Clarification on air discharge requirements in AS 1668.2:2024

Questions have been raised about whether the 6m aggregation noted in the standard is intended to apply only to determining whether a discharge is objectionable under Clause 3.10.1 (against the Type A/Type B thresholds and as written in the 2012 edition), or whether it also governs the minimum separation distances to intakes, openings and boundaries under Clause 3.10.2, as written in the 2024 edition.

Standards Australia Technical Committee ME-062 Ventilation and Airconditioning has provided the following clarification statement:

AS 1668.2:2012 included a number of subjective performance requirements, including Clause 3.10.1, which required exhaust air to be discharged so as not to cause danger or nuisance to building occupants, occupants of neighbouring buildings, or members of the public.

The National Construction Code requires referenced Deemed-to-Satisfy Standards – DTS (See more about DTS and Performance Solutions here – What you need to know | NCC)  to contain prescriptive requirements.

Standards containing subjective performance requirements are therefore unsuitable for this purpose.

For example, compliance with the prescriptive discharge requirements in Tables 3.3 and 3.4 of AS 1668.2:2012 did not, on its own, demonstrate compliance, as Clause 3.10.1 also had to be satisfied.

Accordingly, the discharge provisions in AS 1668.2:2024 have been revised to be fully prescriptive and no longer include subjective performance requirements.

These changes from the 2012 edition were deliberate and carefully considered.

AS 1668.2:2024 underwent the full Standards Australia standards development process, including the additional requirements applicable to primary NCC-referenced Standards.

This process included impact assessment, stakeholder consultation, and public comment.

No adverse comments on Clause 3.10 were received during these consultation stages. Following completion of this rigorous process, AS 1668.2:2024 was referenced in NCC 2025.

The committee responsible for the development and maintenance of the AS 1668 series will consider the inclusion of explanatory diagrams, where appropriate, in a future amendment to the Standard.

Since the publication of NCC 2025, which calls up the new version of AS 1668.2, Standards Australia and industry bodies such as AIRAH have been fielding questions in relation to Clause 3.10.1.

Many questions have centred on the interaction between Clause 3.10.1 and the minimum separation distance provisions that follow in Clause 3.10.2 (particularly relating to commonplace horizontal apartment exhaust configurations with separations less than 6m, that were no longer prohibited when the BCA referenced AS 1668.2:2012 in 2013).

Clause 3.10.1 in the 2012 edition required discharges to be designed “in such a manner as not to cause danger or nuisance” (which aligned with and accommodated BCA Volume 1’s contemporary performance requirement FP4.5, and then the current F6P5) with minimum separation distances set out in Table 3.4 (noting that these minimum separation distances were not deemed to deal with the requirement for not causing a nuisance). This reference to an unquantified outcome (such as nuisance) was considered to be unacceptable for a modern NCC referenced prescriptive standard for Deemed-to-Satisfy provisions.

In order to prevent designers from clustering multiple discharges close together in an arrangement where they effectively operate as one larger discharge, inadvertently applying separation distances in isolation (and without considering the possibility of causing a nuisance), previous requirements involving both separation distances and consideration of nuisance have been resolved into a single, measurable requirement, adopting a familiar separation strategy.

In the 2024 edition, Clause 3.10.1 states: “For the purposes of determination of flow rate, multiple discharges located within a single 6m radius shall be treated as a single discharge.” Table 3.5 (formerly Table 3.4) was also amended, with additional criteria to accommodate the quantified approach.

The specified 6m radius requires consideration of clustering within a simply measured straight line (as could be visualised within a notional sphere, centred at the discharge location), while the separation distances involve the shortest distance between discharges and openings (as could be measured with a piece of string).

Please note that Standards Australia is not a government agency, or a regulator, and is not involved in any regulatory oversight, certification, technical advice, or compliance. It cannot assist with applying a specific standard in an application or provide direction.

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