Choosing the Right Holding Frame for Pocket Filters in Rooftop AHUs

Pocket filters are only as effective as the frame that holds them in place. In rooftop air handling units, the holding frame does more than support the filter bags: it helps maintain a proper seal, keeps the filter bank stable under airflow, and makes routine servicing easier or harder depending on the design.

For mechanical contractors and facility managers, frame choice should be treated as part of the whole filter system, not an afterthought. The best option depends on the AHU layout, access space, dust loading, and the way the unit is maintained over time.

Why the holding frame matters

A pocket filter needs even support around its perimeter. If the frame is a poor fit, the filter can leak around the edges, flex under load, or be awkward to remove and reinstall during service.

In rooftop AHUs, those issues can be more noticeable because units are exposed to heat, wind, dust and access constraints. A frame that looks suitable on paper may still be difficult to seal properly if the cabinet opening is uneven or the service side is tight.

The frame affects three practical outcomes:

• •Sealing: whether air is forced through the filter media rather than around it
• •Rigidity: whether the frame holds shape when the unit is operating and when filters are changed
• •Service access: how easily technicians can remove, inspect and replace filters without damaging the frame or the bags

If you are specifying a new unit or replacing an existing filter bank, it is worth checking the frame details early. If you are unsure how to measure the existing opening, how to measure an air filter is a good place to start.

Common frame types used in rooftop AHUs

Metal holding frames

Metal frames are widely used in commercial systems because they are strong and hold their shape well. They are often chosen where the filter bank needs extra rigidity, or where the frame will be exposed to repeated handling during servicing.

A well-made metal frame can suit rooftop conditions, especially where the unit experiences vibration or the filter section is accessed regularly. It can also help when the filters are relatively large and the frame needs to stay square for a reliable fit.

Things to check with metal frames:

• •edge finish and protection against corrosion in rooftop conditions
• •whether the filter slides smoothly without snagging
• •compatibility with the unit’s existing gasket or seal arrangement
• •whether the frame can be cleaned without damaging the seating surfaces

Galvanised steel frames

Galvanised steel is often selected for its balance of strength and practicality. It is commonly found in commercial air handling units and can be a sensible choice where the frame needs to resist wear while remaining straightforward to fabricate or replace.

For rooftop AHUs, galvanised steel frames are usually preferred where the filter bank is part of a larger custom assembly or where service teams need a robust frame that can cope with regular access.

Aluminium frames

Aluminium frames are lighter and can be easier to handle during installation or replacement. That can be helpful where service access is awkward, or where repeated removal is expected and reducing handling weight matters.

They may be suitable where corrosion resistance and ease of handling are priorities, but the final choice should still consider stiffness and how the frame interfaces with the cabinet. A lighter frame is not always the better frame if it moves or distorts under load.

Plastic or composite frames

Some filter systems use plastic or composite frames, particularly where weight, cost or corrosion resistance are important factors. These can work well in the right application, but they should be checked carefully for rigidity, temperature tolerance and how well they seal against the AHU housing.

In rooftop units, it is especially important to confirm that the frame material is suitable for the environment and the service method. Some filters are designed to be washable - check the product instructions before assuming a frame or filter can be reused after cleaning.

How frame choice affects sealing

A good seal is not just about the gasket on the filter. It also depends on the frame being straight, square and consistent against the cabinet face.

When the frame is poorly matched to the housing, you can end up with bypass air at the edges. That means some of the air stream may move around the filter rather than through it, which undermines the purpose of the filter bank and can create uneven dust loading.

To improve sealing, look for:

• •a frame that matches the actual opening dimensions, not just the nominal size
• •a stable mounting face with enough contact area for the seal
• •a frame design that does not twist when the retaining mechanism is tightened
• •a layout that allows the filter to sit evenly without forcing the bags against obstructions

If the existing bank has recurring sealing issues, replacing the frame alone may not solve the problem unless the cabinet, tracks and retaining hardware are also checked. For custom sizes or unusual housings, custom air filters may be the better path than trying to make a standard frame fit poorly.

Rigidity and airflow stability

Pocket filters need support across the full filter bank so the media can load evenly. If the frame flexes, the pockets may flutter, bow or shift in a way that affects performance and makes servicing more difficult.

Rigidity matters most where:

• •the AHU runs for long periods and the filters progressively load with dust
• •the filter bank spans a wide opening
• •access panels or retaining systems place uneven pressure on the frame
• •rooftop vibration or repeated handling is common

A rigid frame helps the filter bank stay aligned, which reduces the chance of damage during installation and removal. It also makes it easier for technicians to see whether the filter is seated properly after replacement.

That said, excessive stiffness without a proper fit is not enough. The frame still needs to match the cabinet accurately and allow the filter to be installed without forcing it into position.

Service access and maintenance practicality

Service access is often where the best-looking specification fails in practice. If the frame is awkward to remove or reinstall, filters are more likely to be fitted badly or damaged during maintenance.

For rooftop AHUs, consider how a technician will actually work in the space:

• •Is there enough room to pull the filter out without catching the pockets?
• •Does the frame need tools for removal, or can it be opened quickly and safely?
• •Can the filter be inserted straight into the track, or does it need to be twisted into place?
• •Is there enough clearance for inspection of seals and retaining points?

A frame that supports easy access can make inspection and replacement more straightforward, especially where the unit is on a roof with limited working space. If the overall service arrangement is difficult, filter servicing or commercial filter servicing may be worth planning into the maintenance approach.

Matching the frame to the site and standards

The right holding frame depends on more than the filter size. It should suit the AHU design, the rooftop environment, and the maintenance regime for the site.

Useful questions to ask before selecting a frame include:

• •Is the unit new or an existing retrofit?
• •Is access from one side or both sides?
• •Are there vibration or corrosion concerns?
• •Is the opening rectangular, shallow, or non-standard?
• •Will the filters be changed by in-house staff or a contractor?

If the system forms part of a regulated ventilation setup, the mechanical services engineer or building owner should confirm the site requirements, including any relevant application of AS 1668.2:2024. Frame selection should support the specified system design, not replace it.

For some projects, a standard frame is fine. For others, a custom frame or custom filter arrangement is the better fit, especially where existing cabinets have been modified over time or where standard sizes do not seat correctly.

Choosing a practical solution with Total Air Filtration

At Total Air Filtration, a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, we help customers choose filters and holding frames that suit the actual AHU, not just the nominal drawing.

If you need a practical recommendation for a rooftop unit, we can help supply or make the right filters and discuss frame options for the installation. Call 07 3800 3448 or request a quote. You can also browse our products or contact us to discuss the details of your AHU and service access needs.