Custom Filters for Rooftop Units, AHUs and Packaged Systems

Rooftop units, air handling units and packaged systems can be straightforward on paper, but the filter side of the job is often anything but standard. Tight access, unusual filter banks, mixed frame depths and site-specific airflow patterns can make off-the-shelf filters a poor fit, even when the equipment itself is common.

For mechanical contractors, facility managers and industrial buyers, the challenge is not just getting a filter into the slot. It is getting the right construction, dimensions and media into plant that has to run reliably in real-world conditions, with enough access for maintenance and enough consistency to keep pressure drop and dust loading under control.

Why rooftop plant often needs custom filters

Commercial rooftop plant is exposed to harsh conditions. Heat, windblown dust, pollen, leaf litter and general urban contamination can all build up around the intake path, while maintenance access is often limited by roof layout, safety controls and time constraints.

That combination creates a few common problems:

• •The original filter size may no longer be available, especially on older equipment
• •The housing may have been modified over time, leaving non-standard dimensions
• •Access doors can restrict filter depth or frame style
• •Higher dust loads may call for a different media choice than the original specification
• •Vibration and weather exposure can make flimsy frames or poor sealing fail sooner than expected

In rooftop applications, a filter that is close enough is usually not good enough. Small gaps around the frame can let unfiltered air bypass the media, and a poor fit can make removal and replacement awkward for maintenance teams.

The main filtration challenges in AHUs and packaged systems

Air handling units and packaged systems can be deceptively variable. Two units from the same manufacturer may look similar externally, but the filter section can differ depending on the project, the site owner, or later modifications made during service life.

Space, access and depth constraints

Some units only allow a narrow filter pocket or an awkward loading angle. That can limit the media type, the frame depth or the way the filter is sealed into place. When access is tight, it is worth considering how the maintenance team will remove and inspect the filter without damaging the frame or surrounding equipment.

Airflow and dust loading

The right filter is not only about the nominal size. It also needs to suit the airflow rate, the type of contaminant and the site environment. A filter that is suitable for one site may load too quickly on another, especially in dusty industrial areas or locations near busy roads, construction activity or rooftop discharge points.

Pressure drop should be monitored where gauges are fitted, as it gives a practical indication of how the filter is performing in service. Inspection frequency depends on the system, site conditions and dust load, and should follow the filter and equipment manufacturer instructions.

Sealing and frame durability

In commercial plant, the frame matters as much as the media. A well-made frame helps maintain a proper seal and makes the filter easier to handle during servicing. Depending on the system, that may mean a cardboard frame, metal frame, synthetic frame or another construction suited to the application.

For some installations, filters need to be robust enough to withstand repeated handling, vibration and weather exposure. In others, the main requirement is a precise fit with minimal bypass.

How custom sizing helps in the real world

Custom filters are useful when standard dimensions do not match the equipment exactly, or when the existing setup has been altered over time. They can also help where a site wants to keep the existing housing but improve fit, handling or media performance.

A custom approach can be suitable when you need:

• •Non-standard width, height or depth
• •A specific frame style to suit the housing
• •A tighter seal around the perimeter
• •Alternative media to suit heavier dust loading
• •Filters built to fit older or discontinued equipment
• •A particular construction to suit handling, moisture exposure or access limitations

This is especially relevant for rooftop units where every millimetre counts. A correctly sized filter reduces installation frustration and can make routine servicing more predictable.

If you are measuring an existing filter or a filter bank opening, accuracy matters. Our guide on how to measure an air filter is a useful starting point, particularly when the original item has distorted, worn corners or missing labels.

Choosing the right construction and media

There is no single filter type that suits every rooftop unit or AHU. The right choice depends on the system design, the dust challenge and the maintenance approach at the site.

Frame type

Frame selection affects fit, strength and ease of handling. Common needs include:

• •Lightweight frames for easier replacement
• •Stiffer frames where the filter is handled often
• •Better edge integrity for sealing in demanding plant rooms
• •Materials suited to moisture, heat or mechanical stress

Media selection

Different media types can suit different site conditions. Some installations need coarse pre-filtration to catch larger debris, while others require finer filtration stages to capture smaller particles.

For projects involving multiple stages, it can help to think about the filter bank as a system rather than individual items. The pre-filter, final filter and any intermediate stage should work together to suit the equipment and the maintenance plan.

Washable and reusable options

Some filters are designed to be washable - check the product instructions. This can be useful in specific applications, but it is not suitable for every system, and cleaning methods must match the product design. If a filter is not intended to be washed, using water or compressed air can damage the media or frame.

If you are unsure whether a washable option is appropriate, it is best to match the product instructions to the system requirements before ordering.

Matching the filter to the site and the standard

For commercial ventilation and air conditioning work, site requirements should be confirmed with the mechanical services engineer or building owner. Where relevant, AS 1668.2:2024 may inform the ventilation approach, but it does not replace the need to check the actual equipment, installation and maintenance requirements on site.

It is also important not to assume that filter rating systems are interchangeable. MERV, EN 779 and ISO 16890 ratings do not convert directly, so it is better to specify the filter according to the actual application and the equipment documentation rather than rely on a quick conversion.

If you are dealing with legacy plant, you may still see references to G3/G4 or F5-F9 classes. These were EN 779 classes and are still widely used in Australia, even though ISO 16890 is now the current standard reference in many projects.

For higher-efficiency applications, HEPA filters need especially careful specification. H13 is at least 99.95% efficient at the most penetrating particle size under EN 1822, and H14 is at least 99.995%. Those filters are used in specific applications only, so the project requirement should be checked before selecting them.

Practical steps before ordering replacement filters

Before placing an order, it helps to confirm a few details so the replacement suits the equipment and the site.

• •Measure the existing filter carefully, including actual depth and frame type
• •Check whether the filter sits in a bag, panel, rigid or framed arrangement
• •Note any clips, rails, tabs or holding systems used in the housing
• •Record airflow direction if applicable
• •Confirm whether the site prefers standard or custom construction
• •Review the equipment manual and any site maintenance instructions
• •Check whether the filter bank has room for a deeper or sturdier option

If the old filter is missing, damaged or badly compressed, the opening itself may be the better reference point. Our custom air filters page explains the types of builds that can be made to suit unusual equipment.

For ongoing support, regular filter servicing can help keep the system easier to manage, and commercial filter servicing may be a better fit for sites with multiple units or access limitations.

Getting the right fit for rooftop plant

Custom filters are often the most practical answer when rooftop units, AHUs and packaged systems no longer match a standard catalogue size. The goal is simple: a filter that fits properly, seals correctly and suits the dust conditions, access and maintenance needs of the site.

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 filters for your equipment, and we can talk through options for non-standard plant.

Call 07 3800 3448 or request a quote. If you need to discuss a project or confirm an existing specification, you can also contact us or review our products.