What to Consider When Upgrading Filters in Older School HVAC Systems

Older school HVAC systems often need filter upgrades, but the work is rarely as simple as swapping one product for another. In legacy plant, the main challenges are usually mechanical: housing dimensions, access constraints, limited space for higher-efficiency filters and incomplete records of what has been fitted before.

For mechanical contractors and school maintenance teams, a careful site check before ordering can save time, reduce callbacks and help avoid fit-up issues on the day of installation.

Start with the housing, not the filter label

The first question is whether the new filter will physically fit the existing frame, track or holding system. Older equipment may have been altered over time, and the nominal size on a label does not always match the actual clear opening available in the housing.

Before choosing a replacement, check:

• •the exact filter dimensions, including depth
• •the condition of the frame, gasket and retaining hardware
• •whether the filter seats from the front, rear or side
• •whether there is enough clearance to insert and remove the filter without forcing it
• •whether the housing has been bent, patched or adapted over time

If you are unsure about measurements, how to measure an air filter is a useful place to start. For non-standard openings, custom air filters may be the better option than trying to make a standard size work.

Check access before you change the specification

A filter that is theoretically suitable may still be impractical if the plantroom or ceiling void is hard to reach. Older schools often have cramped mechanical spaces, hinged access doors with limited swing, or filters positioned above ceilings, behind ductwork or near other services.

Access matters because it affects whether maintenance can be done safely and cleanly. If a filter is difficult to remove, staff may be tempted to leave it in place longer than intended, or a rushed changeover may damage seals and frames.

When reviewing an older system, consider:

• •whether ladders, platforms or shutdown access are needed
• •whether the access panel is large enough to remove the filter without flexing it
• •whether there is room to inspect the upstream side for dust build-up or debris
• •whether any services, cabling or pipework block the removal path
• •whether the filter can be fitted without bypass gaps at the frame edges

If the access arrangement makes routine servicing awkward, it may be worth discussing modifications with the mechanical services engineer or building owner before increasing filter resistance or changing media type.

Understand what was fitted before

In older school HVAC systems, the filter history is often unclear. Drawings may be out of date, maintenance records may be incomplete, and the installed product may not match the original design intent. It is common to find mixed filter types across different air handling units, or a past change that was never properly documented.

If the previous filter specification is unknown, start with what can be confirmed on site:

• •existing filter dimensions and frame style
• •visible dust pattern, which can show bypass or leakage paths
• •condition of the upstream coil and housing
• •any labels, service tags or part numbers still attached
• •pressure drop readings, if gauges or test points are fitted

Where the previous specification cannot be verified, avoid assuming a higher grade is automatically better. A tighter filter may increase resistance and may not suit an older fan, frame or control arrangement. The safer approach is to confirm the site requirements with the mechanical services engineer or building owner and align the selection with the actual system capability.

Match the filter to the system, not just the room use

Schools have varied HVAC arrangements, from small split systems to larger central air handling plant. The right filter depends on the equipment, the expected dust load and how the system operates, rather than a one-size-fits-all assumption.

For example, a heavily trafficked corridor, dusty workshop area or plantroom intake may place different demands on the filter compared with a general classroom supply system. External conditions also matter, especially where intakes are exposed to leaf litter, road dust or coastal air.

Useful questions to ask before upgrading include:

• •Is the system running continuously or intermittently?
• •Is there visible dust loading on the current filter face?
• •Are pre-filters already in place?
• •Does the fan have enough allowance for additional resistance?
• •Are there existing pressure gauges or differential pressure switches?

Inspection frequency should be set to the system, site conditions and dust load. Follow the filter and equipment manufacturer instructions, and monitor pressure drop where gauges are fitted.

Be careful when moving to higher-efficiency filters

If a school is moving from a coarse filter to a finer one, the upgrade needs to be checked against the capacity of the housing and fan. Higher-efficiency filters can be useful in the right application, but they also need enough space, correct sealing and proper support.

Older equipment may not have been designed for deeper filter banks or multi-stage filtration. In those cases, the main practical issues are often:

• •whether the frame can support the new filter depth
• •whether the gasket seals properly under load
• •whether the unit can maintain airflow with the added resistance
• •whether the service team can still access the filter safely

If the project involves ventilation requirements, AS 1668.2:2024 may be relevant, but site requirements should always be confirmed with the mechanical services engineer or building owner. If you are comparing performance classes, remember that MERV, EN 779 and ISO 16890 ratings do not convert directly.

Don’t overlook maintenance realities

A filter upgrade only works if it can be maintained properly. In older schools, the most suitable product on paper may become a poor choice if it cannot be inspected, changed or reseated without difficulty.

Practical maintenance checks include:

• •whether staff can safely access the filter without special equipment
• •whether the filter can be removed without damaging the frame
• •whether dust is collecting around the edges, suggesting leakage
• •whether a visible pressure rise is prompting timely changeover
• •whether the housing needs cleaning before the new filter goes in

Some filters are designed to be washable - check the product instructions before cleaning or reusing them. If a washable product is being considered, make sure the school has a clear process for inspection, cleaning method and reinstallation, rather than relying on assumptions from the previous filter.

When replacement isn’t a straightforward swap

If the existing filter opening is non-standard, the frame is distorted, or the system has been modified over time, a direct replacement may not be the best answer. In those cases, it may be better to look at a custom solution that fits the actual housing and the access available.

This is especially important where the school wants to improve dust capture but does not have room for a larger or deeper filter. An accurate measure-up, a review of the access path and a check of the fan and housing limits can prevent repeated site visits and short-term fixes.

For help matching a practical filter to an older system, see our products, filter servicing and commercial filter servicing.

Getting the next step right

Upgrading filters in older school HVAC systems is usually a fit, access and maintenance exercise first, and a product selection exercise second. If you confirm the housing dimensions, check service access and understand the existing filter history, you are far more likely to land on a solution that works in the real plantroom.

Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, and we can help supply or make the right filters for legacy systems. Call 07 3800 3448 or request a quote. You can also contact us if you need help with a difficult fit-up or a non-standard replacement.