Need to know: key updates to AS 1668

One of the most important Australian Standards used by HVAC practitioners, AS 1668, is being reviewed. Brett Fairweather, M.AIRAH, from It’s Engineered takes us through the major changes and the key talking points in the drafts of AS 1668.2 and AS 1668.4, which are open for public comment until January 23, 2024.

Why is an update to AS 1668 such a big deal for HVAC practitioners?

The AS 1668 series would be applied directly or indirectly by most HVAC practitioners on most days. These standards prescribe minimum ventilation provisions that our industry is tasked with designing, installing, testing and operating. This includes minimum amounts of ventilating air and some limitations on how it must be applied. Parts of these standards are mandatory when following a Deemed-to-Satisfy compliance pathway through BCA Volume 1.

The requirements of a design based on the 1668 standards then forms the basis for maintenance activities in accordance with standards such AS/NZS 3666 and AS 1851.

An update to AS 1668.2 like this has only occurred twice before in my career, and one of those the 2002 edition) was never referenced in the Building Code. The whole industry now has an opportunity to consider the changes proposed and to provide comments to the committee. I think this is a pretty big deal!

What do AS 1668.2 and AS 1668.4 deal with?

AS 1668.2 establishes rules for mechanical ventilation in buildings, and AS 1668.4 establishes rules for natural ventilation in buildings. These aspects of ventilation were all contained in the one document back in 2002, but were separated in 2012 as part of the criteria to re-establish AS 1668.2 as a referenced BCA standard.

Neither AS 1668.2 or AS 1668.4 deal with matters of fire and smoke control, which are covered by AS 1668.1.

The drafts include simple and detailed procedures for minimum outdoor air calculations. How has this changed from previous versions, and what issues does it address?

Minimum outdoor air requirements in AS 1668.2 are based on principles of dilution and filtration that are necessary to achieve an acceptable indoor air quality. Air quantities and requirements are established during a design for different types of occupancies and enclosures, but a more detailed assessment of other circumstances can be useful in some situations.

In the current 2012 edition, we must establish the minimum amount of “effective” outdoor air that needs to be introduced to a space, then establish what proportion of this air must be “introduced” outdoor air, directly from outside the building. The proportion is influenced by filtration methods, to achieve an acceptable (or “effective”) level of dilution.

The draft proposes to allow this two-step method to be skipped where the designer intends to ventilate a room with air from outside alone, or without considering opportunities available from some methods of filtration (which have been updated, in line with changes in other standards).

What can you tell us about the borrowed ventilation strategy in the draft?

The principle of “borrowing” air from an adjoining room was established in the BCA a long time ago, but is only available where the “borrower” enclosure is naturally ventilated. The draft proposes to permit the same application, but where the primary enclosure is mechanically ventilated, with some limitations on when this is acceptable.

Both borrowed ventilation strategies rely on the primary enclosure being over-ventilated to compensate for the secondary enclosure, and for the semi-natural opening between the enclosures to be oversized to encourage the transfer of air.

This proposed strategy should be particularly useful for small spaces, but will not allow the borrowing of air from enclosures that we’re not allowed to recycle air from, like carparks, anything in Appendix B, some healthcare enclosures, and other spaces that may contain harmful or offensive contaminants.

Carpark ventilation also gets a look in – can you tell us what is changing here and why?

There are quite a few details to consider for carparks in the draft (Section 4 in AS 1668.2).

Like the ventilation in other spaces, carpark ventilation relies on principles of diluting contaminants, particularly those produced by internal combustion engines. The emissions from these engines have improved significantly, and the size and cost of ventilation systems designed to accommodate a fleet of much older engine technology has been found to be unnecessary in many situations.

The draft proposes to reduce ventilation rates accordingly. The base ventilation rate for a single operating engine has been halved. This should enable reductions in the size, cost, energy consumption and use of materials in newer construction.

Treatment of kitchen exhaust seems like a new requirement. What is intended with this?

This is one of the proposed responses to recognise the benefits of some commonplace Performance Solutions, where setting some limits would allow a simpler process for compliance through the Deemed-to-Satisfy pathway.

While any kitchen exhaust that exceeds 1,000L/s can only be discharged vertically to comply with the current standard, this is not always practicable. The BCA allows a Performance Solution to be developed, where the exhaust could be treated and discharge horizontally without causing issues to the building occupants and the community.

AS 1668.2 provides some guidance on concessions that might be considered for these solutions, but doesn’t currently permit a horizontal discharge.

The draft includes an option to treat “Type B” effluent (typically kitchen exhaust air) to reduce the objectionable content in the air by filtration and odour-treatment strategies that are known to be effective. These optional requirements require more design input than what is currently permitted, the applications are limited and will increase equipment costs. This will avoid the costs of the process to prepare some Performance Solutions though, and Performance Solutions are still available through the BCA for other technologies.

Industry has flagged issues around natural ventilation provisions in the NCC. AS 1668.4 deals with natural ventilation, could the updates help address some of the problems we have seen around Deemed-to-Satisfy ventilation requirements?

Natural ventilation is not suited to every situation, but I will always start by looking for opportunities to adopt it. The benefits for energy minimisation, maintenance, cost, etc., are tough to beat.

AS 1668.4 has a lot to offer as when considering natural ventilation of rooms. While the NCC specifies a blunt 5 per cent for all types of spaces and for all types of occupants, AS 1668.4 includes both “simple” and “detailed” procedures for determining the size of openings. The simple procedure allows the 5 per cent (to align with the existing NCC provisions), but the detailed procedure requires adjustments for both the use of the enclosure and the occupant density. A sparsely occupied warehouse, for example, might only require 2.5 per cent, while a densely occupied primary school classroom will be nearing 20 per cent.

I issued some Proposals for Change to the ABCB to consider when revising the NCC for the next edition. One of my proposals encouraged consideration of the adoption of AS 1668.4 to replace BCA Clauses F6D7 and F6D8. The simple procedure should enable a no-cost, like-for-like outcome to existing Deemed-to-Satisfy applications, but the detailed procedure would allow improvements to be adopted without needing to venture into the processes for a Performance Solution. We will need to look at the draft NCC when it is also released for public comment to see what comes of this suggestion.

Any other major changes that practitioners should look out for?

Yes! I’d encourage everyone to have a look at the parts of the standard that they are most familiar with and experienced in. The committee that has prepared the draft includes people from a diverse range of roles, locations and experience in this industry, but the public-comment process allows experience from a wider audience to inform the future of AS 1668.

What are the next steps for AS 1668.2 and 4? Could we expect to see them called up in NCC 2025?

After the public-commenting process has finished early in the new year, the committee will reconvene and be tasked with reviewing and considering the comments that are received. Any adjustments from here would need to remain within the approved scope for the revision, so suggestions that require a shift from minimum practice to best practice will be sidelined. The ABCB will then need to consider the final document for consistency with the approved scope and for its suitability to be referenced in the BCA, and the expected impact of any changes to regulation.

The timing of any such reference will be a matter for the ABCB, and we’ll all have an opportunity to see what’s proposed when the draft NCC is released.

The drafts of AS 1668.2 and AS 1668.4 are available for public comment through the Standards Australia portal until January 23, 2024.

Comments provided are the opinion of It’s Engineered and do not represent advice from Standards Australia or the technical sub-committee involved in updating AS 1668.

Photo by Mitchell Luo on Unsplash