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How to select the correct AHU Filter
Expert commentary #2
Indoor Air Quality
Air filtration plays a significant role in maintaining optimum indoor air quality (IAQ), which is more essential than ever in view of COVID19. However, the growing attention the market has placed on air filtration has also given rise to some misinformation surrounding the air filter selection for air handling units (AHUs).
Let ́s get the facts straight regarding AHU filter selections
Regional manager, Systemair Sweden
At Systemair, we advocate for looking at air filters in the context of the whole system as having a holistic approach is key to ensuring proper IAQ.
The focus should be on ensuring the right filter application considering the entire application and the specific requirements of the project, because the filter itself is not the only solution of importance for preventing virus exposure. A more integrated approach will help provide the correct ventilation, deliver quality IAQ and mitigate the risk and spread of infection.
We should also never forget why we install the filters in the first place, which is to ensure the health of the building inhabitants. However, a good, quality air filter can have high filtration efficiency and good energy efficiency. One factor doesn’t have to negatively influence the other.
We should never forget why we install the filters, which is to ensure the health of the building inhabitants.
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How can you best decide on the right filter selection?
To ensure proper selection of air filtration, customers can also refer to accepted standards and recommendations by international bodies, which provide comprehensive information backed by data.
An important reference point is the industry recommendation Eurovent 4/23 (2020) on the selection of EN ISO 16890 rated air filter classes for general ventilation applications. The Eurovent recommendation introduces a method which matches the recommended minimum filtration efficiency with both the outdoor air and supply air category, as part of efforts to simplify selection procedure, while considering all relevant factors.
The method also refers to limit values recommended by the World Health Organization (WHO) in order to maintain consistency on an international level. Minimum filtration efficiencies recommended in this document refer to various PM particle size ranges, depending on the specific application requirements.
Is there a certification programme for air filters?
Yes. We also urge customers to look at the Eurovent Certification for Air Filters. This accredited third-party certification verifies the performances of air filters both in terms of filtration and energy efficiency. It also offers a handy labelling scheme, allowing users to easily identify the best filters.
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When are HEPA Filters right for you?
Keeping these recommendations in mind would help customers ensure they are implementing appropriate solutions and not always relying on installing High Efficiency Particle Arrestance (HEPA) filters, which are typically chosen for healthcare applications or in projects where there is a high hygiene requirement. HEPA filters can thus be an over-engineered solution for general ventilation applications. In many cases, high quality filters as recommended per manufacturers’ or maintenance instructions for a particular unit tend to be sufficient - provided the correct selection, installation and maintenance processes are observed.
While HEPA filters offer good filtration capabilities, it cannot be arbitrarily installed in every air handling unit (AHU). Most standard AHUs are not designed for HEPA filters and in the event HEPA filters are used, it is often recommended to use a pre-filter to prolong the lifetime, when used for supply air installations. Even with the use of novel filtering materials, air passes through the HEPA filter with significant pressure drop. The pressure values rise as filters get clogged during operations.
Considering Corona viruses exist in diameters of around 120 nanometers (1000 nm = 1 μm), the HEPA filters are capable of blocking the virus – in the event that it reaches the air handling unit at all. In reality, there are other factors to take into consideration that impact the IAQ. In this case, for example, the relative humidity has an impact.
The use of HEPA filters is also justified if a large part of the room air gets recycled. In such cases, the concentration of the particles in the room air can be reduced. However, the air in the room, as well as the surfaces, can still be contaminated by virus particles from infected people in the room.
It is also important to remember that HEPA filters drive high energy cost. As such, there is a need to support the market with clear, accurate and reliable information to ensure selected and installed air filters positively contribute to the health of building inhabitants, without putting unnecessary burden on energy consumption.
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Which general recommendations can you follow?
As part of a holistic approach, we also urge stakeholders to consider the following recommendations on maintaining healthy IAQ from the team of experts at Systemair:
Extend ventilation system’s operating time. It would be ideal to run the system 24/7.
Optimise ventilation by increasing system’s volume flow rate, while ensuring it does not interfere with reaching recommended humidity level.
Maintain minimum humidity levels at a minimum of 40% and not exceeding 60%.
Adjust air volume controls.
Set recirculation according to operation mode.
Refer to the Eurovent recommendations on the selection of air filters.
Ask us if in doubt
If you are ever in doubt, ask your supplier for advice. At Systemair, our experts are always at your service to meet your specific requirements.