How to Specify Replacement Filters for an Older AHU Without a Current Data Sheet

When an older air handling unit has no current data sheet, replacement filters still can be specified accurately with a bit of methodical checking. The goal is to match the filter to the housing, the holding frame and the system requirements, rather than guessing from a worn-out sample alone.

For mechanical contractors and property managers, the safest approach is to verify the physical fit first, then the filter construction, and finally the performance standard the system is meant to meet.

Start with the unit, not the old filter

Before you remove anything, inspect the AHU and the filter section carefully. Old filters can be distorted, trimmed, bowed or compressed, which makes them a poor reference if you rely on them without checking the housing.

Look for the following:

• •The filter bank layout and number of filters fitted side by side or in depth
• •Whether the unit uses flat panel filters, pleated filters, bag filters or a custom-built arrangement
• •The direction of airflow marked on the unit or frame
• •Any retaining clips, knife edges, tracks, clamping bars or door clearances
• •Signs of bypass, such as dust tracks around the frame, damaged seals or bent retainers

If the unit has a service label, take photos before anything is removed. Even incomplete information can help later when you are matching a replacement product.

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

Measure the filter opening carefully

The most common mistake is ordering by the nominal size printed on an old label without checking the actual opening. On older plant, the nominal size and the real dimensions may not match exactly.

Measure the housing opening and the existing filter frame in millimetres. Check width, height and depth, and note whether the filter sits on the inside of a track, against a gasket face, or inside a rigid holder.

Pay close attention to:

• •The true opening size versus the visible face size
• •The available insertion depth
• •Clearance for loading and unloading the filter
• •Whether the filter needs to compress into a gasket or seal against a flat face
• •Any obstructions that would stop a thicker filter from seating properly

If there is any doubt, take photos with a tape measure in place. This helps when discussing the unit with a supplier or when sourcing custom air filters for a non-standard opening.

When a housing is worn or has been modified over time, it is often better to match the measured opening than the old filter label.

Check the frame style and seal method

Two filters with the same dimensions may still be unsuitable if the construction differs. An older AHU may rely on the filter frame itself, the gasket, or the holding hardware to create the seal.

Confirm whether the existing filter uses:

• •Cardboard, metal or plastic framing
• •Foam gasket or knife-edge sealing
• •Single-layer media or pleated media
• •Rigid panel construction or a soft-sided bag design
• •Top-loading, side-loading or front-loading access

If the frame style does not suit the existing hardware, the filter may fit physically but still allow bypass or create maintenance issues.

Identify the performance standard the system is expecting

Once the size and construction are understood, work out what performance level is needed. On many older sites, the original paperwork may refer to a filter grade using an older naming system, or it may not specify a standard at all.

Do not assume that MERV, EN 779 and ISO 16890 ratings are directly interchangeable. They are different systems and do not convert cleanly one-to-one. If the original documentation refers to G3, G4 or F5 to F9, those are EN 779 classes still widely used in Australia, even though ISO 16890 is now more current.

A practical way to proceed is to ask:

• •What is the filter protecting: occupied spaces, plant equipment or a downstream finer filter stage?
• •Is the site designed around a coarse pre-filter, a medium filter, or a finer final stage?
• •Are there site instructions from the mechanical services engineer or building owner that need to be followed?
• •Does the system documentation refer to AS 1668.2:2024 or another building requirement?

If the standard is unclear, confirm the intended filter grade with the mechanical services engineer or building owner before ordering. That is especially important when the filter bank has been altered, or when the AHU has been upgraded without updated paperwork.

When HEPA is involved

If the AHU uses HEPA filtration, be precise about the class. Under EN 1822, H13 is at least 99.95% efficiency and H14 is at least 99.995% efficiency at the most penetrating particle size, or MPPS.

Do not substitute a general high-efficiency filter unless the housing, seals and test requirements are suitable. HEPA installations are more sensitive to leakage, frame quality and installation method than standard pre-filters or medium filters.

Match the filter to the service conditions

The right replacement is not only about dimensions and rating. Site conditions can influence the construction that will work best in service.

Consider the following:

• •Dust load and whether the site experiences heavy or light contamination
• •Whether the unit is indoors, outdoors or exposed to moisture
• •The available resistance to airflow in the system
• •Access for routine inspection and safe replacement
• •Whether the filter will be removed by front access or through a narrow service opening

Some filters are designed to be washable - check the product instructions before assuming a filter can be cleaned and reused. Others are strictly disposable and should be replaced when they reach the condition set by the manufacturer instructions or site maintenance plan.

Inspection frequency depends on the system, site conditions and dust load. Where pressure gauges are fitted, monitor pressure drop and replace or service filters in line with the filter and equipment manufacturer instructions.

If you already have an ongoing maintenance arrangement, filter servicing and commercial filter servicing can help keep records consistent across multiple sites.

What to do when the original paperwork is missing

When the data sheet is incomplete, use a simple evidence-based process rather than trying to reverse-engineer the filter from memory.

Build your specification from:

• •Measured opening dimensions
• •Filter depth and available clearance
• •Construction type and sealing method
• •Any visible labels, part numbers or arrows on the old filter
• •The operating context of the AHU and the intended filtration stage
• •Any direction given by the mechanical services engineer or building owner

If you can, keep a record of the final specification in the site log. Include the measured size, filter type, observed orientation and any notes about seal condition or access constraints. That makes the next replacement simpler and reduces the chance of ordering the wrong product again.

Where the original filter has been superseded, damaged or modified, a custom-made solution may be the most practical option. That can be especially useful for older plant with unusual frames, limited access or non-standard depths.

Getting the replacement right the first time

For older AHUs, the best outcome usually comes from matching the actual hardware rather than relying on assumptions. Measure carefully, confirm the construction, identify the intended standard and check the site’s requirements before you place an order.

If you need help working through an old unit, Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established 2009. We can help supply the right filters or make them to suit the application.

Call 07 3800 3448 or request a quote. You can also browse our products or contact us if you want to talk through a specific AHU before ordering.