Specifying Filters for Older Air Conditioning Systems

Older air conditioning systems often keep running long after the original filter label has faded, gone missing or been replaced by a sticker that no longer tells the full story. That leaves homeowners, contractors and facility managers with the same question: what filter actually fits, seals and performs properly in this unit?

The answer usually comes from careful checking rather than guesswork. Start with the frame size, the filter holding arrangement, the available depth, the direction of airflow and any pressure drop information on the unit. If the original details are unclear, you can still identify the right replacement with the right measurements and a few practical checks.

Start with the equipment, not the old filter alone

If you are dealing with an older system, the first step is to look at the air conditioning equipment itself and not rely only on whatever filter is currently installed. The existing filter may be a temporary substitute, a cut-down panel or a product that was chosen because it was close enough to fit.

Check the air handler, return air grille, filter rack or cassette for any of the following:

• •A model and serial plate on the unit
• •A faded filter size label near the access door or rack
• •Direction arrows on the frame or holder
• •Notes on the cabinet, return grille or maintenance tag
• •Evidence of a slide-in channel, holding clips or a gasketed frame

If the label is missing, the dimensions of the opening and the type of holding system are more useful than guessing from the old filter alone. A filter that is too small can allow bypass air around the edges, while one that is too large may buckle, jam or damage the frame.

If you are unsure how to measure the opening, use our guide on how to measure an air filter.

Measure carefully and record the right dimensions

Older systems often use nominal sizes that do not match the exact measured size. That is why it helps to record the actual opening and the space available for the filter rather than assuming the printed size on a previous filter is correct.

When measuring, note:

• •Width, height and depth of the filter slot or frame
• •The available space for removal and installation
• •Whether the filter sits in a track, frame, grille or side access door
• •Any obstructions such as wiring, hinges, dampers or service panels
• •Whether the filter needs a rigid frame, a panel media style or a bag-style product

It is also worth checking if the previous filter was compressed to fit. A compressed filter may look like the correct size, but compression can affect fit and sealing.

For systems where space is tight, a custom-made replacement may be the best option. Total Air Filtration manufactures and supplies custom air filters for applications where standard sizes are not suitable.

Understand the filter type already in use

Older air conditioning systems may have been set up for a basic panel filter, a pleated panel, a deeper extended-surface filter or a special-purpose arrangement. The wrong filter type can create fit problems or put extra strain on the system.

Look for signs such as:

• •A thin disposable panel in a shallow frame
• •A pleated filter with a cardboard, metal or synthetic frame
• •A washable-style filter with a rigid frame and removable media
• •A pocket filter or bag filter in a deeper commercial housing
• •A purpose-built frame that holds a pre-filter and final filter together

Some filters are designed to be washable - check the product instructions before cleaning or reusing them. Do not assume that every reusable-looking filter can be washed safely, because the frame, media and seals are all product-specific.

If the original product has become unavailable, it is often possible to match the dimensions and filtration style without changing the whole system. That said, the fit must still suit the airflow path and the space available in the cabinet or grille.

Check the filtration level the system was designed for

When a label is missing, it can be tempting to upgrade to a finer filter because it seems like the safest choice. In older systems, that can be a mistake if the fan, coil, ductwork or filter rack was not designed for the added resistance.

If there are notes from previous maintenance, look for references to MERV, EN 779 or ISO 16890. These rating systems do not convert directly, so avoid using a simple conversion table when selecting a replacement. In Australia, you may still see G3, G4 or F5-F9 references from EN 779 on older documentation, even though ISO 16890 is now the current standard.

For commercial systems, the right target should be confirmed with the mechanical services engineer or building owner, especially where ventilation requirements may relate to AS 1668.2:2024. Site conditions, occupancy, outside air levels and maintenance access can all affect the choice.

If you are considering a higher-efficiency product, make sure the system can handle it and that the filter rack can support the correct seal and depth.

Look for pressure drop clues and maintenance records

Pressure drop is one of the most useful clues on an older installation, especially where gauges are fitted or previous service records exist. A filter that loads too quickly, or that was replaced because of poor airflow, may indicate the wrong grade, the wrong size or a sealing issue.

When reviewing the system, look for:

• •Gauge readings at the filter bank
• •Notes from previous servicing visits
• •Visible dust loading patterns across the filter face
• •Signs of air bypass around the edges
• •Discolouration or damage to the frame, gasket or sealant

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. A filter that appears clean at a glance may still be restricted if the system runs in a dusty environment.

If you provide commercial maintenance, our filter servicing and commercial filter servicing pages outline how we support different system types.

When to choose a custom-made filter

A custom replacement is often the most practical option when the original size is unusual, the frame is damaged or the system needs a specific construction to fit correctly. This is common in older buildings where plant rooms have been altered over time, or where original parts are no longer available.

A custom filter may be worth considering if:

• •The opening is a non-standard size
• •The filter rack has been repaired or modified
• •The original frame is missing or distorted
• •The system needs a different depth or frame material
• •The existing filter is allowing visible bypass around the edges

Custom manufacturing can help match the physical dimensions, frame type and media arrangement needed for the installation. It can also make ongoing servicing easier because replacements are made to the same pattern.

If you are managing multiple sites, keeping a record of the measured sizes, frame type and airflow direction for each unit can save time on future replacements. That information also helps when ordering from our products range or arranging a made-to-order solution.

Getting the specification right the first time

For older air conditioning systems, the best approach is to identify the filter by measurement, fit and system requirements rather than by assumption. A few careful checks can prevent repeated call-backs, poor sealing and unnecessary trial-and-error ordering.

If you are unsure what to specify, Total Air Filtration can help assess the opening and supply or make the right filters for the system. We are a family-owned air filter manufacturer and supplier based in Hillcrest, Brisbane QLD, and we work with homeowners, contractors and facility teams across a range of applications.

Call 07 3800 3448 or request a quote. You can also contact us if you need help identifying an older filter arrangement or confirming the next replacement.