Good Filtration

1. Intake (within the MVHR unit): G4 filters or M5/ F5 filters are supplied as standard, but it is generally recommended to use a finer filter of F7 grade, also called pollen filter. These help to keep the supply ductwork clean, as they filter finer dust and particles and bacteria and spores out of the incoming air. PAUL now offeres new F7 filters, which have better air flow properties and can be more easily cleaned.
2. Extract (within the MVHR unit): Typically a coarse dust filter of G4 grade is being used.
3. For areas of high traffic pollution, we can provide active charcoal filters for the supply air, that bind dangerous NOx gases.
4. Kitchen extract (in the room extract point): Typically a dust & grease filter of G3 grade is being used.

Most of our modern systems are volume flow constant, which means that they maintain the same airflow and keep balanced, even when filters start to clog up. When the filters are very soiled, the system will become louder, as the fans have to compensate for the increased resistance. Keeping the filters clean will help to keep the system silent and to maintain a low energy usage.

Smells from a neighboring wood stove or traffic can be a nuisance, when drawn into an MVHR system. The only way to protect yourself from such smells are active charcoal filters. These bind the organic compounds, instead of filtering them out.

If you have the Zehnder ComfoAir Q unit, there is also an option to insert an active charcoal filter into the MVHR unit (instead of the F7 intake filter).

Otherwise a filter box can be added either between MVHR unit and the supply duct work, e.g. manifold, so that the MVHR intake filter serves as a pre-filter.

If there is no space in the supply leg, it could also sit on the intake duct run from the outside. In this case, care should be taken that the filter box is insulated, to prevent condensation and minimize heat losses.

If a filter box is added to a non volume-flow constant MVHR system (most cheaper MVHRs), then the MVHR needs to be re-commissioned, as the flow rates will have dropped through the increased resistance of an additional filter.

Viruses are around 0.1 microns in size, and at that size no standard filter can help. However, viruses are normally bonded to something larger, e.g. droplets. “There is never a naked virus floating in the air or released by people,” said Linsey Marr, a professor of civil and environmental engineering at Virginia Tech who specializes in airborne transmission of viruses. The virus attaches to water droplets or aerosols (i.e. really small droplets) that are generated by breathing, talking, coughing, etc. These consist of water, mucus protein and other biological material and are all larger than 1 micron. “Breathing and talking generate particles around 1 micron in size.”

F7 filters filter out at least 65% of 1 micron sized particles.

MVHR systems that bring in fresh air from the outside filter out most of the air pollution, but mostly clean out the indoor air pollution, as well as air borne bacteria and viruses through the constant air exchange and extraction. The risk of bringing in bacteria and viruses from the outside air is negligible. Air intake terminals for MVHR systems are typically more than 2m above ground and normally not along public paths.

Filters are classified in different grades, which capture particles of different sizes. Whereas the old standard EN 779 tests arrestance of only one particulate size, the new standard ISO 16890 determines arrestance over a spectrum of particulate sizes.

Filter classification according to ISO 16890:

How does the old standard of EN779 relate to the new ISO 16890: