and ventilation insulation
AIRAH DA16, Air Conditioning Water Piping (design methods, pipe sizing, pressure loss data)
AIRAH DA09, Air Conditioning Load Estimation and Psychrometrics (4th edition - cooling load calculation)
ASHRAE Handbook - Fundamentals, Chapter 33: Pipe Sizing (international reference)
ASHRAE Standard 90.1, Energy Standard for Buildings (international reference - delta T and pipe sizing requirements)

Chilled Water Design: Pipe Sizing + Pump Head Guide

What You Need to Know

The Rules

What This Means in Practice

Key Design Decisions

Air-Cooled vs. Water-Cooled Chiller

Primary-Secondary vs. Variable Primary Flow

Standard Delta T vs. High Delta T

Pipe Insulation and Vapour Barriers

Who Needs to Know What

Need this engineered for your project?

References

Related design memos

Air Handling Unit Selection and Specification

Variable Air Volume (VAV) System Design

Plant Room Design and Access Requirements