Air Coming Around the Filter Instead of Through It: How to Spot Bypass in a Commercial AHU

When a commercial air handling unit is running but dust levels, odours or visible dirt are still showing up downstream, the filter may not be the only issue. Air can leak around the filter instead of passing through it, especially where the housing, gasket or access door is not sealing properly.

Bypass is easy to miss because the system still appears to be operating normally. The fan runs, the filter looks installed, and the pressure drop may even seem acceptable, but unfiltered air is finding a path around the media. Here’s how facility managers and commercial contractors can spot the signs and narrow down where the leak is happening.

What bypass looks like in a commercial AHU

Bypass happens when air takes the path of least resistance around the filter rather than through it. In practice, that means the filter is doing part of the work, but not all of it.

Common signs include:

• •Dust tracks around the filter frame or along the holding rails
• •Dirty streaks on the downstream side of the filter bank, especially near the edges
• •Uneven loading on the filter media, where the centre looks dirty but the sides stay relatively clean
• •Gaps that can be seen or felt around the perimeter of the filter
• •Noticeable dust downstream even though the filter has not collapsed or obviously failed

If the system uses higher-efficiency filters, such as HEPA units, bypass becomes even more important to check because even a small leak can undermine the intended filtration path. H13 and H14 HEPA filters are defined under EN 1822, but the filter only performs as intended when the housing and seal arrangement are sound.

Start with the filter seating and frame

A filter can only seal properly if it matches the frame, sits squarely, and is clamped evenly into place. One of the most common causes of bypass is simply a poor fit in the holding frame.

Look for these issues:

• •The filter is the wrong size for the slot or frame
• •The gasket is compressed unevenly
• •The frame is bowed, dented, corroded or out of square
• •The retaining clips, latches or hold-downs are loose or not engaging fully
• •The filter media is damaged at the edge from rough handling during installation

If the filter has been replaced recently and the problem started soon after, measure the filter carefully rather than assuming the old size label is correct. If you need help checking dimensions, how to measure an air filter is a useful place to start.

For systems using bag filters, panel filters or rigid cell filters, the sealing surface matters as much as the media itself. A filter with the right rating still cannot perform properly if the air is slipping past the edge.

Check the gasket and seal condition

The gasket is often the first line of defence against bypass. If it is damaged, compressed, missing sections or no longer contacting the frame evenly, air will leak around it.

Pay close attention to:

• •Flattened or brittle gasket material
• •Cracks, splits or missing pieces
• •Dust trails where air has been escaping past the seal
• •Sealant that has peeled away or failed at the corners
• •Filters reused in applications where the product instructions do not allow it

If the filter is a washable type, only do so when the product instructions specifically say it is designed to be washable. Some filters are designed to be washed, but that does not apply to all products.

A simple field check is to inspect the dirty side after shutdown and look for a clean line around the outer edge of the filter. That clean line can indicate the gasket is leaking and air is bypassing the media rather than loading through it.

Don’t overlook the access door and housing

Bypass is not always a filter problem. In many AHUs, the leak is in the cabinet, access door or filter housing itself.

Signs to check include:

• •Door seals that are cracked, flattened or missing
• •Latches that do not pull the door tight against the frame
• •Hinges that have sagged the door out of alignment
• •Warped panels or housing damage near the filter bank
• •Poorly fitted access covers after maintenance

A quick visual inspection can show whether the door is closing flush. If there is daylight around the perimeter, or if the door flexes when latched, that is a strong indication the seal line is not reliable.

In some units, the filter rack is integrated with the access door. If the door is distorted, the filter may appear to fit correctly but still leave a gap when the system is under pressure.

For ongoing maintenance, filter servicing can help identify whether the issue is the filter, the frame or the housing. For larger sites, commercial filter servicing may be a better fit where multiple AHUs or filter banks need regular attention.

Field checks that help isolate the leak

You do not always need specialist instruments to narrow down bypass. A careful visual and tactile inspection often reveals the source.

Useful checks include:

• •Shut the unit down safely and inspect the filter perimeter for dust trails
• •Check whether the filter is seated squarely and fully home in the rack
• •Examine the gasket for compression gaps, tears or hardening
• •Inspect the door seal and latch pressure along the full height and width
• •Look for signs of air leakage on the downstream side, especially around corners and edges
• •Confirm that the correct filter style and dimensions are being installed for that housing

If the unit has pressure gauges or differential pressure monitoring, use those readings as one part of the picture, not the whole diagnosis. A stable pressure drop does not rule out bypass, especially if air is escaping around the filter rather than through it.

Where site requirements are being set or reviewed, AS 1668.2:2024 may be relevant to the ventilation design, but the correct approach should be confirmed with the mechanical services engineer or building owner for that specific project.

When to replace parts rather than keep chasing the symptom

If the same AHU keeps showing bypass after filter changes, the underlying issue may be a worn frame, damaged door, distorted housing or a sealing method that no longer matches the installation.

At that point, it is worth considering:

• •Replacing damaged retaining hardware
• •Rebuilding or resealing the filter frame
• •Upgrading to a filter arrangement that better suits the housing
• •Ordering filters made to suit the actual slot size or frame depth
• •Reviewing whether the current filter type is appropriate for the site conditions and dust load

This is also where custom sizes can save a lot of rework. If the unit is non-standard, custom air filters may be the practical way to get a proper seal and improve fit in the existing housing.

If you are not sure what the unit needs, it can help to compare the current filter dimensions with the cabinet opening before ordering. our products gives a useful overview of the types available.

A practical way to keep bypass from coming back

Once bypass has been found and fixed, the key is to make sure the same problem does not return after the next service.

Good habits include:

• •Checking filter fit every time a replacement is installed
• •Inspecting seals and latches whenever the access door is opened
• •Keeping a record of housing damage, missing hardware and recurring leakage points
• •Following the filter and equipment manufacturer instructions for installation and maintenance
• •Reviewing inspection frequency based on the system, site conditions and dust load rather than using a fixed schedule

For facilities with mixed plant rooms, tenant fit-outs or dusty environments, regular visual checks are often just as important as measuring pressure drop. The aim is to catch leakage early, before it becomes a repeat maintenance issue.

Need the right fit for your AHU?

Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established 2009. If you need help identifying bypass, matching a filter to the housing, or sourcing the right replacement, call 07 3800 3448 or request a quote.

You can also contact us or view our products if you are comparing options for a commercial AHU, filter bank or custom application.