• Filtration

    Filtration

Filtering the intake air serves primarily the purpose of keeping the duct work clean. Additionally good filters also improve the indoor air quality, which studies of dwellings with MVHR have shown (Schneiders 1994, Feist 1995, Flueckinger 1997). These proved that good filters (e.g. F7 grade) on the air intake substantially reduced the concentration of bacteria and spores in the intake air and the room air, compared to the outside air. In summer when windows were opened more frequently, the internal room concentrations of spores and especially bacteria increased again, partially above the external air concentrations, as these increase with the presence of occupants, pets, plants, foods and others.

Filters are classified in different grades, which capture particles of different sizes.

Below is a table of the fractional separation efficiencies of air filters according to DIN EN 779 (ASHRAE test)

Particle size:
↓Grade
:
10µm 5µm 3µm 1µm 0.5µm 0.3µm 0.1µm
G1 40-50% 5-15% 0-5%
G2 50-70% 15-35% 5-15% 0-5%
G3 70-85% 35-70% 15-35% 5-15% 0-5%
G4 85-98% 60-90% 30-55% 15-35% 5-15% 0-5%
F5/M5 >98% 90-99% 70-90% 30-50% 15-30% 5-15% 0-10%
F6/M6 >99% 95-99% 85-95% 50-65% 20-40% 10-25% 5-15%
F7 >99% >99% >98% 85-95% 60-75% 45-60% 25-35%
F8 >99% >99% >99% 95-98% 80-90% 65-75% 35-45%
F9 >99% >99% >99% >98% 90-95% 75-85% 45-60%

The coding G/M/F stands for Grob (German for coarse)/ Medium/ Fine dust. Average results for clean filters. 1µm = 0.001mm / 1000µm = 1mm.

For comparison, here are typical particle sizes of different types of pollution (Source: Camfil):

 Type of pollutant: Particle size:
Visible to the eye >20µm
Particles entering the lung 0.05-5µm
Human hair 20-100µm
Normal dust 20-200µm
Pollen 10-100µm
Plant/ fungal spores 10-35µm
Bacteria 0.35-10µm
Viruses 0.002-0.06µm
Soot 1-100µm
Oil smoke 0.035-1µm
Tabacco smoke 0.01-1µm
Smog <2µm
Cement dust 3.5-100µm
Sinking dust 1-100µm
Suspended atmospheric dust <1µm