Fan Curve Basics for Buyers of Rooftop Air Handling Units

If you are buying filters for a rooftop air handling unit, the fan curve matters just as much as the filter size. A filter that looks right on paper can still create too much resistance once it is fitted, which can leave the unit short of airflow. The good news is that with a basic understanding of fan curves and pressure drop, you can make better decisions before the unit goes on the roof.

What a fan curve actually shows

A fan curve is a chart from the unit or fan manufacturer that shows how the fan performs at different airflow and pressure conditions. In plain English, it tells you how much air the fan can move when resistance in the system changes.

The important idea is simple: as resistance increases, airflow usually falls. Filters are one of the main sources of resistance in an AHU, so the filter choice directly affects where the fan ends up operating on the curve.

When you look at a fan curve, you are usually trying to answer a practical question:

• •Can this fan still deliver the required airflow once the selected filters are installed?
• •Will the fan have enough spare capacity as the filters load with dust?
• •Is the unit likely to sit in a stable operating range rather than near its limit?

If the fan has no margin, even a sensible filter upgrade can push the system off target.

Why filters change the operating point

Every filter creates a pressure drop. Clean filters have a lower pressure drop than dirty filters, and the system sees both over time. That is why the right filter is not just about capture efficiency or frame style; it is also about how much resistance the fan must overcome.

For rooftop AHUs, this matters because access can be awkward and the system may be exposed to tougher site conditions such as dust, pollen, smoke, salt air or construction debris. A filter that loads quickly can cause the fan to work harder than intended, which affects the airflow available to the space.

Clean pressure drop and dirty pressure drop

When selecting filters, it helps to think about two figures:

• •Initial pressure drop: the resistance of a clean filter at start-up.
• •Final pressure drop: the resistance when the filter has collected enough dust that it needs attention.

Both figures matter. A low initial pressure drop is helpful, but not if the filter loads too quickly for the site. A filter with a slightly higher initial pressure drop may still be the better choice if it stays more stable in service.

How to read the fan curve with filters in mind

To use a fan curve properly, start with the required airflow for the system and then consider the total external static pressure the fan must handle. That total includes ductwork, grilles, coils, dampers and the filter bank.

If you are comparing filter options, the question is not just “which filter?” but “which filter keeps the unit closest to its design point?”

A useful way to approach it is:

• •Identify the required airflow from the mechanical design.
• •Confirm the available fan data for the rooftop AHU.
• •Add the filter pressure drop to the rest of the system resistance.
• •Check where that total lands on the fan curve.
• •Make sure the fan can still deliver the target airflow with some sensible allowance for filter loading.

If the selected filter pushes the system too far to the right on the curve, airflow drops. If the system is forced too far left or too far right of the fan’s preferred operating range, you can also run into noise, instability or poor performance.

Choosing filters that fit the system, not just the frame

A filter should suit the unit, the installation and the site conditions. A rooftop AHU may have limited face area, so a filter with too much resistance for that cabinet can create airflow problems even if it physically fits.

When selecting filters, consider:

• •the filter type and media construction
• •the available filter face area
• •the expected dust load at the site
• •whether the AHU is supplying fresh air, recirculated air, or both
• •any project requirements from the mechanical services engineer or building owner

Some projects call for coarse pre-filters upstream of a finer final filter stage. In many systems, that can help manage dust loading and protect the fan from unnecessary pressure drop rise. The right arrangement depends on the unit and the duty, not on guesswork.

If you are unsure what will fit, it can help to how to measure an air filter before ordering. Getting the dimensions wrong is one of the easiest ways to create unnecessary leakage or resistance.

Practical points for rooftop AHUs

Rooftop units often face harsher conditions than plantroom equipment. Heat, wind, dust and difficult access all affect how the filters perform in real use.

Keep these checks in mind:

• •Make sure the filter rack seals properly so air does not bypass the filter bank.
• •Confirm the filter media and frame can handle the installation environment.
• •Allow for the actual dust conditions at the site, not just the clean indoor design intent.
• •Check whether the unit has enough space for the filter depth you want to use.
• •If gauges or differential pressure indicators are fitted, monitor them and use them to guide maintenance.

Inspection frequency depends on the system, site conditions and dust load. Follow the filter and equipment manufacturer instructions and watch the pressure drop rather than relying on a fixed calendar schedule.

If the project has ventilation requirements under AS 1668.2:2024, confirm the site-specific requirements with the mechanical services engineer or building owner.

Ratings, standards and what they do and do not tell you

Filter ratings help you compare products, but they do not replace airflow checking on the fan curve. MERV, EN 779 and ISO 16890 are different rating systems and do not convert directly into one another.

In Australia, you will still see G3/G4 and F5-F9 referenced on older projects or legacy documentation. Those are EN 779 classes, even though ISO 16890 is now the current European standard.

If a project needs HEPA filtration, the efficiency level must be treated carefully. Under EN 1822, H13 is at least 99.95% and H14 is at least 99.995% at the most penetrating particle size. That does not mean every filter rating is interchangeable, and it does not change the need to confirm the fan can still move the required air.

Also, some filters are designed to be washable - check the product instructions. Washing is a product-specific feature, not something to assume across all filter types.

A simple buying checklist before you order

Before placing an order for rooftop AHU filters, make sure you have:

• •the unit model and fan data
• •the required airflow and operating point
• •the filter size, depth and frame type
• •the expected dust conditions at the site
• •any instructions from the mechanical services engineer
• •the pressure drop limits you need to stay within

If you are replacing an existing filter set, compare the old and new pressure drop characteristics rather than assuming a like-for-like swap will behave the same. A small change in media, depth or frame design can affect the fan curve enough to matter.

Getting the right fit for the job

At Total Air Filtration, we are a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, and we can help match filter selection to the realities of your rooftop AHU, not just the dimensions on a drawing. We can supply standard options or make custom air filters to suit the unit and the airflow target.

If you need help checking compatibility, talk to us on 07 3800 3448 or request a quote. You can also review our products, explore custom air filters, or contact us if you want to discuss a specific rooftop application.