Static and Dynamic Pressure Considerations for HEPA Retrofit Projects

When a filtration retrofit moves a system toward higher-efficiency filters, the biggest mistake is to look only at the filter frame size and media rating. In practice, the change can alter static pressure, airflow and fan operating point enough to affect the whole air-handling system. For mechanical contractors and facility managers, the right first step is a pressure and capacity review before any filtration stage is changed.

Why pressure matters in a retrofit

Every filter adds resistance to airflow, and that resistance changes as the filter loads with dust. In a retrofit, the new filtration set-up may include deeper media, finer prefilters, tighter seals or a HEPA stage, each of which can increase system resistance compared with the original arrangement.

That matters because a fan does not deliver the same airflow at every pressure condition. If resistance increases beyond what the fan can comfortably handle, airflow can fall, balance can shift between branches, and equipment may run outside its intended range.

A retrofit should therefore begin with a simple question: can the existing fan and duct system support the new pressure drop while still moving the required air volume?

Static pressure and dynamic pressure in plain terms

Static pressure is the resistance the fan must overcome to push air through filters, ductwork, coils, dampers and terminals. Dynamic pressure is tied to the velocity of the air moving through the system. In day-to-day retrofit planning, static pressure is usually the main concern because filters directly affect it.

A HEPA retrofit can change both the total resistance and the way air moves through the system. If the filter bank is not designed for the new load, the fan may operate at a point where airflow is lower than expected, noise is higher, or control dampers are forced to work harder to compensate.

For a site that already has tight ductwork, long runs or heavily loaded prefilters, even a modest increase in resistance can be significant. That is why the existing fan curve, motor capacity and system controls should all be reviewed before any change is made.

What to check before changing filtration stages

• •Existing fan curve and motor operating point
• •Current filter bank resistance and the expected resistance of the proposed filters
• •Whether there is enough fan margin for loading over time
• •Ductwork condition, dampers, transitions and any bypass paths
• •Coil and terminal pressure losses that also compete for fan capacity
• •Whether the control system can maintain airflow after the retrofit

HEPA retrofits need more than a filter swap

HEPA filters are designed for very high particle capture, but they are not a drop-in choice for every system. H13 filters are at least 99.95% efficient at the most penetrating particle size under EN 1822, and H14 filters are at least 99.995% efficient at the same point. That level of performance can be useful in the right application, but it usually comes with higher resistance than standard filtration stages.

Because of that, a HEPA retrofit often needs more than just a new filter frame. The system may need a revised filter housing, better gasket sealing, stronger fan capacity, or a change in upstream prefiltration to protect the final stage from loading too quickly.

It is also important not to assume that a rating from one system translates directly into another. MERV, EN 779 and ISO 16890 do not convert directly, and EN 779 classes such as G3, G4 and F5-F9 are still often referred to in Australia even though ISO 16890 is now widely used. The practical question is not the label alone, but whether the proposed filter set suits the actual airflow and pressure conditions on site.

Common retrofit risks when pressure is not reviewed

• •Reduced airflow after the upgrade
• •Uneven air distribution across the building or process area
• •Fans running closer to their limits than intended
• •Control instability when dampers keep opening further to chase airflow
• •Premature loading of the final filter if prefiltration is not suitable
• •Seal leakage if housings are not designed for the higher-pressure arrangement

Matching the filter stages to the system

A well-planned retrofit usually looks at the whole filtration sequence, not only the final stage. In many systems, a robust prefilter arrangement can reduce the burden on the HEPA stage and help preserve airflow for longer. The aim is to build a filtration train that captures dust progressively, rather than forcing the final filter to do all the work.

For some sites, that may mean using a staged approach with coarse, medium and final filters selected to suit the actual dust load. For others, the right answer may be a custom air filter arrangement designed around the existing housing and available fan capacity. If the system is unusual or space is tight, custom air filters can be a practical way to align filtration with the plant that is already in place.

Some filters are designed to be washable - check the product instructions. Even where that is the case, washability does not remove the need to assess pressure drop, sealing and compatibility with the application.

Signs the existing system needs closer review

• •The current fan is already operating near its limits
• •The building has been altered since the original air system was installed
• •There are frequent filter changes due to heavy dust loading
• •The filter bank has visible leakage, poor seating or damaged frames
• •You are planning to add a higher-efficiency final stage without changing the fan

How to approach a retrofit on site

A sensible retrofit process starts with the equipment in front of you, not the product catalogue. Measure the existing filter dimensions carefully, confirm the housing type, and inspect how the filters seat and seal. If the site already has pressure gauges or differential pressure monitoring, review the readings before changing anything.

It is also worth checking the broader system requirements with the mechanical services engineer or building owner, especially where AS 1668.2:2024 is relevant. Site requirements can vary depending on the building use, plant configuration and performance targets, so the retrofit should be based on confirmed project needs rather than assumptions.

Practical steps that help avoid trouble include:

• •Confirming the available fan capacity before selecting the new filtration stages
• •Checking whether the housing can handle the proposed pressure profile
• •Reviewing prefilter options so the final stage is not overloaded
• •Making sure seals, clips and access doors are in good condition
• •Monitoring pressure drop where gauges are fitted after the retrofit is in service
• •Following the filter and equipment manufacturer instructions for inspection and replacement planning

If you need help identifying the current filter size or housing fit, how to measure an air filter is a useful starting point before ordering replacements.

Planning for long-term service, not just the install

The best retrofit is the one that still performs after dust has built up and the system has settled in. That means thinking beyond the installation day. A filter arrangement that looks acceptable when new may behave very differently once pressure drop rises, seals bed in and the fan moves to a new operating point.

For facility managers, that is why monitoring matters. For contractors, it is why the handover should include the expected pressure profile, the filter sequence used, and any checks the site team should carry out. Inspection frequency depends on the system, site conditions and dust load, so it is better to follow the equipment and filter manufacturer instructions and watch the pressure readings rather than rely on a fixed calendar.

Where a site needs ongoing support, filter servicing and commercial filter servicing can help keep the system aligned with the original design intent.

Talk to Total Air Filtration about retrofit-ready filters

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 on practical filtration solutions. If you are planning a HEPA retrofit, we can help supply or make the right filters for the housing, airflow and application.

Call 07 3800 3448 to discuss your project, or request a quote. You can also browse our products or contact us if you want to confirm the best next step for your site.