Air Handling Unit Selection and Specification
What You Need to Know
An AHU (air handling unit) filters, heats, cools, and moves air through a building. It sits at the centre of every ducted air conditioning system. Pick the wrong AHU and you waste energy, blow the budget, or fail NCC compliance. The rules below cover how to size and specify an AHU for commercial projects in Australia.
The Rules
- The AHU fan must meet minimum efficiency grades set by NCC 2025 Part J6, Table J6D5a. Grades range from 42.0 to 65.0 depending on fan type (NCC 2025 J6D5(2))
- Any system moving more than 1,000 L/s with variable supply air needs a variable speed fan (NCC 2025 J6D3(1)(e))
- An economy cycle (free cooling with outdoor air) is required when airflow exceeds the threshold for your climate zone - for example, 2,000 L/s in climate zone 6 and 9,000 L/s in climate zone 2 (NCC 2025 J6D3(1)(c), Table J6D3)
- Average duct pressure drop must stay at or below 1 Pa/m across the index run. Flexible duct is limited to 6 m per run (NCC 2025 J6D5(3))
- Each AHU component has a maximum pressure drop allowance: coils 30 to 220 Pa, standard filters 55 to 110 Pa, intake louvres 30 to 60 Pa, fire dampers 15 Pa open (NCC 2025 Table J6D5e)
- Outdoor air rates for each room type must comply with AS 1668.2:2024. The AHU outdoor air intake must be sized to meet these minimums
- Ductwork connected to the AHU must be sealed per AS 4254 and insulated to R1.2 minimum in conditioned spaces, or R2.0 in unconditioned spaces (NCC 2025 J6D6)
What This Means in Practice
Start with the cooling load. AIRAH DA09 sets out the method: calculate heat gains from the building envelope, occupants, lighting, equipment, and ventilation air. The total cooling load sets the required airflow and coil capacity. A typical commercial office needs about 80 to 120 W/m² of cooling. For a 500 m² floor plate, that is 40 to 60 kW of cooling, which translates to roughly 2,500 to 3,500 L/s of supply air.
The cooling coil does the heavy lifting. Cooling coil face velocity should sit between 2.0 and 2.5 m/s. Above 2.5 m/s, the risk of water droplet carryover from the coil increases. Industry practice typically limits coils to 6 rows and 10 fins per inch for cleanability. Coils with more than 4 rows and tight fin spacing are almost impossible to clean in service.
Filters protect the coil and the occupants. A two-stage setup is standard: a G4 prefilter (roughly equivalent to ePM10 ≥50% under AS/ISO 16890) catches coarse dust, followed by an F7 final filter (ePM1 ≥50%) for fine particles. The prefilter extends the life of the more expensive final filter. Allow 55 to 110 Pa clean pressure drop for filters in your fan static calculation (NCC 2025 Table J6D5e).
The fan you pick must pass NCC efficiency checks. Backward curved plug fans with EC motors are now standard in the Australian market. They run efficiently across a wide speed range and include built-in speed control. Size the fan for the total system static pressure: duct losses at 1 Pa/m maximum, plus coil drop, filter drop, louvre drop, and fitting losses. Add 10 to 15% margin for dirty filters and system degradation.
Key Design Decisions
AHU Type: Packaged vs. Custom Built
For systems under 5,000 L/s, packaged modular AHUs work well. They are factory-tested, quicker to deliver (6 to 10 weeks), and typically cost $80 to $130 per L/s installed. Custom AHUs suit larger systems or unusual configurations. They typically cost $130 to $260 per L/s and take 12 to 20 weeks to manufacture. Pick custom when you need non-standard coil arrangements, specific acoustic targets, or tight spatial constraints.
Face Velocity: Standard vs. Low
Standard face velocity (2.0 to 2.5 m/s) gives a compact AHU footprint. Low face velocity (1.25 to 1.5 m/s) cuts coil pressure drop by 40 to 50% and improves dehumidification. This reduces fan energy and helps meet NCC Section J fan power limits.
Economy Cycle: When to Include It
NCC 2025 requires an economy cycle when your AHU airflow exceeds the climate zone threshold (Table J6D3). The cycle uses outdoor air for free cooling when conditions suit. It needs motorised outdoor air, return air, and relief air dampers plus temperature and humidity sensors.
Casing and Leakage Class
Specify double-skin, thermally broken casing with 50 mm polyurethane-filled panels. Target Class L1 leakage (per EN 1886) and TB1 thermal bridging. Single-skin casings sweat in humid climates and waste energy through conduction.
Who Needs to Know What
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References
- National Construction Code 2022, Volume One, Part J6 — Air-conditioning and ventilation
- AS 1668.2:2024, The use of ventilation and airconditioning in buildings — Part 2: Mechanical ventilation in buildings
- AS 4254.1:2021 / AS 4254.2:2012, Ductwork for air-handling systems in buildings
- AIRAH DA09, Air Conditioning Load Estimation and Psychrometrics (4th Edition)
- ASHRAE Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings (international reference)
- ANSI/AHRI Standard 430, Performance Rating of Central Station Air-Handling Unit Supply Fans (international reference)
- EN 1886, Ventilation for buildings - Air handling units - Mechanical performance (international reference)
- AS/NZS 1657:2018, Fixed platforms, walkways, stairways and ladders — Design, construction and installation