What to Check Before Ordering Replacement Filters for an AHU

If you are reordering filters for an air handling unit, the part number alone is not always enough. A filter that looks close can still fail to seal properly, fit the track incorrectly, or perform differently from the original unit.

Before you place the order, it is worth checking the key details of the existing filter and the AHU itself. A short procurement review can save time, reduce installation issues, and help ensure the replacement matches the system requirements.

Start with the exact dimensions

The first check is the filter size, and this needs to be more than a rough estimate. Measure the filter, the holding frame, and the clear opening if the old filter is missing or damaged.

What to confirm

• •Nominal size and actual size
• •Length, width and depth in millimetres
• •Whether the filter is a standard size or a custom size
• •Any tolerance or fit note on the old label, carton, or frame
• •Orientation of the airflow through the filter

If the filter sits in a tight track or slide, a small difference in depth or edge profile can stop it from seating correctly. If you are unsure how to measure, use how to measure an air filter as a reference, or send us photos and dimensions through contact us.

It also helps to check whether the frame or housing has been modified over time. Replacement parts sometimes need to match the current opening, not just the original drawing.

Check the frame type and construction

The frame style affects how the filter installs, seals and holds its shape in service. Before reordering, note the frame material and the way the filter is built.

Common points to verify

• •Cardboard, metal, plastic or rigid box frame
• •Panel, pleated, bag, mini-pleat or rigid format
• •Whether the filter uses a header, separator, wire support or other internal structure
• •Presence of reinforcing mesh on one or both sides
• •Any directional markings, arrows or installation notes

For some AHUs, the frame must match a specific retainer or holding rail. Even if the media and rating are correct, the wrong frame can lead to bypass around the edge or a filter that is awkward to install.

If the unit uses a custom format, custom air filters may be the better option than forcing a near match.

Confirm the media and filtration grade

The media is the working part of the filter, so it should suit the system’s airflow, dust load and required filtration level. Look at the original specification, the existing label, and any notes from the mechanical services engineer or building owner.

Things to record

• •Whether the media is fibreglass, synthetic or another material
• •Whether it is a coarse pre-filter, medium filter or fine filter
• •The rating system used on the existing product, such as MERV, EN 779 or ISO 16890
• •Any special requirement for low pressure drop, higher dust holding, or finer particle capture
• •Whether the filter is washable, if the product instructions specifically say so

Do not assume that ratings convert directly from one standard to another. MERV, EN 779 and ISO 16890 are not interchangeable, and they should be reviewed as specified for the equipment rather than guessed from a rough equivalent.

If the application requires HEPA filtration, check the exact grade. Under EN 1822, H13 is at least 99.95% efficiency at the most penetrating particle size, and H14 is at least 99.995% at MPPS. For broader system design, site requirements should be confirmed with the mechanical services engineer or building owner, and with reference to AS 1668.2:2024 where relevant to ventilation arrangements.

Review the gasket, seal and mounting details

A good filter can still underperform if the seal is wrong. The gasket style needs to suit the frame, the housing and the method of clamping or compression used in the AHU.

Check for

• •Foam gasket, rubber gasket, gel seal or other sealing arrangement
• •Location of the gasket on the frame
• •Whether the gasket is continuous or joined
• •Condition of the old seal if you are matching an existing unit
• •Signs of flattening, cracking or gaps that might suggest the mounting hardware also needs attention

The mounting face matters as much as the gasket itself. Inspect for uneven rails, bent retainers, damaged clips or a dirty sealing surface. If the filter is installed in a service-accessible section, note whether the door clearance makes it difficult to fit a deeper or stiffer replacement.

Where the system uses a specialised seal arrangement, check the product instructions closely before ordering. A simple visual match is not enough if the sealing method is different.

Match the AHU and site requirements

Replacement filters should be chosen in the context of the whole system, not in isolation. A filter that fits physically may still be wrong for the airflow, access, loading pattern or operating environment.

Ask these questions before ordering

• •What type of AHU is it, and what is the filter stage in the system?
• •Is the filter serving comfort cooling, process air or a higher-cleanliness area?
• •What is the available face velocity and pressure drop range for the unit?
• •Is there enough room for the same depth and dust holding capacity as the original?
• •Are there any access constraints, such as narrow plant rooms or side-entry frames?
• •Has the system had recurring sealing, dust loading or handling issues?

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 so you know when service is due.

If you are managing multiple units, it can help to record each filter position, size and rating separately rather than treating all AHUs as the same. That is especially useful when maintenance is shared across several buildings or tenancies.

Put the order together with the right information

Before sending a purchase order, make sure the supplier has enough detail to supply the correct replacement without back-and-forth delays.

Include

• •Filter size and quantity
• •Frame type and depth
• •Media type and filtration rating as written on the existing product or specification
• •Gasket style and any seal notes
• •Airflow direction and installation orientation
• •AHU model, if known
• •Any photos of the old filter, housing and label

If you are unsure whether the current product is still the best match, it can help to compare options with our products or speak with a supplier that also offers filter servicing and commercial filter servicing. That can be useful when the filter arrangement, housing condition or access method has changed over time.

Need help matching a replacement filter?

Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, and we can help you check the details before you order. Whether you need a standard replacement or a made-to-measure solution, we can supply or make the right filters for your AHU.

Call 07 3800 3448 or request a quote.