Key changes in AS 1668.2:2024 as NCC 2025 adoption begins

20 July 2026

AS 1668.2:2024: What HVAC practitioners need to know for the NCC 2025 transition

As NCC 2025 is progressively adopted across Australia's states and territories (ABCB NCC 2025 state and territory adoption page), the HVAC industry is beginning its transition to a new set of Deemed-to-Satisfy requirements. Central to this transition is the adoption of AS 1668.2:2024, which replaces the 2012 edition as the referenced mechanical ventilation standard.

The 2024 edition introduces several important changes to the design, sizing and documentation of mechanical ventilation systems. Some are subtle but carry significant compliance implications if overlooked, while others provide welcome clarification or opportunities for more efficient system design.

The following highlights some of the most significant changes practitioners should be aware of.

1. Discharge Grouping Under Clause 3.10.1

In the 2012 edition, the rule for grouping co-located exhaust discharges sat as a Note to Table 3.3. In AS 1668.2:2024 it has moved into the body of Clause 3.10.1 as normative text. The shift from an informative note to a normative requirement is easy to miss, and interpretations across industry have differed, with a real risk of misapplication. The placement matters: Clause 3.10.1 is intended to apply, for the purposes of determining flow rates, to both objectionable discharges (when applying Table 3.4) and non-objectionable discharges (when applying Table 3.5).

For non-objectionable discharges, the flow rate versus separation distances in Table 3.5 are applied using the air flow rate determined in accordance with the third paragraph of Clause 3.10.1, aggregating, rather than individual discharge rates. This is most relevant to apartment and hotel projects, where exhaust points are commonly grouped or stacked on facades. Applying individual discharge rates instead of the aggregated flow understates the separation required, which can leave insufficient clearance between discharge points and nearby openings or air intakes, and a corresponding risk of non-compliance.

2. Changes That Add Value or Remove Ambiguity

The remaining changes are mostly to the practitioner's advantage:

Carpark ventilation rates updated for modern vehicles. Contaminant generation rates have been revised for lower-emission vehicle fleets and can reduce the required ventilation rate. On large multi-level carparks, where fan and duct sizing drives cost, that can mean smaller plant and lower energy use. It is worth re-running carpark calculations to the 2024 rates on projects still in early design.

  • Simple and detailed outdoor airflow methods. Minimum outdoor airflow can now be set by either a simple or a detailed method. The simple method suits standard commercial fitouts. The detailed method accounts for occupancy profiles and contaminant sources, and can sharpen system sizing on complex buildings where airflow drives plant selection.
  • Borrowed ventilation. The standard now allows outdoor air to be drawn from an adjacent room under defined conditions, which can simplify ductwork for internal spaces that are hard to duct directly. It is conditional: the source room needs an adequate outdoor air supply, and the transfer path must meet the standard's requirements.
  • Prescriptive values replace non-quantified criteria. Several performance provisions that previously lacked numerical values, and relied on individual judgement, now carry defined values. That removes ambiguity and makes designs more consistent between practitioners.

3. Healthcare Ventilation: Improved Air Quality and Occupant Safety

For those involved in Healthcare projects, ventilation requirements have been substantially revised to improve infection control and occupant safety. New HEPA filtration requirements, isolation room air changes raised from 6 to 12 per hour, and additional pressure categories for birthing suites and anaesthetic rooms all lift indoor air quality in clinical settings. The trade-off is greater plant and ductwork capacity, so members with healthcare projects in design should check ventilation schedules against the 2024 requirements early.

Practical Next Steps for Members

  • Confirm with the certifier which edition applies on each project. Work already documented to the 2012 edition can usually continue under it through to certification.
  • Design new projects to the 2024 edition from the outset.
  • Update in-house calculation templates and spreadsheets still referencing 2012 clause numbers and values.
  • Flag the edition in use, and its effect on plant space and facade coordination, to the wider project team early.

AMCA Technical Services will keep monitoring how the 2024 edition is being applied. For questions on the standard, or feedback on the discharge grouping provision above, contact [email protected].