How to Interpret Initial Resistance on an Air Filter Test Report

If you are comparing air filters for a commercial HVAC system, the first number to look at is not always the efficiency rating. Initial resistance, sometimes called initial pressure drop, tells you how much airflow resistance a new filter adds to the system at a stated air volume. Understanding that figure can help you choose a filter that suits the equipment, the site, and the maintenance plan.

For mechanical contractors and facility managers, this matters because two filters with a similar size or rating can behave very differently once they are installed. A filter with a lower initial resistance may be a better fit where fan capacity is limited, while a higher-resistance filter may still be suitable if the system is designed for it. The key is to read the datasheet in context, not in isolation.

What initial resistance actually means

Initial resistance is the pressure drop across a new, clean filter when air is flowing through it at a specified rate. It is usually shown in pascals or millimetres of water gauge, depending on the document and manufacturer.

In practical terms, it is a measure of how hard the filter makes the fan work at the start of service. Every filter adds some resistance to airflow. The question is not whether there is resistance, but whether the level is suitable for the equipment and operating conditions.

A lower initial resistance can be helpful when:

• •the system has limited fan capacity
• •the application has long duct runs or other components already adding resistance
• •the owner wants to minimise strain on the air handling system
• •the filter bank has multiple stages and the upstream filter load matters

A higher initial resistance may still be acceptable when the system has been designed for it and the filter performance benefits are required for the application.

How to read the number on a test report

A test report or datasheet usually lists initial resistance alongside airflow, dimensions, media type, and efficiency rating. The most important point is that the pressure drop figure only makes sense when you know the test airflow.

If one filter shows a lower initial resistance than another, check that both figures are based on the same airflow and similar test conditions. A pressure drop figure measured at a different airflow is not directly comparable. If the airflow is higher, resistance usually increases. If the airflow is lower, resistance usually decreases.

When reviewing a report, look for:

• •the stated airflow or face velocity used for the test
• •the filter size and number of pleats or surface area, where shown
• •the rating standard or test method used to assess performance
• •whether the figure is for the clean filter at installation, not a loaded filter

It also helps to check whether the datasheet refers to the filter alone or to a complete filter assembly. Frames, seals and construction details can all affect real-world pressure drop.

Why initial resistance matters to contractors and site teams

Initial resistance affects how a filter will behave from day one. If the number is too high for the system, the fan may have to work harder than intended, or the airflow may fall below what the plant is meant to deliver. If the figure is low, the filter may be easier on the system, but you still need to make sure the media grade and dust-holding needs are right for the job.

For mechanical contractors, this is especially important during replacement work. A new filter that looks similar to the old one may have a different pressure drop because of media design, pleat packing, or frame construction. That can change the operating point of the fan and affect system balancing.

For facility managers, initial resistance is a useful reference point when planning maintenance and monitoring filter condition. Once a gauge or BMS trend is in place, the clean starting point gives you a baseline for judging how the filter is loading over time. Where gauges are fitted, monitor pressure drop and follow the filter or equipment manufacturer instructions. Inspection frequency depends on the system, site conditions and dust load.

Comparing candidate filters the right way

When you are choosing between candidate filters for the same system, compare more than one line item on the datasheet. A filter that looks attractive on efficiency alone may not be the best match if its pressure drop is too high for the available fan static pressure.

A useful comparison should include:

• •initial resistance at the stated airflow
• •final resistance or recommended change-out limit, if provided
• •filter dimensions and fitment details
• •efficiency rating and particle capture characteristics
• •frame material and seal type
• •whether the product is intended for pre-filtration, final filtration, or general HVAC use

It is also worth confirming the rating language. MERV, EN 779 and ISO 16890 do not convert directly, so avoid assuming that two filters with different rating systems will perform the same way. G3/G4/F5-F9 are EN 779 classes and are still widely used in Australia, even though EN 779 has been superseded by ISO 16890.

If you are working to a building specification, confirm site requirements with the mechanical services engineer or building owner. For ventilation design and compliance considerations, AS 1668.2:2024 may be relevant to the broader system, but the right filter choice still depends on the actual application and equipment details.

What else on the datasheet deserves attention

Initial resistance is important, but it should never be the only deciding factor. A filter with low pressure drop but poor fit or the wrong media type can create problems in service. Likewise, a higher-resistance filter may be worthwhile if the system is designed around it and the filtration requirement calls for that construction.

Other details to check include:

• •the exact nominal or actual size, including depth
• •whether the filter is designed for panel, bag, or cartridge use
• •the frame style and whether it matches the existing housing
• •the sealing method used in the filter bank
• •any notes about operating orientation or installation direction

Some filters are designed to be washable - check the product instructions. Do not assume a washable claim applies to every product in a range. If a filter needs special handling, the manufacturer documentation should make that clear.

If you need help measuring an existing filter before ordering, our guide on how to measure an air filter can help avoid fitment issues. For replacement planning or ongoing maintenance, see our filter servicing and commercial filter servicing options.

How Total Air Filtration can help

Total Air Filtration is a family-owned air filter manufacturer and supplier based in Hillcrest, Brisbane QLD, and we work with contractors and facility teams who need practical advice on selecting the right replacement. We can supply standard products, make custom air filters where required, and help you match the filter to the system rather than guessing from a single datasheet figure.

If you want to compare options for a specific plant room, rooftop unit, or commercial site, please request a quote or contact us. You can also review our products or see the industries we supply.

For direct help, call 07 3800 3448.