Odd-Depth Filters: When 38 mm, 50 mm or 70 mm Is the Real Requirement

In commercial HVAC work, the filter size on paper is only part of the story. If the cabinet, access door or available plant room clearance is tight, the real requirement may be a non-standard depth such as 38 mm, 50 mm or 70 mm rather than a more common frame depth.

For mechanical contractors and facility managers, getting the depth right can save a lot of back-and-forth on site. It affects fit, sealing, access and how the filter sits in the frame, so it is worth checking early rather than assuming a standard replacement will do the job.

Why odd-depth filters matter

Most people focus on face dimensions first, then discover the depth is the part that causes trouble. In packaged units, compact AHUs, fan coil cabinets and custom enclosures, there may simply not be enough room for a deeper panel or pleated filter.

A filter that is too deep can interfere with the door closing, foul on clips or rails, or leave insufficient clearance for removal and inspection. A filter that is too shallow may sit loosely, bypass around the edges or require packing that was never intended for the unit.

That is why odd-depth filters are often less about being unusual and more about being practical. A 38 mm, 50 mm or 70 mm depth may be the exact dimension needed to suit:

• •shallow access doors
• •tight side-clearance cabinets
• •older equipment with non-standard filter tracks
• •retrofits where the housing cannot be modified
• •replacement units where the original filter was made to suit a specific slot

Where standard depths run out of room

Cabinets and compact air handling units

Cabinet style units often have a fixed filter slot with very little tolerance. Once the door, latch, gasket and frame are all accounted for, the available depth can be less than expected.

In these situations, the filter is usually selected to match the housing, not the other way around. If the depth is wrong, the filter may bow, rub or fail to seat properly.

Access doors and removable panels

Access doors are a common pinch point. Even if the filter itself would fit inside the casing, it still has to be removed and reinstalled without catching on the door opening or adjacent components.

A shallower filter can make servicing easier, especially where there is limited swing space or where other services sit close to the access point. It can also reduce the chance of damage during changeout.

Tight plant room clearances

Plant rooms often have pipes, cable trays, dampers and insulation that limit working space. A filter that is easy to handle on a bench may be awkward in the field if there is only a narrow opening to work through.

When clearance is limited, depth becomes part of the serviceability check. It is not just about whether the filter can be inserted, but whether it can be gripped, removed and replaced safely.

What to measure before ordering

The safest approach is to measure the actual housing rather than relying on old packaging or verbal descriptions. Filter call-ups are often written down in shorthand over the years, and that shorthand can drift away from the physical opening.

Before ordering, check:

• •face dimensions of the filter slot or frame
• •actual clear internal depth available
• •any rails, tabs, clips or stop points inside the housing
• •door clearance when closed
• •whether the filter seats on a gasket, frame lip or retaining edge
• •whether the existing filter is compressed, bowed or altered

If you are unsure how to check the dimensions, our guide on how to measure an air filter is a useful starting point.

It also helps to inspect the old filter for clues. Look for wear patterns on the frame, compressed corners, air bypass staining or damage to the media. These signs can tell you whether the size is genuinely correct or whether the unit has been running with a compromised fit.

Why depth affects performance and maintenance

A correctly sized filter does more than simply fit in place. It helps the air move through the intended media area and improves the chance of a proper seal in the frame. When the depth is wrong, the filter can be harder to secure and may not sit squarely in the housing.

That can create practical maintenance issues such as:

• •filters jamming in the frame during removal
• •covers or doors not closing cleanly
• •gaps at the edges where unfiltered air can pass around the media
• •damaged filter frames from forcing an oversized product into position
• •added time on site while trying to make a standard depth work

For systems where pressure drop is monitored with gauges, the inspection frequency should be based on the system, site conditions and dust load, and the filter or equipment manufacturer instructions should be followed. A filter that fits well will give a more reliable basis for that monitoring than one that has been squeezed into an unsuitable slot.

If the application falls under broader ventilation design requirements, site requirements should be confirmed with the mechanical services engineer or building owner, including reference to AS 1668.2:2024 where relevant.

Choosing the right construction for the job

Odd-depth filters can be supplied in different media and frame styles depending on the application. The depth alone does not tell you everything about how the filter will behave in service, so it is worth matching the product to the system, not just the dimensions.

Consider:

• •whether the application needs a panel, pleated or rigid style
• •the airflow direction through the housing
• •whether there is enough support for the filter media
• •the available sealing surface in the frame
• •whether a deeper or shallower option will make servicing easier

Some filters are designed to be washable - check the product instructions before attempting to clean them. In other applications, disposable replacement is the better option. The right choice depends on the equipment and the manufacturer’s guidance.

If you are selecting from a broader range, our our products page is a helpful place to compare options, and custom air filters may be the answer when the housing is non-standard or the dimensions are no longer available off the shelf.

Getting the fit right on site

For contractors, the main challenge is often not finding a filter, but finding one that fits first time. For facility managers, the priority is usually keeping the system serviceable without repeated adjustments or temporary workarounds.

A good process is to:

• •confirm the measured dimensions on site
• •compare them with the existing filter and housing clearances
• •check whether the door or panel closes without force
• •make sure there is enough room to remove the filter later
• •record the exact depth for future maintenance

If the existing arrangement is unusual, a photo set and measured sketch can save time later. That record is especially useful when the filter is hidden in a cabinet or when the original part was never clearly labelled.

Need a 38 mm, 50 mm or 70 mm filter made to suit?

Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established 2009. If you need an odd-depth filter for a cabinet, access door or tight plant room clearance, we can help supply or make the right filter for the space.

Call 07 3800 3448 or request a quote. If you also need support with maintenance planning, our filter servicing and commercial filter servicing pages may help, or you can contact us to discuss the best fit for your system.