What to Do When the Existing Filter Size Is No Longer Available

If an air filter size is no longer available, it does not automatically mean the system needs a major overhaul. In many cases, there are workable paths forward, but the right choice depends on the equipment, the housing, the filter duty and the site conditions.

For facility managers, mechanical contractors and industrial buyers, the key is to treat the problem as a fit-and-function issue first, then look at replacement options that maintain airflow, sealing and access for servicing.

Why old filter sizes disappear

Legacy filter sizes are often tied to older plant, custom-built units or equipment models that are no longer manufactured. Over time, product ranges change, materials change, and suppliers rationalise stock lines to focus on the most commonly used sizes and constructions.

That can leave you with a filter that is technically simple, but awkward to source. Before assuming the system is obsolete, it is worth checking:

• •whether the original dimensions were nominal rather than exact
• •whether the frame type, depth or gasket arrangement is standard enough for a close substitute
• •whether the holding frame has any wear, distortion or damage that is affecting fit
• •whether the filter is a pre-filter, panel filter, bag filter, cartridge filter or another type with more than one possible replacement format

If you are unsure how to confirm the dimensions, how to measure an air filter is a useful starting point.

Start with the dimensions, not just the label

Old labels can be misleading. A size written on the frame or packaging may not match the actual outside dimensions, inside dimensions or depth once the filter has been in service and the frame has seen wear.

When checking a legacy size, measure the opening carefully and compare:

• •length, width and depth
• •frame material and rigidity
• •gasket location and compression
• •direction of airflow, if marked
• •whether there is enough clearance for removal and insertion

This is especially important when the filter sits inside a restrictive casing, a bank of filters, or a tray system with little spare room. A replacement that is close on paper can still be unusable if it cannot seal properly or be fitted safely.

Options when the exact size is no longer made

There are usually three practical routes: a substitute size, an alteration to the holding arrangement, or a custom-made filter.

1. Use a suitable substitute size

If the housing can accommodate a nearby size, a substitute may be the simplest path. The aim is not to force a near match into place, but to choose a filter that fits securely and performs as expected in the system.

A good substitution should consider:

• •physical fit in the frame or slot
• •sealing method and gasket contact
• •filter media type and construction
• •dust-holding needs and service access
• •pressure drop and airflow impact under the system’s normal conditions

In some cases, a slight change in size is workable if the sealing arrangement is appropriate. In others, even a small mismatch can create bypass around the edges, leading to poor performance and a frustrating maintenance cycle.

2. Alter the frame, tray or housing

If the legacy size is very close to a current standard, modifying the holding arrangement may be the neatest long-term solution. This can be a sensible option when the system is being refurbished or when repeated special orders are becoming difficult to manage.

Possible alterations include:

• •adjusting or replacing the filter retainer
• •modifying the tray depth or side rails
• •updating the frame to suit a standard modern size
• •improving access so the replacement can be installed without damaging the media or gasket

Any alteration should be checked by the mechanical services engineer or building owner, particularly where the system is part of a larger ventilation arrangement. If the site has ventilation requirements under AS 1668.2:2024, those requirements should be confirmed before changes are made.

3. Order a custom-made replacement

When the size is genuinely unique, a custom-made filter is often the best answer. This is common for plant with older housings, specialised machinery, dust collection equipment, or imported systems where local stock sizes do not align.

A custom filter can be made to suit:

• •unusual width, height or depth
• •special frame materials
• •pleat geometry or media selection
• •seal type or mounting arrangement
• •higher dust loads or unusual operating conditions

If the system needs a non-standard product, custom air filters may be the most direct path to a proper replacement without compromising fit.

What to check before choosing a replacement

Before selecting a substitute or custom solution, it helps to gather a few details about the system and its operating environment. The more complete the information, the more likely the replacement will fit cleanly and work properly.

Useful information includes:

• •the existing filter dimensions and photos
• •the equipment make and model, if known
• •the filter type and frame style
• •whether the filter is used for supply air, return air, recirculation or process air
• •the dust type, contamination level and operating conditions
• •whether the system uses a pressure gauge or other indication of loading

If the application is in a commercial building, workshop or industrial process, the dust load can change the selection significantly. A filter that is adequate in one site may not be suitable in another, even if the frame size is the same.

Avoid common mistakes with legacy sizes

When trying to replace a discontinued filter, it is easy to focus only on making it fit. That can lead to problems later, especially if the filter is forced into place or chosen without checking the seal and media type.

Common mistakes include:

• •choosing a size that is close, but not close enough for a proper seal
• •ignoring frame depth and clip or rail clearance
• •replacing a filter with a different construction that changes airflow behaviour
• •assuming a different filter rating or media type is interchangeable without checking system needs
• •overlooking damaged housings that make every replacement seem the wrong size

It is also worth remembering that MERV, EN 779 and ISO 16890 do not convert directly. If you are matching a replacement for a specific application, compare the actual product details rather than relying on a simple conversion shortcut. G3/G4/F5-F9 are EN 779 classes and are still widely used in Australia, even though EN 779 has been superseded by ISO 16890.

When washability or higher efficiency matters

Some filters are designed to be washable - check the product instructions before reusing any filter. Others are intended for single use only, and washing them can damage the media or reduce performance.

If the application calls for a higher efficiency filter, make sure the replacement is selected for the actual system requirement rather than the legacy label alone. For HEPA applications, H13 is at least 99.95% efficient and H14 is at least 99.995% efficient at the most penetrating particle size under EN 1822. That level of specification is only relevant where the system is designed for it.

If people on site have health concerns, they should seek medical advice. The filter selection itself should still be based on the system’s technical requirements and the conditions at the site.

Getting the right replacement without wasting time

If you are dealing with an unavailable filter size, the fastest path is usually to identify the exact dimensions, check the housing, and decide whether a substitute, alteration or custom-made option is the most practical. For ongoing sites, it can also be useful to keep a record of the measured dimensions and the final replacement specification so future orders are simpler.

For help with standard products, see our products or arrange filter servicing if the system needs attention beyond the filter itself.

Need a legacy size replaced?

Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, and we can help source, supply or make the right replacement for your site.

Call 07 3800 3448 or request a quote. If you need site-specific support, you can also contact us or explore industries we supply.