Carbon Filters in Rooftop Packaged Units: Space, Weight and Access Considerations

Rooftop packaged units leave very little room for extras, and gas-phase media can be one of the hardest additions to fit well. Once you add carbon or other adsorptive filtration, the practical questions quickly become about space, weight, access and how much resistance the system can handle.

For mechanical contractors and facility managers, the right answer is rarely “just add a carbon filter”. It usually depends on the cabinet layout, fan capacity, service access and the actual odour or gas-phase control goal for the site.

Why rooftop units are difficult places for carbon filters

Carbon filters need more than just a slot in the frame. They need enough depth for the media, enough face area for air to pass through without excessive resistance, and enough clearance for removal and replacement.

In many rooftop packaged units, those conditions are already tight. The cabinet may be designed around standard particulate filters, coils, dampers and access doors, leaving limited room for a deeper gas-phase section.

Common constraints include:

• •shallow filter bays that only suit basic panel or pleated filters
• •existing access doors that are too small for a deeper carbon cell
• •limited clearance above or beside the filter rack
• •cramped return-air sections where media cannot be replaced safely
• •unit housings that were never intended for added weight or extra resistance

If the filtration arrangement is being redesigned, it is worth checking the actual unit dimensions and available service space early, not after the equipment is already on site. If you need help measuring the available space, see how to measure an air filter.

Space and depth: the first design limit

Carbon filters used for odour control or gas-phase adsorption often require more depth than a standard dust filter. That depth helps create enough contact time between the air stream and the media.

In rooftop units, depth can be limited by:

• •the filter track depth
• •the position of the fan and coil sections
• •adjacent pipework or cabling
• •access panels that swing into the filter path
• •structural members inside the unit casing

If the cabinet cannot accept a deeper cell, a contractor may need to look at a different filter format, a modified frame, or an external arrangement that suits the equipment better. In some cases, a custom-built filter solution is the practical option. You can explore custom air filters when a standard size or format will not fit.

It is also worth remembering that more media is not always better if the system cannot accommodate it. A cramped installation can make removal difficult, increase the risk of air bypass, and create maintenance issues later.

Weight matters more than many teams expect

Carbon media and the supporting frame can add noticeable weight compared with a light-duty particulate filter. That matters in rooftop units because filter sections, access panels and supports are often built for compact, low-mass components.

Before specifying a carbon solution, check:

• •whether the filter rack and supports can carry the added load
• •whether the access door hinges and latches are suitable
• •whether the frame can be handled safely by a technician on the roof
• •whether the unit’s internal supports are already carrying other components

This is especially important where filters are installed in elevated or awkward positions. A filter that is technically suitable but difficult to handle can slow maintenance, increase the risk of damage, and encourage poor fitting.

If a retrofit is being considered, it is sensible for the mechanical services engineer or building owner to confirm site requirements before the final selection is made. AS 1668.2:2024 may be relevant to the broader ventilation design, but the exact arrangement still needs to suit the equipment and the site.

Access and serviceability should be designed in, not hoped for

A rooftop filter that is hard to reach is one that is more likely to be neglected or damaged during changeout. Carbon filters are particularly sensitive to poor service access because the media can be bulky, and the seal around the frame matters.

Good access means:

• •the filter can be removed without forcing it past pipework or framing
• •there is enough room to inspect the seal and the bypass edges
• •maintenance staff can work without unsafe body positions
• •the access door can be opened fully and closed properly afterwards

If access is tight, ask whether the filter can be staged in sections or fitted into a serviceable cassette rather than as a one-piece insert. A well-designed arrangement reduces the chance of bypass and makes routine inspection more straightforward.

For ongoing support, see filter servicing and commercial filter servicing.

Pressure drop: the part that affects the whole system

Carbon filters generally introduce more resistance than a basic dust filter, and that resistance matters in rooftop packaged units. If the pressure drop is too high for the fan and system layout, airflow can suffer and the unit may not perform as intended.

When evaluating a carbon option, check:

• •the clean pressure drop of the chosen filter
• •how the pressure drop will change as the media loads or approaches exhaustion
• •whether the fan has spare capacity for the added resistance
• •whether other components in the air path are already causing a heavy pressure loss

Pressure drop should be checked against the equipment design and the site condition, not guessed. Where gauges or monitoring points are fitted, they should be used to track the filter condition over time. Inspection frequency depends on the system, the site environment and the dust load, and should follow the filter and equipment manufacturer instructions.

If a rooftop unit is already operating near its fan limit, adding carbon media without reviewing pressure drop can create more problems than it solves. In that situation, it may be better to change the filter arrangement or use a different housing that is purpose-built for gas-phase media.

Choosing the right carbon solution for the job

Not every odour issue needs the same filter. The right product depends on what is being targeted, how much air is moving, and how much room the unit has for the filter section.

Useful questions to ask before ordering include:

• •Is the main goal odour control, gas-phase adsorption, or a combination with dust filtration?
• •Is the rooftop unit being used in a return-air or outdoor-air path?
• •Is there enough depth for the selected media format?
• •Can the frame be sealed properly to prevent bypass?
• •Will maintenance staff be able to change it without special tools or unsafe access?

Some filters are designed to be washable - check the product instructions. However, that does not mean every washable product is suitable for every rooftop application, and it is important to match the product type to the operating conditions.

For wider product options, see our products or industries we supply.

Practical takeaway for contractors and facility teams

Carbon filters can work well in rooftop packaged units, but only when the system has room for them and the fan can handle the resistance. The best installation is the one that fits the cabinet, can be serviced safely, and performs without pushing the unit beyond its intended limits.

If you are planning a retrofit, replacement or custom arrangement, Total Air Filtration can help specify or make the right filters for the space available. Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established 2009.

Call 07 3800 3448 or request a quote. You can also contact us if you need help working through an existing rooftop unit layout.