How Fan Curves Change After a Filter Upgrade in an Office AHU

When an office AHU gets a filter upgrade, the job is not always as simple as swapping one panel for another. If the new filter has a higher resistance to air movement, the fan may no longer operate where it did before, and that can change delivered airflow, noise levels and room comfort.

For mechanical contractors and facility managers, understanding the fan curve is the practical way to predict what happens next. It helps you see whether the system still has enough capacity, whether the fan is being pushed into a less efficient part of its range, and whether pressure readings are pointing to a filter issue or a broader airside problem.

What a fan curve actually tells you

A fan curve shows how much air a fan can move at different static pressures. In simple terms, the fan does not deliver the same airflow no matter what you put in front of it. As resistance rises, airflow usually falls.

In an office air handling unit, that resistance comes from several parts of the system:

• •Ductwork layout and fittings
• •Coils, dampers and grilles
• •Air filters
• •Dirt build-up on components over time

The fan only operates where its curve intersects with the system resistance curve. That intersection is the operating point. If you change the filter and add resistance, the operating point shifts. The fan may move to a lower airflow and a higher static pressure, depending on how the system is set up.

This is why two filters that both fit the same frame can behave differently. The size, media type, construction and dust-holding design all matter.

Why a more restrictive filter changes the operating point

A filter upgrade often means better dust capture, but it can also mean more resistance to airflow. That extra resistance may be small at first, then increase as the filter loads with dust. In a fan curve, that shift shows up as the system curve moving upward.

What happens in practice

If the fan is fixed speed, the system usually settles at a new point with:

• •Lower airflow than before
• •Higher fan static pressure
• •More fan noise in some cases
• •Reduced air delivery to some zones

If the fan has variable speed control, the controller may try to maintain airflow or pressure by speeding the fan up. That can help maintain supply air performance, but it may also increase noise and energy use, depending on how the system is tuned.

With office AHUs, this matters because occupied spaces are often sensitive to both under-supply and over-supply. Too little airflow can leave rooms stuffy or uneven, while too much can create draughts, noise and balancing issues.

The most common signs after a filter upgrade

After installing a more restrictive filter, the changes are not always obvious at first. You may notice them during commissioning, after a maintenance visit, or when tenants report a change in room conditions.

Watch for these clues

• •Supply airflow readings that drop below the expected setpoint
• •Increased fan sound at the plantroom or at diffusers
• •More complaints about draughts in some areas and stagnant air in others
• •Higher pressure drop across the filter bank than the previous product
• •Fan speed or drive output increasing to compensate

A pressure gauge or differential pressure switch is especially useful here. If gauges are fitted, monitor the pressure drop across the filter and compare it with the equipment instructions and the filter manufacturer’s guidance. That tells you whether the issue is the new filter, dust loading, or a broader system imbalance.

If the building has variable air volume zones, even a modest airflow change can alter how those zones behave. A filter that is too restrictive can make balancing harder, particularly if the fan was already close to its limit before the upgrade.

Noise and comfort are often the first complaints

A filter change can affect comfort in ways that are not immediately linked to filtration. When airflow changes, the noise signature of the AHU can change too. A fan running against higher resistance may generate a different tonal noise, and outlets may hiss or sound more prominent if the distribution pattern changes.

Comfort issues that can follow include:

• •Uneven temperatures between zones
• •Less effective fresh air distribution
• •Draught complaints near terminals serving higher-airflow areas
• •Stale-feeling rooms if supply air drops too far

These are mechanical issues rather than occupant preferences. In office buildings, small changes at the AHU can be noticeable across open-plan areas, meeting rooms and enclosed offices. It is worth checking whether the new filter is suitable for the fan’s available pressure margin before assuming a control fault or balancing problem.

How to choose a filter without upsetting the system

The right filter is not just about a filter class on paper. It also needs to suit the fan, the housing and the operating conditions of the site. A higher-performing filter may be the right choice, but only if the AHU can carry the added resistance without compromising airflow.

Good questions to ask before changing filter type

• •What is the current fan static pressure at the normal operating point?
• •How much spare pressure capacity does the fan have?
• •Is the system fixed speed or variable speed?
• •What dust load does the site actually see?
• •Are the existing frames and seals in good condition?

For some projects, the answer may be a deeper media filter, a better pre-filter arrangement, or a different combination of stages rather than a single more restrictive product. In other cases, a custom air filters solution is the best way to match the filter to the plant.

If you are working from a maintenance plan, remember that inspection frequency depends on the system, site conditions and dust load. Follow the filter and equipment manufacturer instructions, and use pressure drop readings where gauges are fitted rather than relying on a fixed calendar.

Standards, fitment and practical site checks

For occupied buildings, airside changes should be considered alongside the ventilation requirements of the project. If AS 1668.2:2024 is relevant to the site, confirm the requirements with the mechanical services engineer or building owner before making changes to the filter arrangement.

A few practical checks can save time after a filter upgrade:

• •Confirm the filter dimensions and sealing method are correct for the frame
• •Check that the filter is seated properly and not bypassing air around the edges
• •Compare pressure drop readings before and after the change
• •Verify the fan control strategy is still appropriate for the new resistance
• •Review whether the air handling unit can still meet the required airflow at peak load

If you are not sure how to measure a filter correctly, our guide on how to measure an air filter is a useful starting point before ordering replacements.

When to review the whole system, not just the filter

Sometimes a filter upgrade exposes an existing issue. The fan may have been operating close to its limit all along, and the extra resistance simply makes that visible. In other cases, dust build-up on coils, damaged dampers or poor commissioning can be part of the problem.

It is worth reviewing the full airside path when:

• •The pressure drop rises faster than expected
• •Airflow falls off after the filter change
• •Fan noise changes significantly
• •Occupants notice comfort changes soon after maintenance

That broader review can help you decide whether to keep the new filter, adjust controls, change the filter stage, or look for a fault elsewhere in the system. If the site uses a product with a specific media design, check whether some filters are designed to be washable and follow the product instructions if that applies.

Need the right filter for the fan curve?

Total Air Filtration is a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established 2009. If you need a filter that suits the fan, the housing and the site conditions, we can supply or make the right fit for your AHU.

Call 07 3800 3448 or request a quote. You can also view our products or get in touch through our contact us page.