Custom Pleated Filters for Non-Standard HVAC Systems

When an HVAC system does not use a standard filter size or frame, off-the-shelf options can be awkward, noisy, hard to seal, or simply unusable. Custom pleated filters are a practical way to match unusual return air grilles, rooftop units and legacy equipment while still capturing airborne dust and other particles the system is designed to collect.

For homeowners, facility managers and mechanical contractors, the key is not just finding a filter that fits the opening. It is choosing a filter that fits the equipment, the airflow path and the site conditions, so the system can operate as intended.

Why non-standard systems need custom pleated filters

A filter works best when the frame, media and sealing arrangement suit the housing. If the size is even slightly off, air can bypass the filter, dust can build up where it should not, and access can become awkward for maintenance.

Non-standard systems often show up in older buildings, renovated homes, mixed-use sites and plant rooms with equipment that has been replaced over time. Common examples include:

• •return air grilles that are wider, narrower or deeper than common retail sizes
• •rooftop units with limited access or unusual filter tracks
• •older ducted systems with obsolete frame dimensions
• •equipment that has been modified by previous owners or contractors
• •plant enclosures where standard panel filters do not seat properly

In these situations, pleated filters are often a sensible choice because the pleated media gives more filter area within a compact frame. That can help the filter suit the available space without forcing a redesign of the ductwork or housing.

Where custom pleated filters are commonly used

Unusual return air grilles

Return grilles in homes and light commercial spaces are not always standard. Some are custom joinery sizes, some sit in tight wall or ceiling cavities, and some were made to match older architectural features. A custom pleated filter can be built to the exact dimensions needed so it sits securely and is easier to remove and reinstall.

When measuring a grille installation, it helps to confirm:

• •the clear opening
• •the available depth behind the grille
• •the direction of airflow
• •whether the filter seats in a slot, frame or ledge
• •whether there is room for a handle, tab or gasket

If you are unsure how to measure, our guide on how to measure an air filter is a useful starting point.

Rooftop units

Rooftop units can present a different problem: access. The filter may need to be changed in a narrow service path, through a side access door, or from a maintenance platform. The filter also needs to fit the housing so it does not flex, rattle or leave gaps when the unit is running.

For rooftop applications, it is worth considering:

• •the exact internal dimensions of the filter rack
• •the available insertion space
• •whether one large filter or several smaller filters will be easier to handle
• •the frame material best suited to the environment
• •whether the filter needs added strength at the edges for repeated handling

Legacy HVAC equipment

Older air handling units and ducted systems can be difficult to service when the original filter format is no longer available. Some systems still use dimensions that are not commonly stocked, or a frame style that needs to be reproduced to keep the equipment usable.

In these cases, custom pleated filters can help preserve the existing system layout while allowing a replacement filter to be made to the right size and construction.

What to measure before ordering

The best custom fit starts with careful measurement. Because many filter housings are not perfectly square or may have been altered over time, it is better to confirm the actual opening and clearances rather than assume the nominal size is still correct.

Useful details to record include:

• •length, width and depth of the filter slot or opening
• •the exact point where the filter seals or rests
• •any lip, rail or frame that reduces the usable opening
• •the direction of airflow through the system
• •the level of dust loading and how often the filter is inspected
• •any access constraints for changing the filter safely

If the equipment has a label or maintenance manual, keep a copy with the measurements. If the system is part of a larger installation, the mechanical services engineer or building owner should confirm the site requirements, especially where ventilation design or operating conditions are involved. If relevant, AS 1668.2:2024 may need to be considered as part of the broader system design.

Choosing the right pleated filter construction

Not every custom pleated filter is the same. The right option depends on the equipment, the available space and the filtration level the system is designed to use.

Frame and media options

Custom pleated filters can be made in a range of frame materials and media combinations. Common considerations include:

• •cardboard, plastic or metal frame styles
• •standard or reinforced edges
• •pleated media for increased surface area
• •gasketed edges where a tighter seal is needed
• •special dimensions for shallow or deep housings

Some filters are designed to be washable - check the product instructions. If a washable option is being considered, it is important to confirm it is suitable for the actual site conditions and maintenance approach.

Compatibility with the equipment

A filter should suit both the physical housing and the airflow resistance the system was designed for. An overly restrictive filter can affect how the system behaves, while an undersized or poorly fitted filter may allow bypass. This is why matching the dimensions and construction matters just as much as choosing the nominal size.

If you are comparing options across different rating systems, note that MERV, EN 779 and ISO 16890 do not convert directly. EN 779 classes such as G3, G4 and F5-F9 are still widely used in Australia, even though ISO 16890 is now the current European standard family. For high-efficiency applications, H13 and H14 HEPA filters are defined under EN 1822, with H13 at 99.95% and H14 at 99.995% efficiency at the most penetrating particle size. These ratings should only be used where the system is designed for them.

Maintenance and replacement planning

Inspection frequency depends on the system, site conditions and dust load. In some environments, filters need checking more often than in cleaner spaces, while in others the interval can be longer. Follow the filter and equipment manufacturer instructions, and monitor pressure drop where gauges are fitted.

Good maintenance practice usually includes:

• •checking for damaged frames, loose edges or bypass paths
• •confirming the filter is seated correctly after installation
• •noting visible dust build-up or airflow changes
• •replacing the filter when it can no longer perform as intended
• •keeping spare custom sizes on hand for critical equipment

For commercial sites, coordinated maintenance can be arranged through filter servicing or commercial filter servicing where appropriate.

Why custom supply matters for homeowners and contractors

For homeowners, a custom pleated filter can mean the difference between a system that is easy to maintain and one that is awkward to service. For contractors, it can reduce callbacks caused by poor fit, rattling filters or improvised installations. For facility managers, having a reliable custom source can make maintenance planning much simpler across different plant types.

Custom supply is especially helpful when:

• •the original filter size is no longer available
• •a site has been refurbished and the openings no longer match standard sizes
• •access is tight and the filter needs to be made to suit the installation
• •a building has multiple non-standard systems that need consistent ordering

If you manage several properties or equipment types, it can also help to keep a record of each filter size, frame type and site location so replacements are easier to organise later. You can review our products or see the industries we support through industries we supply.

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 or make the right custom pleated filters for unusual grilles, rooftop units and older HVAC equipment.

Call 07 3800 3448 or request a quote to discuss your filter size, application and site requirements. You can also contact us if you need help working out what information to provide.