How to select AHU air filters: G4, F7 and HEPA explained
A practical guide to filter stages, ISO 16890 classes, pressure drop and sizing for air handling units in Egypt and the Gulf.
Gabtic engineering team · · 7 min read
Start with the job the air has to do
Filter selection starts with the space, not the catalogue. An office needs comfort and coil protection. A hospital ward needs fine particle control. An operating theatre or sterile filling room needs HEPA. Write down the space type, the airflow of each AHU and any standard the project must meet before choosing a single filter.
Use stages: pre-filter, fine filter, final filter
Each stage protects the next one. A cheap pre-filter catches coarse dust and extends the life of the expensive fine filter behind it.
- Pre-filter: G4 panel or washable cell (ISO Coarse).
- Fine filter: F7 bag or pleated filter (ISO ePM1 50–65%) for most commercial buildings.
- Final filter: F9 for hospitals and labs, H13/H14 HEPA for clean rooms and theatres.
Read the class correctly: EN 779 vs ISO 16890
EN 779 (G4, M5, F7, F9) was withdrawn in 2018 and replaced by ISO 16890 (ISO Coarse, ePM10, ePM2.5, ePM1). Many specifications in the region still say F7. As a working guide F7 is roughly ISO ePM1 50–65% and F9 is ePM1 above 80%, but the test methods differ, so always ask for the ISO 16890 test report.
Size for airflow and pressure drop
A 592 × 592 mm face is rated at about 3,400 m³/h for a bag filter. Running above the rated flow raises initial pressure drop and fan energy. Check the AHU frame dimensions, pocket depth that fits the section, and the final pressure drop at which the filter will be changed — typically 200 Pa for G4 and 450 Pa for F7.
Dusty climates need a stronger first stage
In Egypt and the Gulf, sand and fine desert dust load filters fast. A washable metal or synthetic pre-filter at the intake, followed by G4 and F7, usually costs less over a year than replacing F7 bags every few weeks.