Keeping Track of Non-Standard Filter Sizes on Older Rooftop Units

Older rooftop units often keep running long after the original filter sizes have disappeared from standard catalogues. When that happens, the job is not just finding a replacement once — it is making sure the exact dimensions, frame style and fitting notes are recorded so the same filter can be reordered without guesswork.

For mechanical contractors and industrial buyers, a simple, consistent record for each unit can save time on future maintenance visits and help avoid repeated errors when the original drawings are missing or the old filters have been cut down, modified or superseded.

Why older rooftop units create filter ordering problems

Older rooftop units are often a mixed story of replacement parts, site modifications and incomplete documentation. Over time, original equipment schedules can go missing, a different filter frame may be installed during a service call, or the housing may have been altered to suit a locally available size.

Common issues include:

• •Non-standard face dimensions that do not match common catalogue sizes
• •Shallow filter depths where clearance is tight around the access door or holding frame
• •Split-bank arrangements where multiple smaller filters sit inside one compartment
• •Custom-made retainers or slide tracks that affect the overall fit
• •Units where the actual opening size differs from the nominal filter size on the label

The result is usually the same: a site needs the filter again, but no-one is certain which size was last supplied. That is where good records matter.

Record the right dimensions, not just the label size

One of the most useful habits is to record both the nominal size and the measured size. The printed size on the frame may help when reordering, but the measured size is often what confirms whether the filter will actually fit the housing.

When you inspect an older rooftop unit, note:

• •Length, width and depth in millimetres
• •Whether the size refers to the filter media frame or the complete outer frame
• •The direction of airflow through the unit
• •Frame material such as cardboard, galvanised steel, aluminium or plastic
• •Whether the filter sits in a fixed rack, slide rail or clamp system
• •Any visible bypass gaps, compression marks or signs of distortion

If a filter has been removed and the original label is still readable, photograph it before disposal. A clear image of the label, frame profile and orientation can be invaluable later.

If you are not sure how to measure the opening or the filter itself, our guide on how to measure an air filter is a practical place to start.

Create one record format for the whole site

A consistent record is easier to use than a pile of photos and handwritten notes. Whether you keep it in a service report, asset register or maintenance file, use the same fields every time.

Useful details to capture include:

• •Unit tag or asset number
• •Roof level or plant location
• •Filter bank position if there are multiple banks
• •Measured size and nominal size
• •Filter type, such as pleated panel, bag or rigid cell
• •Media grade or rating, if known
• •Date the sample was taken
• •Any special notes about access or fitment

That simple structure makes it easier for a buyer, contractor or service team to order the same item later without relying on memory.

Keep a sample reference for obsolete or unusual filters

For non-standard or obsolete sizes, a retained sample can be more useful than a written description alone. If the filter is still intact enough to identify, keep one labelled sample on site or store a clear reference photo set with the service records.

A good sample reference should include:

• •A full front-on photo of the filter
• •A side view showing depth and frame style
• •A close-up of the label, if present
• •A photo of how the filter sits in the unit
• •Notes about any special clips, gaskets or sealing strips

If the filter has been discontinued, a sample may help identify whether a standard size can be adapted, whether a custom replacement is needed, or whether the housing itself has been modified over time. For some sites, custom air filters are the most practical route when standard stock does not line up with the opening.

Mark which details must stay unchanged

Not every detail matters equally. Some features can vary, while others should be held constant to preserve fit and function.

When writing replacement notes, clearly mark:

• •Must match exactly: overall dimensions, depth, sealing method, frame orientation and airflow direction
• •Can vary if approved: media type, frame colour, label placement and packaging format
• •Check before substitution: filtration grade, rigidity, gasket style and initial resistance

That distinction helps avoid confusion when different people place orders across a long maintenance cycle.

Replacement notes that help the next person

Good replacement notes should read like instructions, not a memory aid. They should explain what was fitted, what was changed, and what must be checked before the next reorder.

Useful notes might include:

• •“Fit only with access door removed”
• •“Rotate filter to clear compressor pipework”
• •“Must use gasket on downstream edge”
• •“Original opening is larger than label size”
• •“Two filters required per bank”
• •“Sample held in maintenance store”

If the original unit no longer matches the installed filter, note the difference clearly. For example, if a previous service team trimmed a frame or changed the filter thickness to suit the rack, that should be recorded so the same workaround is not repeated without understanding the impact.

This also matters where multiple trades are involved. Mechanical services contractors, maintenance teams and purchasing staff may all touch the same asset at different times. A concise note in the right file can prevent the next person from ordering the wrong item.

When to involve the supplier or mechanical services engineer

Some older rooftop units are straightforward once measured accurately. Others need a second opinion, particularly if the housing has been altered, the filter location is cramped, or the intended air volume and filtration arrangement are unclear.

It is sensible to involve the mechanical services engineer or building owner when:

• •The existing filter size no longer aligns with the unit opening
• •The unit has been modified without documentation
• •The filter bank looks damaged or poorly sealed
• •The site has special ventilation requirements under AS 1668.2:2024
• •You are considering changing the filter type, depth or frame style

Site requirements should be confirmed before changing the filter arrangement. If you need regular support with site records, filter servicing and commercial filter servicing can help keep the documentation consistent.

Keep procurement notes tied to the asset

When you place an order, link the filter reference to the actual asset number rather than only the order history. Orders can be duplicated, but asset records are easier to verify years later.

A good purchasing note should include:

• •Asset number and location
• •Verified filter dimensions
• •Last known supplier reference, if available
• •Whether the size is standard, special or custom-made
• •Any handling or access notes for site delivery

If you manage several buildings or plant rooms, the category pages under our products and industries we supply can also help align the right filter type with the right application.

Make reordering easier before the next service call

The best time to solve a non-standard filter problem is before the next change-out is urgent. Once the dimensions, sample references and replacement notes are recorded properly, reordering becomes much simpler and the chance of a mismatch drops.

For older rooftop units, that usually means creating a repeatable record, keeping a sample or detailed photo set, and noting anything unusual about the frame, fit or sealing arrangement. It is a small piece of organisation that pays off every time the filter needs to be ordered again.

Need help identifying an odd-sized filter?

Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established 2009. If you need help matching an unusual size, replacing an obsolete filter or documenting a unit for future reorders, call 07 3800 3448 or request a quote.

You can also contact us to discuss a sample, photos or site notes, and we can help supply or make the right filters for the job.