Selecting Filters to Support AS 1668.2:2024 Compliance in Practice

When specifying filtration for an Australian HVAC system, the filter is only part of the story. The right selection depends on how the air-handling unit is designed, what the system needs to achieve, and how the filters perform as loading increases.

For mechanical contractors, facility managers and consultants, the practical challenge is balancing air quality, fan performance, access for maintenance and the site’s operating conditions. AS 1668.2:2024 is a key reference point for ventilation design, but the final filter selection should always be confirmed with the mechanical services engineer or building owner.

Start with the system requirements, not the filter catalogue

A good filtration specification begins with the building and the HVAC system, not with a product code. The right filter grade for one installation may be a poor fit for another if the fan, ductwork, housing space or maintenance access is different.

Before selecting a filter, consider:

• •the intended use of the building and occupied spaces
• •outdoor air quality and local dust conditions
• •whether the system is supplying general comfort air or a more demanding application
• •available fan capacity and acceptable pressure drop across the system
• •space for pre-filters, final filters and safe access for servicing
• •whether the filter frame, housing and sealing arrangement suit the application

This is where close coordination matters. A filter that performs well on paper can become difficult to maintain if the housing is too tight, the access point is awkward, or the fan was not designed for the added resistance.

If you are still measuring existing filters for a replacement, our guide on how to measure an air filter can help avoid ordering errors.

How filter grades affect performance and maintenance

Filter grade is the first specification point most teams look at, but it should be chosen alongside system resistance and dust loading. In Australia, you may still see references to older EN 779 classes such as G3, G4 and F5-F9, even though ISO 16890 is now the current standard in many contexts. The important point is that these ratings do not convert directly, so avoid treating one class as an automatic substitute for another.

For some systems, a coarse pre-filter is used to capture larger dust before it reaches the downstream filter. In other applications, a finer final filter is needed where the system and the building call for tighter particulate control. The right balance depends on what the system is trying to do and how much resistance the fan can handle.

A practical way to think about filter grades is:

• •coarser filters generally have lower initial resistance and are often used to protect equipment or act as a first stage
• •finer filters can capture smaller particles but may introduce higher resistance
• •multi-stage filtration can improve system design flexibility by spreading the load across more than one filter stage
• •higher efficiency is not automatically better if the fan, housing or maintenance access cannot support it

For projects needing purpose-built solutions, our custom air filters page shows how tailored sizing and construction can help align the filter with the system.

Pressure drop is a design issue, not just a maintenance note

Pressure drop is one of the most important practical factors in filtration selection. It affects fan performance, air delivery and how the system behaves over time as the filter loads with dust.

A clean filter has an initial resistance. As dust accumulates, resistance rises. If the system was selected with too little allowance for that rise, air flow can drift away from the intended operating point long before the filter is physically full.

That is why specification should consider both the initial pressure drop and the expected loading profile. This is especially important in systems with:

• •limited fan margin
• •long duct runs
• •high outdoor air volumes
• •sites with elevated dust or debris
• •housings with restricted filter face area

Where pressure gauges or differential pressure monitors are fitted, they should be used to track filter loading rather than relying on guesswork. Inspection frequency depends on the system, the site conditions and the dust load. Follow the filter and equipment manufacturer instructions, and replace or service the filter based on condition and measured resistance, not on a fixed calendar alone.

If your site needs ongoing support, our filter servicing and commercial filter servicing pages outline the type of maintenance support we can provide.

Matching the filter arrangement to the air-handling unit

Even a well-chosen filter grade can underperform if the surrounding system is not designed for it. Housing design, sealing, access, airflow distribution and filter face area all influence how the filter works in practice.

Good system design should aim to:

• •keep the filter sealed correctly so air does not bypass around the frame
• •provide enough filter face area to manage resistance
• •allow safe, practical access for inspection and change-out
• •avoid turbulence or uneven loading across the filter face
• •protect downstream components from unnecessary dust exposure

In some cases, staged filtration is the best way to support both performance and maintenance. A pre-filter can take the load off the finer filter stage, making servicing more predictable and helping the system stay within its intended operating range. In other cases, the available space or fan capacity means the design must be simplified, with careful attention to filter choice and housing layout.

For facilities that rely on repeatable supply arrangements across multiple sites, the industries we supply page is a useful starting point for understanding the types of environments we work with.

Special applications and higher-efficiency filters

Some projects require higher-efficiency filtration, particularly where the building design or occupancy calls for greater control of fine particulate. In those cases, it is important to specify the exact performance needed and to confirm that the system can accommodate the resistance and installation requirements.

If HEPA filtration is being considered, the relevant EN 1822 ratings are H13 at greater than or equal to 99.95% efficiency at the most penetrating particle size, and H14 at greater than or equal to 99.995% efficiency at the most penetrating particle size. These grades are not interchangeable with general HVAC filter classes, and they should only be used where the system, housing and maintenance approach are designed for them.

Washability is another point that needs care. Some filters are designed to be washable - check the product instructions before cleaning or reusing any filter. Do not assume a filter can be washed simply because it looks robust.

Where a site has unusual dimensions or a non-standard housing, a made-to-order solution may be the safest way to achieve the right fit and performance. Our our products page and custom air filters page can help with specification discussions.

A practical specification process for AS 1668.2:2024 projects

For teams working on new builds, refurbishments or ongoing maintenance, a disciplined specification process reduces rework and makes maintenance easier later on. A useful approach is to:

• •confirm the site requirement with the mechanical services engineer or building owner
• •identify the system’s airflow targets, available fan capacity and physical constraints
• •choose a filter grade that suits the application and dust load
• •check initial and expected pressure drop against the fan and housing design
• •confirm access, sealing and replacement method before ordering
• •plan how condition will be monitored in service, including pressure gauges where fitted

This approach is especially helpful when multiple trades are involved and the filter decision needs to fit within a broader ventilation design. It also helps avoid the common problem of selecting a filter that looks suitable on paper but is difficult to maintain in the real installation.

Talk to Total Air Filtration

Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established 2009. We can help supply the right filters, including standard and made-to-order options, for Australian HVAC systems.

If you need support choosing the right filter grade, managing pressure drop, or fitting a filter to an existing housing, call 07 3800 3448 or request a quote. You can also contact us if you want to discuss a project in more detail.