What to Consider When a Heritage Office Building Has Older AHUs

Heritage office buildings often have character, but their ageing air handling units can create practical problems when it is time to replace filters. Original housings, access doors and cassette sizes may not match modern standard dimensions, so choosing the right replacement is usually about fit as much as filtration performance.

For building managers, property managers and mechanical contractors, the main task is to identify what the unit actually needs on site, then match that to a filter that will install correctly, seal properly and perform as intended. If the dimensions are unusual, custom manufacture may be the simplest path.

Why older AHUs create different filter challenges

Older air handling units were often built to suit the project available at the time, rather than current standard filter formats. That means two buildings with similar age can still have very different filter arrangements.

Common issues in heritage or older commercial buildings include:

• •Non-standard filter depths or face dimensions
• •Shallow filter tracks that limit frame depth
• •Access doors that make rigid filters difficult to remove or install
• •Filter banks with mixed sizes in the same AHU
• •Corrosion, bent rails or worn gaskets that affect sealing
• •Legacy clips, retainers or slide channels that no longer align neatly

These issues matter because a filter that is close but not quite right can be hard to fit, may bypass air around the edges, or can damage the frame, door or holding hardware over time.

Start with the existing filter and the AHU housing

Before ordering replacements, inspect the actual unit and the filter in place. In older buildings, records are often incomplete, so physical checking is usually more reliable than relying on drawings alone.

Useful checks include:

• •Measure the filter frame, not just the nominal size shown on an old label
• •Check the depth available in the filter rack or holding frame
• •Note how the filter is retained: slide-in, clamped, hinged, or held by a removable access panel
• •Look for gaps, worn seals, damaged corners or warped metalwork
• •Confirm whether the filter sits upstream or downstream of fans, coils or other components

If you are unsure how to capture the right dimensions, how to measure an air filter is a practical place to start. In many cases, the hardest part is identifying whether the current filter was trimmed, adapted or already a non-standard replacement.

Matching the replacement to the space you actually have

Once the existing arrangement is understood, the next step is choosing a replacement filter that suits the unit, the access conditions and the required air quality objectives for the site.

Standard size is not always the best answer

A standard commercial filter size can work well if the housing was designed around it. But older systems often need a more specific solution.

Consider:

• •Whether the filter can be supplied to the exact external dimensions required
• •Whether a slightly different frame style would improve sealing or handling
• •Whether the unit needs rigid panel filters, pleated filters, pocket filters or another form
• •Whether the holding frame needs adjustment before a new filter will sit correctly

A custom solution can be especially useful when an original AHU has been modified over time, or when access limitations make a modern standard frame awkward to install.

If the system needs a purpose-made solution, custom air filters may be the better option than forcing a near match into a housing that was never designed for it.

Frame style, seal and rigidity all matter

With older equipment, the frame itself is often as important as the media. A filter can have the right nominal size but still perform poorly if the frame flexes, bows, or cannot seal against the rails.

When reviewing options, ask:

• •Will the frame hold its shape during installation and service?
• •Is there enough rigidity for the available access opening?
• •Does the filter need a gasket, foam seal or other edge treatment?
• •Can the filter be removed without snagging on the door frame or surrounding ductwork?

These practical details can save time during routine maintenance and help avoid repeated call-backs.

How air quality requirements affect filter choice

The right replacement is not only about fit. It also needs to suit the building’s ventilation design, internal use and operating conditions.

For heritage office buildings, the required performance may depend on factors such as:

• •The type of tenancy and occupant density
• •Nearby road dust or construction activity
• •Whether the building contains sensitive equipment or archives
• •The condition of the ductwork and AHU internals
• •The maintenance access available to service staff

If the site has specific ventilation requirements, confirm them with the mechanical services engineer or building owner. AS 1668.2:2024 may be relevant to the broader ventilation design, but the correct filter selection still needs to suit the actual system and site constraints.

Where higher-efficiency filtration is being considered, it is important to understand the rating language being used. MERV, EN 779 and ISO 16890 ratings do not convert directly, so avoid treating one label as a simple match for another. EN 779 classes such as G3, G4 and F5-F9 are still widely used in Australia, even though the standard has been superseded by ISO 16890.

For specialised applications, HEPA filters may be relevant. H13 and H14 classes under EN 1822 are defined at the most penetrating particle size, with H13 at at least 99.95% and H14 at at least 99.995% efficiency. These products are for specific applications and should only be selected where the system and housing are suitable.

Maintenance access and service practicality should not be overlooked

Older AHUs can be difficult to service if the replacement filter is awkward to handle, heavy, or needs excessive force to install. Even a well-chosen filter can become a maintenance problem if the access path is tight.

Think about the servicing process, not just the installed result:

• •Can the filter be inserted without bending the frame?
• •Is there enough clearance to close and latch the access door?
• •Will service staff be able to remove the used filter cleanly?
• •Are there sharp edges, corrosion or debris in the housing that should be addressed first?

If the AHU is showing signs of broader wear, filter servicing or commercial filter servicing may be useful to keep the setup practical and tidy. In some cases, a small hardware repair or seal replacement can make a much better difference than changing filter type alone.

Washable filters and heritage buildings

Some filters are designed to be washable - check the product instructions. That can be useful in certain legacy systems, but it is not a universal answer, and the filter still has to suit the application, airflow and maintenance routine.

A practical way to approach older AHU replacements

A reliable replacement process usually starts with inspection, then measurement, then a product match that reflects both the housing and the building’s operating needs.

A sensible workflow is:

• •Inspect the AHU and note the existing filter arrangement
• •Measure the actual opening and frame dimensions carefully
• •Check access, retention method and sealing surfaces
• •Decide whether a standard filter or custom-made option is the better fit
• •Confirm the site’s ventilation requirements with the relevant engineer or owner
• •Keep the replacement simple enough for routine servicing

If the existing setup is badly worn, the best result may involve more than a new filter. Sometimes the frame, seals or holding rails need attention first so the replacement can work properly.

Talk to Total Air Filtration about the right fit

Total Air Filtration is a family-owned air filter manufacturer and supplier based in Hillcrest, Brisbane QLD, established 2009. We can help supply or make the right filter for older commercial equipment, including non-standard sizes and legacy housings.

If you are dealing with an older AHU in a heritage office building, call 07 3800 3448 or request a quote. You can also explore our products or contact us to discuss the dimensions and access conditions on your site.