Filter Selection for Make-Up Air Units in Dirty or Coastal Environments

Make-up air units often sit on the front line of a site’s environment. If the intake is exposed to construction dust, road grime, salt spray or other outdoor pollutants, the filter selection needs to match the real conditions around the building, not just the nominal duty of the unit.

The right filter arrangement can help keep debris out of ductwork and air handling equipment, while also supporting more stable system operation. For mechanical contractors, facility managers and industrial buyers, the key is to choose filters based on airflow, contamination level, housing space and maintenance access.

Start with the site exposure, not the catalogue alone

The first step is understanding what the make-up air unit is drawing in and where the intake is located.

A coastal site may deal with salt-laden air, which can be more aggressive to metal components and can leave residue on filters and internal surfaces. An industrial or roadside site may face fine dust, soot, fibres, pollen or other outdoor pollutants. A construction-adjacent site can see much heavier dust loading during certain phases of work.

Before selecting a filter, look at:

• •The proximity to roads, loading docks, quarries, construction zones or the shoreline
• •Whether the intake is at roof level, wall level or near a contamination source
• •The available filter face area and the depth the housing can accept
• •Whether the system has one stage or can accommodate pre-filtration
• •How easily maintenance staff can inspect and change filters

If the site is complex, it can help to review the equipment layout with the mechanical services engineer or building owner. Site requirements should also be considered alongside AS 1668.2:2024 and the unit manufacturer’s instructions.

Choose a filter sequence that suits the environment

A single coarse filter may be enough in some protected locations, but exposed sites often benefit from staged filtration. The aim is usually to catch the larger debris first, then finer particles before they reach the unit or ductwork.

Coarse pre-filtration for heavy outdoor loading

In dirty environments, a coarse pre-filter is often used to intercept larger dust, lint, insects and wind-blown debris. This can reduce the load on any downstream filter stage and make maintenance more manageable.

For many applications, you may see legacy EN 779 classes such as G3, G4 or F5-F9 referred to in Australia. These terms are still widely used, even though ISO 16890 has superseded EN 779. If you are comparing options, be careful not to assume that ratings from different systems convert directly.

Finer downstream filters where the system needs cleaner supply air

If the make-up air unit supports a finer second stage, that downstream filter can capture smaller particles before air enters the conditioned space or process area. The right level depends on the application, the housing and the pressure drop the fan can handle.

The goal is not to specify the finest filter possible. It is to choose a filter that suits the contamination level, fits the system and can be maintained without creating excessive resistance.

Coastal environments need attention to corrosion and loading

Salt spray changes the conversation. In coastal areas, the issue is not only particle capture, but also how salt and moisture interact with the filter and surrounding equipment.

When specifying for a coastal site, consider:

• •Whether the intake is exposed directly to sea air or partly shielded
• •How quickly the local environment can load the filter media with salt residue and dust
• •Whether the filter frame, seal and housing materials are suitable for the location
• •Whether the unit may need extra protection such as a weather hood, louvre or better intake placement

If corrosion resistance is important, do not focus only on the filter media. The filter frame, supporting hardware and housing condition matter as well. A good filter can still be undermined by poor sealing or an unsuitable enclosure.

Fit, seal and face area matter as much as media choice

A well-chosen filter can still perform poorly if it does not fit properly. Air will always take the path of least resistance, so bypass around the edges is a common issue in make-up air units that have been modified, poorly maintained or fitted with the wrong size.

When checking the filter arrangement, make sure to confirm:

• •Correct dimensions of the existing filter and housing
• •The depth the frame can accommodate
• •The sealing method used in the unit
• •Whether the filter is installed in the correct airflow direction
• •Whether the filter frame is stiff enough to stay seated under operating pressure

If you are replacing an existing filter, measuring accurately is essential. Our guide on how to measure an air filter can help when the original label is missing or unreadable.

A larger face area can also help because it may spread the dust load over more media, but the housing must support that choice. If a deeper or larger filter is not possible, a staged arrangement may be more practical than forcing a finer single filter into a restrictive slot.

Maintenance planning should match dust load and site access

On a dirty or coastal site, filter selection should be tied to how the system will actually be serviced.

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. If a gauge or differential pressure reading shows the filter is loading faster than expected, the site may need a different filter arrangement or a better intake position.

A sensible maintenance plan considers:

• •Whether staff can access the unit safely and easily
• •Whether tools are required to open the housing
• •Whether the site has the labour to inspect filters regularly
• •Whether filter change-outs cause downtime for the process or building
• •Whether the dust pattern changes with seasons, winds or nearby works

Some filters are designed to be washable - check the product instructions. That can be useful in certain applications, but it is not a blanket solution for every dusty or coastal environment. The housing, media type and cleaning method all need to suit the product.

When custom filters are the better option

Standard sizes work for many make-up air units, but custom filters are often the better answer when the housing is unusual, the site is harsh or the system has been modified over time.

Custom options can help when:

• •The existing filter size is obsolete or hard to source
• •The intake housing has limited depth
• •The unit needs a specific frame style or seal arrangement
• •The site exposure calls for a tailored filter build
• •A replacement is needed for older plant with non-standard dimensions

If you need a made-to-order solution, see our custom air filters and our products pages for the kinds of filters we supply.

Specifying the right filter for the job

For make-up air units in dirty or coastal environments, the best specification is usually the one that balances capture, airflow, fit and serviceability. Start with the site exposure, then decide whether the unit needs coarse pre-filtration, a finer second stage or a custom-built solution.

If you are unsure what will fit the housing or what level of filtration is practical, Total Air Filtration can help supply or make the right filters. We’re a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established 2009. Call 07 3800 3448 or request a quote.

If you also need ongoing support, we can assist with filter servicing and commercial filter servicing, depending on the site and system requirements.