Managing Filter Bypass in Older Hotel Rooftop Units

Older hotel rooftop package units can be good workhorses, but age brings wear in places that are easy to miss until dust build-up, odour complaints or uneven airflow start showing up. One of the most common issues is filter bypass: air taking the path of least resistance around the filter instead of through it. In older plant, the problem is often not the filter media itself, but the way the frame, housing and seal all work together.

For mechanical contractors and hotel engineers, the first step is to treat bypass as a physical fit-and-seal problem, not just a filter selection issue. A well-matched filter can still underperform if the unit is distorted, the holding frame is damaged, or the filter is the wrong size for the compartment.

What filter bypass looks like in the field

Bypass is air leakage around the filter edges, through gaps in the frame, or across damaged sealing surfaces. In a rooftop package unit, that air can carry dust, lint and other particles straight into the supply airstream without passing through the filter media.

Common signs include:

• •Dust tracks along the housing or downstream side of the filter section
• •Visible daylight at frame edges when the unit is open
• •Uneven dust loading, where only part of the filter face is dirty
• •Mould or grime build-up on coils, fan sections or duct entry points downstream
• •Filters that rattle, sit crooked or appear to shift when the unit is running

If a filter looks dirty but the edges are relatively clean, that can be a clue that air is slipping around rather than through the element. In older units, bypass may be happening at several points at once.

The most common causes in ageing rooftop units

Worn housings and distorted frames

Over time, rooftop cabinets can twist, corrode or lose alignment. Heat, vibration and maintenance access can all affect the shape of the filter compartment. Even a small distortion can prevent the filter from sitting flat against the sealing surface.

Look for:

• •Bent filter rails or retaining channels
• •Corrosion on the frame or support ledges
• •Missing fasteners, clips or clamps
• •Gaps where the filter bank no longer sits squarely in the housing

If the filter is being forced into place, or if there is visible movement once installed, the housing may need attention before another filter is fitted.

Poor or missing seals

A sealed edge matters just as much as the media. Deteriorated gaskets, compressed foam, missing seal strips or poor-quality patch repairs can all create a path for air to leak around the filter.

Check for:

• •Hardened, cracked or flattened gasket material
• •Seal strips that have peeled away from the frame
• •Gaps at corners, joints and access door edges
• •Temporary sealant or tape that has failed with age or heat

In some older rooftop units, the filter door or access panel is part of the seal path. If that panel is warped or the latch no longer pulls it tight, bypass can occur even with a correctly sized filter.

Mismatched or poorly measured filters

One of the simplest causes of bypass is a filter that does not match the compartment properly. This can happen when a replacement is chosen from an approximate size, an old part number, or a measurement taken only from the media and not the full frame.

Before ordering, confirm the actual opening size and how the filter is retained. If needed, use a proper measuring method rather than guessing from the old filter. Our guide on how to measure an air filter can help with the basics.

A filter may be the right nominal size and still fit poorly if:

• •The frame depth is wrong for the holder
• •The corner construction interferes with the rails
• •The gasket does not match the sealing surface
• •The filter is intended for a different style of track or bank

If the existing unit has been modified over the years, a standard off-the-shelf replacement may not sit as intended.

What to inspect first on site

When you are called to an ageing hotel rooftop unit, start with the areas most likely to cause leakage rather than immediately replacing filters.

Focus first on:

• •The condition of the filter frame and support rails
• •The integrity of the gasket or sealing strip
• •The fit of the filter inside the compartment
• •The access door closure and latch tension
• •Any sign of corrosion, warping or vibration damage
• •Dirt patterns that show where air is escaping

If the unit has differential pressure gauges or ports fitted, check pressure drop in line with the filter and equipment manufacturer instructions. A pressure reading can help tell you whether airflow resistance is increasing, but it will not by itself reveal bypass. You still need a visual inspection of the sealing surfaces and frame condition.

It is also worth checking whether any recent maintenance work disturbed the filter bank. A filter may be installed correctly one visit and shift slightly after service access, a panel replacement or a rushed reassembly.

Choosing the right replacement approach

In older equipment, the best fix is not always a like-for-like replacement of the old filter. Sometimes the real solution is to address the frame, seal or retention method at the same time.

You may need to consider:

• •A custom size to suit a worn or non-standard compartment
• •A different gasket style to improve contact with the housing
• •A stronger frame construction where the existing holder is distorted
• •A revised filter design that better suits the access method and airflow path

If the filter section has been altered over time, custom air filters can be a practical way to restore a better fit. See our custom air filters page for options.

For some sites, particularly where rooftop access is awkward, it may also make sense to review the whole service approach. Our filter servicing and commercial filter servicing pages outline the kind of support available when repeated fit issues need a closer look.

When bypass is really a unit maintenance issue

Not every bypass problem is solved by changing filter media. In many hotel plants, the root cause is broader equipment wear.

Consider further investigation if you find:

• •Recurring gaps even after correct installation
• •Door seals that do not hold under fan vibration
• •Corrosion that prevents the filter frame from sitting flush
• •Repeated damage to filters during installation or removal
• •Uneven airflow that suggests internal leakage or fan-side issues

At that point, the question becomes whether the unit can still hold a filter properly without repair. If the housing is too compromised, the most carefully selected filter will still struggle to do its job.

For hotel engineers, this is also where system context matters. Site requirements should be confirmed with the mechanical services engineer or building owner, and if the system is tied to ventilation design requirements under AS 1668.2:2024, the filter arrangement needs to suit the actual equipment and application rather than a generic part number.

Getting the fit right the first time

The best way to reduce bypass in older rooftop package units is to inspect the compartment before ordering a replacement. Measure carefully, check the seal path, and look for signs that the frame has changed shape over time. If the unit is a non-standard size or the existing filter arrangement has been patched together over the years, a purpose-made filter may save a lot of avoidable return visits.

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 ageing rooftop units, including unusual sizes and fit-critical applications. Call 07 3800 3448 or request a quote, and if you need to review available options first, see our products or contact us.