What to Check Before Reordering Filters for an Older Air Handling Unit

Older air handling units can be straightforward to service until it is time to reorder the filters. That is often when missing drawings, faded labels and non-standard sizes become a problem. A quick visual check is rarely enough, so it helps to confirm the filter details carefully before placing an order.

Start with the filter frame, not the old label

On older AHUs, the label on the existing filter is not always reliable. Labels may be worn, handwritten, stuck over, or referring to a previous modification rather than the unit as it exists now.

Before you order, check the actual filter frame and the holding arrangement in the unit.

Look for:

• •The overall outside dimensions of the filter
• •The depth of the filter media pack or frame
• •Whether the filter sits in a side-access rack, front-load holding frame or bag holder
• •Signs that the housing has been altered, repaired or packed out over time
• •Any locking clips, rails, springs or access restrictions that affect the fit

If the unit has had work done over the years, the label may not match the current opening. In that case, the measurement of the opening and the old filter together is more useful than the printed description alone.

Measure the opening carefully

For legacy plant, the most common mistake is measuring only one dimension or measuring the wrong part of the filter. A filter can appear to be a standard size while the actual frame dimensions differ by a few millimetres that matter.

If possible, remove one filter and measure it directly. If that is not practical, measure the opening and compare it to the old filter after removal.

Useful checks include:

• •Overall width, height and depth
• •Internal opening size versus outside flange size
• •Rebate depth or seating depth in the holding frame
• •Direction of airflow and any arrow markings on the frame
• •Whether the filter is designed to be inserted from the front, side or back

If you need a refresher on the practical side of measuring, our guide on how to measure an air filter is a useful starting point.

Allow for frame wear and site modifications

Older AHUs often have worn gasket faces, distorted frames or bent rails. That means the measurement on paper may not be enough on its own.

Check for:

• •Bent or damaged retaining bars
• •Worn gaskets that have compressed over time
• •Corrosion around the filter bank
• •Gaps caused by previous packing materials or improvised spacers
• •Evidence that a non-original filter has been made to fit

If the housing has moved or been repaired, the correct replacement may not be the same as the label suggests. In some cases, a custom size is the best solution. You can view our custom air filters service if the opening is non-standard.

Confirm the filter type and performance requirement

Once the size is clear, confirm the filter type. Legacy AHUs may have been fitted with panel filters, pleated filters, pocket filters or pre-filters, and the application may have changed since the system was installed.

Ask whether the replacement needs to match:

• •The original filter type
• •The current operating condition of the AHU
• •The required dust capture level for the site
• •The upstream and downstream stages already in place
• •The maintenance access available in the plant room

If you are dealing with a commercial or industrial site, it is worth checking the ventilation requirements with the mechanical services engineer or building owner, especially where AS 1668.2:2024 may be relevant to the overall system design.

If the filter class marking is still visible, note it carefully, but do not assume it converts directly to another rating system. MERV, EN 779 and ISO 16890 do not translate one-to-one, and G3/G4/F5-F9 are EN 779 classes that are still commonly referenced in Australia even though EN 779 has been superseded by ISO 16890.

Check the label details that matter most

Older labels can be confusing, especially where the same unit has had multiple maintenance teams over time. Focus on the details that affect fit and performance rather than just the product name.

Useful label information includes:

• •Manufacturer name or series name
• •Nominal and actual dimensions
• •Media type and frame material
• •Direction of airflow
• •Seal type or gasket details
• •Resistance rating or pressure drop information, if shown
• •Any special instructions for handling or installation

If the label is missing or unreadable, photographs can help. A clear set of photos of the filter face, edge, seal, latch points and the installed position can make it much easier to match the replacement correctly.

Don’t overlook the surrounding hardware

A filter replacement is only as good as the hardware that holds it in place. In older AHUs, the filter bank itself may be the source of the problem rather than the filter media.

Check the following before reordering:

• •Whether the filter rack is square and intact
• •If there are broken clips, missing seals or damaged guides
• •Whether the access door closes properly after the filter is installed
• •If the filter can be inserted without forcing it
• •Whether there is visible bypass around the edges

Bypass can occur when the filter is the wrong size, the frame is distorted, or the sealing surfaces are worn. A replacement that matches the original opening more accurately may solve an ongoing issue with poor fit.

Consider how the filter is maintained on site

The right filter for an older AHU is not only about size. It also needs to suit the way the system is maintained.

Some filters are designed to be washable - check the product instructions. Others are intended to be replaced when they reach their limit. For most commercial systems, inspection frequency depends on the system, the site conditions and the dust load, and you should follow the filter or equipment manufacturer’s instructions. Where gauges are fitted, monitor pressure drop so the filter can be changed at the right time for that installation.

If the unit is difficult to access, or the plant room is tight, a product that is easier to handle may be a better fit for the maintenance team. If the issue is ongoing, filter servicing or commercial filter servicing may also help keep the system organised and reduce repeat ordering errors.

When to ask for help

If the original drawings are missing, the housing has been modified or the label is unclear, it is worth getting support before you place the order. A small measurement error can lead to delays, poor sealing or a filter that simply will not fit.

You should seek help when:

• •The opening is non-standard or irregular
• •The old filter has been cut down or altered
• •There is evidence of multiple filter stages in one bank
• •The airflow direction is unclear
• •The label and the actual dimensions do not match

In those cases, photos, measurements and a clear description of the AHU layout are usually enough to identify a suitable replacement or determine whether a custom solution is needed.

Talk to Total Air Filtration

Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, and we help mechanical contractors and facility managers match replacements to older AHUs when the original documentation is missing.

If you need help identifying the right size or type, call 07 3800 3448 or request a quote. You can also explore our products or contact us through contact us for assistance with a legacy filter setup.