What Pressure Drop Readings Mean for a Filtered Make-Up Air Unit

Filtered make-up air units are common in commercial and industrial buildings because they bring in outdoor air that has been filtered before it enters the space. In these systems, pressure drop readings are one of the simplest ways to see how hard the unit is working and whether the filter is loading up with dust. Reading the gauge correctly helps mechanical contractors and site teams decide when to inspect, clean or replace the filter, rather than relying on guesswork.

What pressure drop actually tells you

Pressure drop is the difference in air pressure across the filter. As air passes through the filter media, resistance increases. A clean filter usually has a lower pressure drop, while a dust-laden filter has a higher one.

On a make-up air unit, the reading is typically shown on a gauge, manometer or differential pressure sensor. The number itself is not the whole story. What matters is how the reading compares with the system’s normal operating range and the limits set by the filter or equipment manufacturer.

A pressure drop reading can help you understand:

• •whether the filter is loading normally
• •whether airflow resistance is increasing
• •whether the system needs inspection
• •whether another issue, such as leakage or a blocked duct, may be affecting performance

It is worth noting that a pressure reading alone does not tell you the full condition of the filter. Visual inspection still matters, especially where there is visible dust loading, damaged media or a bypass issue around the frame.

How to interpret a rising gauge reading

A gradual rise in pressure drop is usually expected as a filter captures more particles. In many make-up air systems, that rise is the clearest sign the filter is doing its job and gradually becoming loaded.

If the reading is increasing faster than expected, that can point to site conditions rather than a filter fault. Common causes include:

• •heavier dust load from nearby works or process areas
• •airborne debris from plant rooms, loading docks or vehicle movement
• •a filter that is undersized for the application
• •incorrect installation, allowing air to bypass or press unevenly across the media
• •moisture, oil mist or other contaminants affecting the filter surface

A sudden jump in pressure drop is different from a steady rise. That may indicate a blocked pre-filter, debris caught at the face of the filter, a damaged component upstream, or a gauge issue. If the number changes abruptly, it is worth checking the system rather than assuming the filter is the only cause.

What a low reading can mean

A low pressure drop is not always a sign that everything is fine. In a filtered make-up air unit, a reading that stays unusually low can mean one of several things.

Possible reasons include:

• •the filter has only recently been installed and is still clean
• •the gauge or sensor is faulty or out of calibration
• •the filter is missing, incorrectly seated or damaged
• •air is bypassing the filter because of gaps in the frame or housing
• •the system is running differently from normal because of fan or control changes

If a filter looks dirty but the gauge remains low, that is a useful warning sign. The gauge may not be measuring the true pressure differential across the filter, or the filter may not be sealing correctly. For contractors, this is a prompt to inspect the housing, seals and sensor lines before making a replacement decision.

Reading the gauge in context

Pressure drop readings should always be interpreted alongside the system design and operating conditions. A value that is normal for one unit may not be appropriate for another, even if the filters look similar.

When reviewing a make-up air system, consider:

• •the filter type installed in the unit
• •whether the system uses a single stage or multiple stages of filtration
• •the fan speed and airflow setting
• •the building’s operating hours and dust exposure
• •whether doors, loading bays or process activities change the outside air load

The filter manufacturer’s instructions are the first reference point. If the building has a mechanical services engineer, their guidance should also be followed, especially where the installation forms part of a broader ventilation strategy. For some sites, AS 1668.2:2024 may be relevant to the ventilation design, but site requirements should be confirmed with the mechanical services engineer or building owner.

If you need replacement filters or a better fit for the housing, see our products or custom air filters.

Practical checks when the reading changes

A gauge reading is most useful when it leads to a clear inspection routine. If the pressure drop is higher than expected, or if the system is not performing as it should, these checks are a good starting point.

Check the filter installation

Make sure the filter is seated properly, the frame is intact and the seals are forming an even fit. A poor seal can let unfiltered air pass around the media and also distort the pressure reading.

Check the gauge or sensor lines

Blocked, kinked or disconnected pressure tubes can give misleading results. A damaged gauge can also make a healthy filter look overloaded, or vice versa.

Check for upstream contamination

In some units, the pre-filter or intake area carries the heaviest loading. If the pre-filter is blocked, the main filter may see more resistance than expected.

Check the surrounding conditions

Construction dust, process dust, fibres and outdoor debris can all affect loading rates. If a site has changed its operations, the filter monitoring approach may need to change too.

For ongoing support, filter servicing and commercial filter servicing can help with inspection, replacement and system upkeep.

Choosing the right filter for the reading you are seeing

If a make-up air unit is regularly reaching high pressure drop sooner than expected, the issue may be a mismatch between the filter and the site. The right choice depends on the unit, the dust load and the level of filtration required.

Useful considerations include:

• •particle size and type in the air stream
• •whether the system needs a coarse pre-filter, finer final filter or both
• •the available space in the housing
• •air velocity through the filter face
• •access for maintenance and changeout

In Australia, you may still hear filters described with older EN 779 classes such as G3, G4 or F5 to F9, even though ISO 16890 is now widely used. These ratings do not convert directly, so it is best to match the filter to the actual application rather than rely on a simple equivalent. If the system requires high efficiency filtration, HEPA options may be relevant; under EN 1822, H13 is at least 99.95% efficient and H14 is at least 99.995% efficient at the most penetrating particle size.

If you are unsure what fits the unit, how to measure an air filter is a useful place to start before ordering a replacement.

When to replace rather than reset

A pressure drop reading can help you decide whether a filter is nearing the end of useful service in that application. But replacement should be based on the condition of the system, not only on a number.

Consider replacement when:

• •the reading remains high after inspection and cleaning of the surrounding components
• •the filter media is visibly loaded, damaged or distorted
• •airflow through the unit is no longer as expected
• •the housing or seals are compromised
• •the filter has reached the manufacturer’s recommended limit for that product and system

Some filters are designed to be washable - check the product instructions before attempting to clean them. For most systems, though, the safer approach is to follow the filter manufacturer’s guidance and the site’s maintenance procedure rather than assume all filters can be reused.

Need help matching the filter to the reading?

If you are managing a make-up air unit and the pressure drop readings do not make sense, Total Air Filtration can help supply or make the right filters for the job. We are a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, and we work with commercial and industrial applications across a range of sectors.

Call 07 3800 3448 or request a quote. You can also explore industries we supply for more context on the types of sites we support.