How to Match a Replacement Filter to an Obsolete AHU with Missing Documentation

If you are dealing with an older air handling unit and the paperwork has gone missing, you are not alone. The good news is that a replacement filter can usually still be identified with careful inspection, a few key measurements, and a clear understanding of how the existing filter sits in the unit.

The aim is not to guess the part number. It is to match the filter to the AHU’s physical dimensions, sealing method, airflow requirements and filtration task so the new filter fits properly and performs as intended.

Start with the unit, not the label

When documentation is unavailable, begin by looking at the filter arrangement inside the AHU itself.

Open the access panel and inspect how the current filter is held in place. Note whether it slides into a track, sits in a holding frame, clamps into a gasketed door, or uses a panel-type retaining system. Older units often have been modified over time, so the visible arrangement is more reliable than assumptions based on the cabinet make or model.

Check for any identification on the filter frame, holding rack, access door, or nearby service records. Even partial markings can help, but do not rely on them alone. If the original manufacturer name is still present, it may indicate the style of filter used, yet the final match still needs to be confirmed by measurement.

You should also look for signs that the current filter has been replaced with a non-original size or style. Common clues include:

• •gaps around the frame
• •compressed or damaged seals
• •bent holding clips or rails
• •dust bypassing around the filter edge
• •filters that sit loosely or need force to fit

If bypass dust is visible, the issue may be the fit rather than the media itself.

Measure the opening carefully

Accurate measurements are the foundation of a correct replacement. If you need a refresher on measuring technique, see how to measure an air filter.

Measure the filter or the clear opening where the filter sits, depending on which is more reliable. Record the nominal size and the actual size, and note whether the filter is designed to fit inside a recess or against a face seal. A small mismatch can make installation difficult or reduce sealing effectiveness.

Measure all three dimensions where relevant:

• •length
• •width
• •depth

For panel filters, also check the thickness of the media pack and the depth of the frame. For bag or pocket filters, measure the front face and the available depth behind the frame so the pockets have room to open properly.

Do not assume that the nominal size on an old carton or scribbled service note is correct. Over time, AHUs can be altered, replacement rails can be added, and local fabrication can change the true dimension. If possible, take photographs of the installed filter from several angles before removing it.

Identify the filter type and construction

Once you know the size, work out what sort of filter you are trying to replace. This matters because two filters with similar dimensions can behave very differently.

Look at the frame material and the media shape. Is it a flat panel, pleated panel, bag filter, rigid cell, or a special frame made to suit a proprietary holder? Check whether the media is cardboard, galvanised steel, plastic, aluminium, or another material. The frame type often points to the right replacement format.

Also note the following:

• •whether the filter has a front or rear gasket
• •whether there is a knife-edge or flat-frame seal
• •whether the media is disposable or intended for specific cleaning instructions
• •whether there is a header frame, side guide, or terminal housing connection

Some filters are designed to be washable - check the product instructions. Do not assume washability from appearance alone.

If the original filter is damaged, compare it with adjacent, similar filters in the building, if there are any. In some plant rooms, the same AHU family may have been installed in different sizes but with identical filter holding systems.

Match the performance need as well as the fit

A replacement filter should fit the unit, but it also needs to suit the application. The right level of filtration depends on the system design, outdoor air load, occupant requirements and what the mechanical services engineer or building owner specifies.

If you are working to a building requirement, check whether the site is aligned with AS 1668.2:2024, and confirm the intended filter arrangement with the mechanical services engineer or building owner. Do not assume that a similar-looking filter from another project will be suitable.

In practice, you may be asked to match one of several common rating systems. Remember that MERV, EN 779 and ISO 16890 ratings do not convert directly, so avoid trying to “translate” one system into another without proper technical confirmation. EN 779 classes such as G3, G4 and F5-F9 are still widely used in Australia, even though ISO 16890 is now the current standard in many contexts.

If the application requires HEPA filtration, be precise about the grade. Under EN 1822, H13 is at least 99.95% efficient and H14 is at least 99.995% efficient at the most penetrating particle size, MPPS. If the existing filter is part of a HEPA or high-efficiency stage, confirm the filter frame, sealing arrangement and housing compatibility before ordering a replacement.

Decide whether a custom filter is the best option

Older AHUs often use dimensions or holding frames that no longer match standard catalogue sizes. In those cases, a custom filter may be the most practical solution.

A custom filter can be specified to match:

• •non-standard dimensions
• •unusual frame depths
• •special flange or lip details
• •gasketed or knife-edge sealing surfaces
• •bag length or pleat depth constraints
• •specific airflow and dust-loading needs

If the housing is worn or distorted, custom manufacture may also help compensate for minor irregularities, provided the final dimensions are based on actual measurements and the unit is checked before sign-off.

When requesting a custom filter, provide as much detail as you can:

• •photos of the installed filter and housing
• •measured dimensions of the opening and old filter
• •the style of frame and seal
• •any markings on the old filter
• •the AHU make and model, if known
• •the service environment, such as office, industrial, kitchen, or plantroom conditions

If the filter is part of a broader maintenance plan, it may help to discuss filter servicing or commercial filter servicing at the same time, so the replacement choice suits ongoing access and maintenance needs.

Common mistakes to avoid

The most common error is ordering by description alone. “Looks like the old one” is not enough when a unit is old, modified, or missing paperwork.

Avoid these pitfalls:

• •measuring only the face size and ignoring depth
• •forgetting to confirm the sealing method
• •choosing a filter that fits loosely and allows bypass
• •assuming the same rating system means the same performance
• •replacing a filter with a different style that changes resistance or fit

It is also worth checking the condition of the filter frame, rails and access door. If the unit itself is damaged, a correct replacement filter may still not seal properly until the housing is repaired or adjusted.

If you are unsure whether the issue is the filter or the AHU, compare the unit against our products to see the range of formats that may suit the application, or speak with a supplier who can interpret the measurements and housing details.

When to ask for help

If the filter arrangement is unusual, the dimensions are non-standard, or the unit has been altered over time, it is often quicker to have the filter identified by someone who works with custom manufacture regularly. That is especially true when the site needs a specific seal, frame profile or airflow-friendly construction.

Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established 2009. We can help identify the right replacement from measurements, photos and physical samples, and then supply or make the filter to suit the application.

Call 07 3800 3448 or request a quote. You can also contact us if you want to talk through the AHU, the existing filter, or the best way to document the opening before ordering.