How to Specify a Custom Filter When the Old One Was Installed Backwards or Sideways

If you are replacing a filter and the old one was installed backwards or sideways, do not assume the visible face tells the full story. The safest way to specify a custom replacement is to work out how the system actually moves air, how the frame sits in the filter rack, and which edges seal against the housing.

That matters for homeowners, property managers and contractors alike, because a sample can be physically correct in size but still be wrong in orientation. A good replacement should match the real installation, not just the way the old filter happened to be facing on removal.

Start with the airflow, not the old arrow

The arrow printed on a filter frame is only useful when the old filter was installed correctly. If it was fitted backwards, crossed in the rack, or turned sideways, the arrow may no longer match the system.

To work out the actual airflow direction, look at the equipment and the ductwork, not just the old filter:

• •Find the supply side and return side of the air handling unit, fan coil, split system add-on housing or filter cabinet.
• •Check which side the fan is on, because air is usually being pulled towards the fan.
• •Look for dust patterns on the upstream side of the old filter and the cleaner, more compacted side on the downstream face.
• •If there are multiple filter stages, note where each stage sits and which direction the air passes through them.
• •Check any labels, service notes, or drawings already attached to the unit.

If you are unsure, a mechanical services technician or the building owner may be able to confirm the intended airflow path. For new work, site requirements should be checked against the equipment instructions and, where relevant, AS 1668.2:2024 and the mechanical services engineer.

Read the sample like a piece of equipment, not just a size

When a sample comes in from site, we look at more than width, height and depth. The frame shape, media pack, seals, and the way the filter was seated all help determine how the replacement should be built.

A sideways or backwards sample can still give the right answer if you identify the working faces carefully. Pay attention to:

• •Which side has the retaining lip, gasket, or compression seal.
• •Whether the frame has a closed edge, open edge, or a support wire that must face the downstream side.
• •Where the media is supported and where the air first meets the filter.
• •Whether the old unit sat in a track, clamp, holding channel, or spring-loaded frame.
• •Any marks caused by air bypass, frame rubbing, or compression from the wrong side.

If the sample is damaged, bent, or heavily loaded with dust, tell us that too. We can often make sense of a worn sample when we know how it was removed and how it sat in the equipment.

Distinguish between filter face, frame orientation and seating

A common mistake is to treat all “faces” of a filter as interchangeable. In practice, there are three different things to identify: the airflow direction, the physical orientation of the frame, and the way the filter seals in the housing.

Airflow direction

This is the direction air should pass through the media. It matters for performance and for how the media is supported. If the old filter was installed the wrong way around, the arrow may be unreliable, so the system needs to be checked directly.

Frame orientation

Some filters can physically fit in more than one direction, especially when the frame is square or rectangular. That does not mean every direction is correct. Side rails, stiffeners, pleats, support mesh and seal faces may be designed to sit only one way in the rack.

Correct seating

The filter must sit flat and seal properly against the holding frame. If it is too loose, too tight, upside down or turned sideways, air can bypass the media. That can happen even when the outer dimensions are technically right.

When measuring for a replacement, use the actual opening and the way the filter is retained in the unit. If you need a refresher, see how to measure an air filter.

What to tell the manufacturer when the old filter was fitted wrong

If you are ordering a custom filter from a sample, clear notes save time and reduce guesswork. The more specific you are about the installation, the easier it is to make the replacement fit the real application.

Include details such as:

• •The equipment type and location.
• •Whether the sample was removed from a return air path, supply air path, or cabinet.
• •The direction the air was moving when the filter was in service.
• •Which edge faced upstream and which edge sealed against the frame.
• •Any markings, clips, tabs, gaskets or support pieces on the sample.
• •Whether the filter was standing upright, laid on its side, or mounted in a non-standard orientation.
• •Photos of the rack, not just the filter itself.

If you have more than one identical unit on site, compare them. Sometimes one correctly installed unit can show you what the other one should look like.

Common situations where the old filter is misleading

Backwards or sideways installations are more common than many people expect, especially after maintenance work, tenant changes, emergency repairs, or when a filter is swapped quickly and the old one is used as a pattern without checking the system.

Watch for these situations:

• •The arrow on the old filter points in a direction that does not match the air path.
• •The filter appears to fit, but the sealing edge is on the wrong side.
• •The frame is the right size, but the media is bowing in the wrong direction.
• •The old unit was trimmed, forced, or packed into a non-standard opening.
• •The rack has been modified, so the current installation no longer matches the original design.

If the system has been altered, a replacement made strictly from the old sample may repeat the same problem. In that case, it is better to identify the actual opening and the correct installation method before manufacturing.

Working with custom filters for homes and commercial sites

For homeowners, the aim is usually a clean, neat fit that matches the equipment and avoids air leakage. For contractors, the goal is often a replacement that accounts for odd housings, altered frames, or legacy setups that no longer match standard dimensions.

At Total Air Filtration, a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, we can help build from a sample and interpret the installation details when the original filter was fitted incorrectly.

If the filter is for a building system, remember that 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. Some filters are designed to be washable - check the product instructions before cleaning or reusing any item.

You can also browse our products or learn about custom air filters if the existing filter is no longer a standard size or style.

Get the replacement right the first time

A backwards or sideways sample does not have to create a bad replacement order. If you can identify airflow direction, frame orientation and correct seating, the custom filter can be built to suit the way the system actually runs.

If you would like help interpreting a sample, Total Air Filtration can supply or make the right filters for your setup. Call 07 3800 3448 or request a quote. If you need follow-up support, you can also contact us.