Choosing Air Filters for Older School and Community Building HVAC Systems

Older school and community buildings often have HVAC systems that were designed around different operating assumptions, different filter classes and, in some cases, a very different maintenance routine. When it is time to upgrade the filters, the challenge is rarely just finding the right nominal size. It is usually about fitting a better filter into an older frame, keeping airflow within the system’s limits and making sure the filter can actually be serviced without turning a routine task into a shutdown.

For contractors, facility managers and maintenance teams, a good retrofit starts with understanding what the system can handle now, not what the original drawings might suggest. The right choice balances capture performance, pressure drop, physical access and the practical realities of an ageing plant room or rooftop unit.

Start with the existing system, not the filter label

Before changing filter type, identify what is already installed and how the housing is built. Older systems may have non-standard filter racks, shallow access doors, warped frames or mixed components from previous repairs.

Check the following on site:

• •The exact filter dimensions, including depth, not just the nominal size
• •Whether the frame is rigid, clipped, sliding or made for panel filters
• •The available space before doors, guards, dampers or coils
• •The direction of airflow and whether the filter seats correctly in that direction
• •Signs of bypass, such as dust tracks around the frame or damaged seals

If a filter sits loosely or has to be forced into place, it is not a proper retrofit, even if the media grade looks suitable. In older systems, a filter that does not seal well can undermine the whole upgrade.

If you need help checking dimensions before ordering, use how to measure an air filter.

Match the filter to the airflow the system can support

One of the biggest retrofit risks in older buildings is choosing a filter that is too restrictive for the fan and ductwork. Better capture performance often comes with higher resistance, so the system must be assessed as a whole.

A filter change can affect:

• •Fan loading and available airflow
• •Terminal delivery to occupied rooms
• •Noise levels from working fans harder than intended
• •Pressure drop across the filter bank
• •The rate at which the filter loads with dust

If the system already struggles to move enough air, a higher-resistance filter may create operational problems even though it looks like an upgrade on paper. In these cases, a staged approach can work better, such as using a more suitable pre-filter arrangement before moving to a finer final filter where the system allows it.

When considering ratings, remember that MERV, EN 779 and ISO 16890 do not convert directly. G3/G4 and F5-F9 are EN 779 classes that are still widely referenced in Australia, even though ISO 16890 is now the current standard. Select based on the equipment, intended use and what the mechanical services engineer or building owner confirms is appropriate for the site.

Build the retrofit around access and maintenance reality

Older school and community buildings often have difficult access conditions. Filters may be mounted above ceilings, behind locked plant rooms, on rooftops or in narrow service corridors. A technically suitable filter is only useful if the maintenance team can remove and replace it safely.

Think about access before finalising the specification:

• •Is there enough clearance to open the access door fully?
• •Can the filter be inserted without bending or damaging the media?
• •Is there space for hands, tools and safe working posture?
• •Can pressure drop be checked where gauges are fitted?
• •Will the replacement require shutdowns that are difficult to schedule around building use?

If access is poor, a custom frame or modified configuration may be the most practical solution. In some older housings, a purpose-made filter can reduce leakage and make servicing simpler than trying to adapt a generic product to an awkward space. See our custom air filters if the existing setup does not suit standard sizes.

Consider the building use and dust conditions

Schools, halls, libraries, sports centres and community rooms do not all load filters in the same way. The type of use, surrounding environment and activity level all affect how quickly filters collect dust and debris.

Common factors to consider include:

• •Construction work nearby
• •High foot traffic during the day
• •Carpets, cleaning products and stored materials
• •Outdoor dust, pollen, leaf litter or smoke exposure
• •Kitchen, workshop or art room exhausts nearby

A filter that performs well in one area of the building may not be the best choice for another. It can be useful to review whether the system has separate zones, mixed occupancies or seasonal use patterns that justify different filter selections in different air handling units.

Inspection frequency should depend on the system, site conditions and dust load. Follow the filter and equipment manufacturer instructions, and monitor pressure drop where gauges are fitted. That approach is far more reliable than relying on a fixed calendar alone.

When washable or high-efficiency options make sense

Some filters are designed to be washable - check the product instructions. That can be useful in certain older systems where access is limited or where a reusable product is genuinely suited to the housing and operating conditions. However, washable does not automatically mean the best option for every site, and the cleaning process itself must be practical and consistent.

If the building needs a finer grade of filtration, a high-efficiency filter may be appropriate, but only if the system can support it. For specialised applications where HEPA filtration is being considered, remember that H13 is at least 99.95% efficient and H14 is at least 99.995% efficient at the most penetrating particle size under EN 1822. Those products are not a casual retrofit choice; they need careful system assessment, sealing and maintenance planning.

For most school and community HVAC retrofits, the question is not how far filtration can be pushed, but what the plant can reliably support over time.

Plan the changeover around standards and site responsibility

Where ventilation requirements are being reviewed, AS 1668.2:2024 is a relevant reference point, but site requirements should still be confirmed with the mechanical services engineer or building owner. Older buildings often have a history of modifications that are not obvious from the original drawings, so assumptions can be risky.

Before changing filter specification, document:

• •Current filter sizes and frame type
• •Any signs of bypass or damaged seals
• •Existing pressure readings, where available
• •Access constraints and safety requirements
• •Any related duct, fan or coil issues that may affect performance

This record helps avoid repeating problems later and makes it easier to identify whether the issue is the filter, the housing or the system as a whole.

Get the retrofit right the first time

A successful upgrade in an older school or community building is usually about fit, airflow and serviceability working together. The best filter is the one that suits the system, can be installed properly and can be maintained without unnecessary disruption.

If you need help selecting or making the right filters for an ageing HVAC system, Total Air Filtration can supply and make filters for a wide range of retrofit situations. We are a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, and you can call 07 3800 3448 to discuss the job or request a quote.

You can also view our products or explore filter servicing and commercial filter servicing for support with ongoing maintenance.