Air Leaks at the Filter Frame: Gasket Problems in Older Rooftop Units

Older rooftop units often look serviceable from the outside while quietly leaking air around the filter frame. When gaskets become compressed, hardened, split or simply go missing, unfiltered air can bypass the filter bank and move straight into the supply airstream.

For mechanical contractors and facility managers, this is one of the most common and most overlooked causes of poor filter performance in ageing plant. The filter itself may be intact, but if the seal around it is not sound, the system will not perform as intended.

Why gasket condition matters in older rooftop units

In older rooftop units, the filter section is often exposed to heat, vibration, roof movement, weather and repeated maintenance access. Over time, these conditions can take a toll on foam, rubber and other sealing materials.

A gasket is meant to create a tight seal between the filter frame and the filter media or holding frame. When that seal weakens, air takes the path of least resistance. Instead of being drawn through the filter, some air slips around it.

That leakage can happen for several reasons:

• •the gasket has flattened and no longer recovers its shape
• •the gasket has gone brittle from age or heat exposure
• •the gasket has torn during filter changeover
• •the filter frame is bent or distorted
• •the holding rail, clip or access door does not close evenly
• •the replacement filter is the wrong size or the wrong sealing type for the frame

In practice, a filter that looks acceptable can still allow bypass if the surrounding seal is poor. That is why a visual check of the media alone is not enough.

Signs of bypass at the filter frame

Air leakage around the filter bank can be difficult to spot unless you know what to look for. In older rooftop units, the signs often show up during routine servicing, odour complaints, dust accumulation or pressure drop checks that do not match the apparent condition of the filters.

Common signs include:

• •dust trails or dirty marks around the filter edges
• •visible gaps between the filter frame and the housing
• •loose or missing compression seals
• •uneven seating of the filter in the frame
• •damaged clips, rails or access panel seals
• •blackened, hardened or sticky gasket material
• •localised dust build-up downstream of the filter section

If the unit uses side-loading filters, end-loading filters or a bank arrangement with multiple sections, leaks may only affect part of the frame. That can make the problem easy to miss unless every section is checked.

A pressure gauge can still be useful, but pressure drop alone does not tell the whole story. A filter bank may show a reasonable reading and still be bypassing air around damaged seals.

What causes gasket failure in rooftop plant

Age is only part of the story. Rooftop systems face harsher conditions than many indoor plantrooms, and that exposure accelerates wear on sealing materials.

Heat, UV and weather exposure

Rooftop units can be exposed to strong sun, wind-driven dust, rain and temperature swings. If access covers or filter housings are not well protected, sealing materials can dry out, shrink or lose resilience.

Vibration and movement

Fans, compressors and general mechanical vibration can loosen holding frames over time. Even a small amount of movement may reduce the compression on a gasket and create a leak path.

Poor filter fit

If a replacement filter is not the correct dimensions or sealing style, it may appear to fit while still leaving a small but important gap. This is especially relevant in older units where original parts may no longer be standard.

Repeated maintenance access

Frequent servicing is necessary, but repeated opening and closing of filter doors can damage seals, clips and mounting surfaces. If the same frame is reused for years, small defects can accumulate.

What contractors should inspect during service

A thorough service of an older rooftop unit should include more than a simple filter swap. The frame, seal surfaces and holding hardware need attention as well.

Check the gasket material itself

Look for cracking, flattening, brittleness, missing sections or compression that no longer rebounds. If the gasket is bonded to the frame, check whether it has lifted at the edges or failed at the corners.

Check the mating surfaces

Even a good gasket will struggle if the contact surface is dirty, dented or warped. Remove dust and debris, then inspect for corrosion, panel distortion or damage caused by previous filter changes.

Check seating and retention

Confirm that the filter sits squarely in the frame and that holding clips, rails or retainers apply even pressure. A filter that is slightly cocked can create an air gap on one side.

Check the replacement filter type

Not all filters seal the same way. Some rely on frame compression, some on foam edges, and some on specific gasket arrangements. If the existing setup is no longer doing its job, the replacement may need a different sealing approach.

If you are measuring a replacement filter for an older unit, use the correct dimensions and measure carefully. Our guide on how to measure an air filter may help when confirming the size and fit.

Practical ways to reduce bypass

Once a rooftop unit has a gasket problem, the right fix depends on the condition of the housing, the type of filter bank and the air quality needs of the site.

Useful corrective actions can include:

• •replacing hardened or damaged gasket material
• •cleaning and preparing the seal surface before refitting
• •straightening or repairing distorted frames where practical
• •replacing worn clips, rails or retainers
• •choosing a filter with the correct sealing profile for the housing
• •adding or changing access panel seals if the leak is not limited to the filter frame
• •reviewing the housing layout if the same problem keeps returning

In some cases, the best solution is not just a new filter but a properly matched filter and frame arrangement. For older rooftop units, that can mean sourcing a standard replacement or arranging custom air filters to suit a non-standard opening, legacy casing or unusual sealing setup.

Some filters are designed to be washable - check the product instructions. If a washable product is being considered for a particular application, confirm that it is appropriate for the housing and the site’s maintenance process rather than assuming any reusable filter will suit.

Matching the filter to the system, not just the dimensions

A leak at the frame can tempt teams to focus only on size. But sealing performance also depends on the filter type, the installation method and the condition of the unit itself.

For example, older rooftop plant may have originally used coarse grades such as G3 or G4, or finer pre-filter and final-filter arrangements depending on the application. These EN 779 classes are still widely used in Australia, even though EN 779 has been superseded by ISO 16890. MERV and EN 779 or ISO 16890 ratings do not convert directly, so avoid assuming equivalence when specifying replacements.

If the site has a ventilation design requirement, or if there is a need to confirm whether the current setup remains suitable, refer to AS 1668.2:2024 and confirm site requirements with the mechanical services engineer or building owner. The right filter arrangement should suit the system as installed, not just the nominal rating on paper.

For some applications, you may also need to consider whether the existing unit can accept a deeper frame, a better sealing edge or a different retention method. That is where a practical review of the housing often pays off.

Keeping older rooftop units serviceable

Older plant can still perform well if the details are looked after. The main point is to treat gasket condition and frame integrity as part of normal filter maintenance, not as an afterthought.

A good inspection should look at:

• •gasket condition
• •frame alignment
• •filter fit
• •retention hardware
• •access door sealing
• •evidence of dust bypass downstream

Inspection frequency depends on the system, site conditions and dust load. Follow the filter and equipment manufacturer instructions, and monitor pressure drop where gauges are fitted. If a unit is in a dusty location, or if doors and frames are opened often, the sealing surfaces may need closer attention.

For facilities managing several ageing units, it can help to document which rooftop plant has recurring leakage issues and which filter style is fitted to each one. That makes it easier to plan replacements and avoid fitting the wrong product to the wrong frame.

Need the right filter for an older rooftop unit?

Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, and we can help you match replacement filters to older rooftop units, including non-standard sizes and sealing arrangements.

If you need advice on a filter fit, a legacy frame or a custom replacement, call 07 3800 3448 or request a quote. You can also view our products or contact us through contact us if you want to talk through the installation first.