Variable Air Volume (VAV) System Design
What You Need to Know
A VAV system cools a building by changing how much air flows to each zone. Instead of blowing the same amount of air all day and changing its temperature, a VAV system keeps the air cool and sends more or less of it where needed. This saves energy. NCC 2025 Section J and AS 1668.2 set the rules for how these systems must work in Australia. Get the design wrong, and you waste energy, fail code, or leave occupants too hot or too cold.
The Rules
- Air-conditioning systems above 1,000 L/s must use a variable speed fan (NCC 2025, J6D3(1)(e))
- The pressure drop across a VAV box with the damper fully open must not exceed 100 Pa for boxes with electric reheat or 25 Pa for other boxes, excluding coil losses (NCC 2025, J6D5(4)(5))
- Duct pressure drop must not exceed 1 Pa/m averaged across the index run (NCC 2025, Part J6)
- Outdoor air must meet at least 10 L/s per person for offices and never drop below 0.35 L/s per m² of floor area (AS 1668.2-2012, Table A1 and Cl 3.3)
- Economy cycles are required when total airflow exceeds the thresholds in Table J6D3, ranging from 2,000 to 9,000 L/s depending on climate zone (NCC 2025, J6D3(1)(c)), except in climate zone 1 or where dehumidification control is needed
- A time switch must control mechanical ventilation systems above 1,000 L/s (NCC 2025, Part J6)
- Supply air diffusers must not exceed 40 Pa pressure drop; balancing dampers must not exceed 25 Pa when fully open (NCC 2025, Part J6)
What This Means in Practice
Picture a 2,000 m² office floor with 200 people split across 15 zones. Each zone gets a VAV box connected to a central AHU (air handling unit). The AHU sends cool air at about 13°C through a main duct. Each VAV box has a motorised damper that opens or closes to match the cooling load in its zone. When a meeting room fills up, the box opens wider. When the room empties, it closes down.
The AHU fan runs on a variable speed drive (VSD). As VAV boxes close, the duct pressure rises. The BMS (building management system) senses this and slows the fan. Less fan speed means less power. Fan energy drops roughly with the cube of the speed reduction, so cutting fan speed by 20% saves about 50% of the fan energy at that operating point.
At minimum airflow, the system still needs to deliver enough outdoor air. For an office zone of 100 m² with 10 people, the per-person rate gives 100 L/s. The area-based floor gives 35 L/s (100 m² × 0.35 L/s/m²). The per-person rate controls at full occupancy. But if only 2 people occupy the space, the per-person rate drops to 20 L/s, and the 35 L/s floor takes over. The VAV box minimum must be set high enough to deliver this outdoor air fraction through the mixed supply air.
Key Design Decisions
VAV Box Minimum Airflow Setting
The minimum airflow setting on each VAV box is the single most important decision. Set it too high (say 50% of peak), and you waste energy reheating overcooled air. Set it too low, and you risk poor air mixing, drafts, or inadequate ventilation. For interior zones, a minimum of 20–30% of peak airflow works well. For perimeter zones with reheat, 30% of peak is common practice. Every box minimum must deliver at least the AS 1668.2 outdoor air requirement for that zone.
Cooling-Only vs. Reheat VAV Boxes
Interior zones with stable loads can use cooling-only boxes. Perimeter zones near glazing need reheat boxes to handle heating loads in winter. Fan-powered boxes (parallel type) are an option for perimeter zones because they mix warm plenum air with cool supply air, reducing the reheat load.
Duct Static Pressure Reset
Instead of holding duct static pressure at a fixed setpoint, reset it down until at least one VAV box damper is nearly wide open (say 95%). This saves fan energy because the fan only works as hard as the worst-case zone demands. Studies show this saves about 14% of fan energy compared to a fixed setpoint.
Supply Air Temperature Reset
Raising the AHU supply air temperature from 13°C to 16°C during mild weather reduces the cooling load on the chiller and cuts reheat at VAV boxes. The BMS monitors zone damper positions: if no zone is calling for full cooling, the SAT can rise.
Who Needs to Know What
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References
- AS 1668.2-2012, The use of ventilation and airconditioning in buildings — Part 2: Mechanical ventilation in buildings
- AS 1668.2:2024, The use of ventilation and airconditioning in buildings — Part 2: Mechanical ventilation in buildings (updated edition)
- National Construction Code 2022, Volume One, Part J6 — Air-conditioning and ventilation
- National Construction Code 2022, Volume One, Part F6 — Light and ventilation
- AIRAH DA09, Air Conditioning Load Estimation and Psychrometrics (4th edition)
- ASHRAE Standard 62.1-2022, Ventilation and Acceptable Indoor Air Quality (international reference)
- ASHRAE Standard 90.1-2022, Energy Standard for Buildings (international reference)