How Pleat Geometry Affects Pressure Drop in Small Commercial AHUs

In a small commercial air handling unit, filter choice is not just about efficiency rating or frame size. Pleat geometry also matters, because the way a filter is folded can change how air moves through it, how quickly resistance builds, and how often the system needs attention.

For mechanical contractors and facility managers, the practical question is simple: will this filter fit the AHU, hold its shape, and keep pressure drop within an acceptable range for the system? The answer often comes down to pleat angle, pleat depth and pleat spacing.

Why pleat geometry matters in smaller AHUs

Small commercial AHUs usually have tighter casing space, shorter filter tracks and less margin for airflow resistance than larger plant. When a filter is installed in a compact frame or shallow bank, the pleat design has to do more of the work.

A well-designed pleated filter spreads dust load across more media area. That can help the filter operate more steadily as it loads with dust, rather than becoming restricted too quickly in one area. But if the pleats are packed too tightly, collapsed unevenly or not suited to the air velocity in the unit, the filter can create avoidable resistance.

In practice, the geometry affects:

• •how much effective media area is exposed to airflow
• •how evenly air enters and leaves the filter
• •how much the pleats flex under load
• •how quickly dirt bridges across the pleat valleys
• •whether the filter maintains shape in a compact frame

The result is that two filters with the same nominal size and similar rating can behave differently in the same AHU.

Pleat angle: how air enters the media

Pleat angle describes the shape and pitch of the folds. In a compact AHU, this affects how smoothly air passes through the filter face and into the pleat pockets.

When the pleat angle is too steep for the application, airflow can concentrate on the leading edges of the pleats. That may increase local loading and create a less even dirt pattern. If the angle is too shallow, the media may not present enough usable area for the available frame depth, which can increase resistance sooner than expected.

A practical way to think about pleat angle is balance. The pleats need to present enough surface area without crowding each other so closely that air struggles to enter the fold.

Contractors should pay particular attention where:

• •the AHU has limited filter depth
• •the supply fan is already operating near its limit
• •the system serves spaces with variable dust load
• •the filter bank has poor upstream or downstream airflow distribution

If you are assessing a replacement, compare the installed filter style with the actual unit geometry rather than assuming any pleated filter will behave the same.

Pleat depth: more depth is not automatically better

Pleat depth refers to how far the pleat extends from face to peak. Greater depth can increase media area, but only if the AHU has enough space and the pleat structure is supported properly.

In smaller AHUs, deeper pleats can be useful when the casing allows them and the filter is designed to hold its form. However, if depth exceeds what the frame and airflow pattern can handle, the pleats may compress, touch each other or distort under load. That reduces the usable media area and can increase pressure drop.

Depth should also suit the frame construction. A filter with a deeper pleat pack may be harder to install cleanly if the holding rails, seals or door clearance are tight. Poor fit can cause bypass, distortion or early loading at the frame edges.

When selecting depth, check:

• •the clear depth available in the filter section
• •access space for removal and insertion
• •whether the filter sits in a rigid frame or a bag-style housing
• •whether the unit has even airflow across the face

For replacement planning, measuring the existing filter carefully is a good starting point. If the dimensions are uncertain, see how to measure an air filter before ordering.

Pleat spacing: the hidden factor in resistance

Pleat spacing is often overlooked, but it has a strong influence on how a filter loads over time. If pleats are too close together, dust can bridge across adjacent folds and block airflow pathways earlier. If they are too far apart, the filter may not make full use of the available media area.

In small commercial AHUs, this matters because the system often has less tolerance for a sudden increase in pressure drop. A filter with poor pleat spacing may start out acceptable but become restrictive more quickly as dust accumulates.

Spacing also affects stability. Narrow spacing can make pleats more vulnerable to flexing or contact between folds, especially if the air velocity is uneven across the face. Wider, well-supported spacing can improve air path consistency, provided the filter is designed for that style of loading.

Useful checks include:

• •looking for collapsed or touching pleats after installation
• •checking whether dust is loading evenly across the face
• •confirming the filter is not bowing in the frame
• •reviewing whether the upstream side has obstructions that create uneven airflow

If a system repeatedly shows unusual pressure rise, the issue may not be just the media grade. The pleat geometry may simply be wrong for that unit.

Reading pressure drop in the real world

Pressure drop is the clearest operational sign that filter geometry and system conditions are interacting. A new filter should be assessed against the system’s expected operating condition, not just its physical fit.

For smaller AHUs, good practice is to inspect the filter and review gauge readings where fitted, then compare those readings with the manufacturer’s guidance for the filter and the equipment. Inspection frequency depends on the system, the site conditions and the dust load. Sites with more dust, construction activity or outdoor contamination may need closer monitoring than cleaner environments.

A few practical points help when troubleshooting:

• •confirm the filter is installed in the correct airflow direction
• •check for frame leaks, damaged seals or bypass paths
• •look for dirt build-up on coils, dampers or prefilters upstream
• •ensure the fan and controls are operating as intended
• •review whether the filter type suits the available face area and depth

If a higher-efficiency filter is being considered, remember that MERV, EN 779 and ISO 16890 ratings do not convert directly. In Australia, G3/G4/F5-F9 are still commonly used EN 779 references, but site requirements should be confirmed with the mechanical services engineer or building owner, particularly where AS 1668.2:2024 is being used as part of the design basis.

Choosing the right filter geometry for a compact AHU

For small commercial units, the best filter is not always the one with the highest nominal media area on paper. It is the one that suits the available cabinet space, airflow pattern and maintenance access.

When specifying or replacing a filter, consider:

• •the actual filter pocket or frame depth available
• •whether the unit benefits from a shallower but more open pleat design
• •whether a rigidly supported pleat pack is needed to avoid collapse
• •whether the current arrangement is causing uneven dust loading
• •whether a custom size or format would fit better than a forced standard size

Some filters are designed to be washable - check the product instructions before re-use. If the system needs a special size, a different frame style or a non-standard arrangement, custom manufacturing may be the best way to match the geometry to the AHU rather than forcing the unit to adapt to the filter.

You can see more of what we supply on our products or explore custom air filters if standard sizes are not giving you the fit or performance you need.

Practical maintenance advice for contractors and facility managers

A good maintenance program is about observing how the filter and AHU behave together. If pressure drop rises faster than expected, or if the filter appears distorted, the geometry may be part of the problem.

Keep an eye on:

• •whether the pleats remain open and evenly spaced after installation
• •whether the filter frame seals properly in the holding track
• •whether the unit has signs of bypass dust around the edges
• •whether replacement filters are matching the original installed geometry
• •whether access for inspection is good enough to prevent rushed fitment

Where servicing is needed, it helps to work with a supplier who understands both the product and the application. Filter servicing and commercial filter servicing can also help keep a closer watch on fit, loading and replacement requirements across a site.

Getting the right filter for the unit

If a small commercial AHU is running harder than it should, pleat geometry is one of the first things worth checking. The right angle, depth and spacing can make the difference between a filter that fits neatly into the system and one that creates ongoing resistance issues.

Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, and we can help supply or make the right filters for your application. Call 07 3800 3448 or request a quote.

If you need help identifying the right option for a site, you can also contact us or view the industries we supply.