Research project

Investigation of the filter effectiveness and the indoor particle concentrations with alternating residential ventilation systems

Building envelopes must be airtight as per the standards of the German Energy Saving Ordinance (EnEV) and therefore allow for a minor natural ventilation only. To achieve the required hygienic minimum air change, mechanical ventilation is necessary in most cases. Alternating residential ventilation systems are an easy and cost-efficient possibility to cover the ventilation expenditure. The equipment is suitable mainly as an easy and cost-efficient solution for building refurbishment. There are no expensive air duct systems with complex installation required but only a few drilled holes for the positioning of the equipment in the outside wall and the circuitry.

Per housing unit, two or more devices are installed. The operating mode of the equipment is alternating: one device provides the supply air in the room in one phase and the same device switches to the extract air mode in the next phase. Thus, heat recovery is possible, with a regenerator in the form of a ceramic unit. Depending on the flow direction, the heat is recovered behind or in front of the ventilator. Based on today’s knowledge, this principle is energy-efficient.

Besides permanently installed indoor air purifiers, since 2020, these systems have been more and more used in the context of Covid-19 and the ventilation of rooms with several frequent occupants. The systems reduce the aerosol concentrations in rooms and the risk of an infection with the Covid virus. The advantage compared to the majority of commercial air cleaners with circulating air treatment is the fact that outside air is supplied and carbon dioxide is dissipated at the same time. Overconcentrations of carbon dioxide in rooms should be avoided due to the reduction of the ability to concentrate, etc. Frequent manual window ventilation is not necessary for these systems.

The easy and cost-efficient possibility to install alternating residential ventilation systems in refurbished buildings unfortunately includes also some disadvantages. In conventional residential ventilation equipment with constant flow direction, the filters are always loaded from one side (e.g. from the room side).

For alternating systems, the filter unit is loaded and potentially unloaded with particles on both sides due to flow reversals. It is currently uncertain if settled particles during airflow direction changes adhere to the filter or if they are released again. This issue raises critical questions regarding air quality and shall therefore experimentally investigated in this research project.

The objective is to gain insight via experimental investigations how settled particles in a filter of alternating residential ventilation systems behave upon airflow reversal. In a test bench to be specifically planned for this research topic, particles shall be generated in a controlled manner and their concentrations and size distribution shall be measured, resulting in answers to the issue raised above. If an assumed particle release turns out to be a weak point, the equipment manufacturers shall receive recommendations for the development of optimized filter units.

Draft of test bench

Project duration

07/2021 – 06/2023 (two years)



Teufel-Stiftung, Tuttlingen, Germany


Project partner

Institute of Combustion and Power Plant Technology (IFK), University of Stuttgart, Germany


This image shows Lukas Siebler, M.Sc.

Lukas Siebler, M.Sc.


Academic employee

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