Use two stages filtration for Air Handling Unit (AHU) in office buildings


Malpractices of application of air filters
(2) Use metal mesh filter for Fan Coil Unit

Pre-filter (metal mesh or panel filter) with primary filter (bag filter or panel filter) are commonly used on AHU at office buildings in Hong Kong. It is believed that the pre-filter could extend the working life of primary filter at downstream. In fact, “It is rarely economical to place a coarse filter in front of a fine filter. Not only will this increase the total filter pressure drop, but it will usually only slightly increase the life of the fine filter” (AIVC Technical Note 65, International Energy Agency”.

 For a coarse filter, like metal mesh filter, to work to its desired performance, momentum contributes to its filtration effect. 

For this reason, coarse pre-filter is not suitable for a system with a considerable variation of air flow rate like variable air volume (VAV) system.

Air filter constitutes a large portion of pressure drop of a ventilation system. With reference to the Building Energy Code 2021, the system fan motor power required for a constant air volume air distribution system for a conditioned space should not exceed a limit of 1.6 W per litre per second (L/s) of supply system air flow and 2.1 W per L/s for a variable air volume air distribution system. 

From perspective of energy saving, it needs to be carefully considered if two stages filtration with a pre-filter is really effective and economical.


In fact, it has been proven in some landmarked buildings in Hong Kong that a single stage filtration with mini-pleated filter (MERV 13-14) installed on an AHU of a VAV system has even marginally improved the IAQ and significantly saved fan energy due to a lower system pressure drop. At the same time, there is no noticeable effect on working life of the filter. It can still last for 12 months’ time or even longer upon the air filter pressure drop reaches the recommended final pressure drop of 250-350 pa.

Proven case ‐ Pre‐filter and Cartridge Filters ‐> mini‐pleated filte

While from ESG perspectives, it reduces consumption of water for routine cleaning of pre-filter and cooling coil that saves precious natural resources. In addition, NanoFIL mini-pleated filter has an antiviral efficacy of over 99% against COVID-19 Omicron Variant Virus as certified by Virology Research Services (UK) and antibacterial efficacy of 99.99% for various spices of bacteria commonly found in air as certified SGS, it protects people against pandemic risk in workplace.


Apart from ESG, replacing pre-filter + primary filter together with one stage mini-pleated filter could reduce the Total Cost of Ownership (TCO) including the energy costs. It is a major concerns of building owners and operators.


Instead of use of the price as the only criterion for cost analysis, TCO calculator of NanoFIL could help their clients to have a deeper and more accurate cost analysis for evaluating different configurations of air filters. 

Nanofil TCO Calculator

The filter testing rig according to EN779 standard at NANOFIL’s factory could also demonstrate the fractional filtration efficiencies and pressure drop of different types of air filters to their clients in order that they could be assured the performance and specifications of filters they buy are conforming to what they specified. 

Filter Testing Rig according to EN 779

 Challenge the status quo! 

Call us if you want to save energy, reduce maintenance cost 

and improve IAQ.

 Nanofil can help you! 

Email: info@nanofil.com.hk   WhatsApp: (+852) 6273 0573