Mechanical ventilation refreshes the air in the house continuously, if properly applied and regularly maintained. Don't confuse this with a ceiling ventilator or an extractor hood; a mechanical ventilation withdraws air from the entire house, but does not put fresh air back in. The tenant should take care of this himself by means of natural ventilation.
Modern mechanical ventilation is regulated with a sensor. Most common are sensor that measure the concentration of CO2 or the humitidy, compare this with a benchmark value and subsequently switch on or switch off the ventilation system.
Cleaning and maintaining the filters in a mechanical ventilation system is the responsibility of the tenant. The grid is best cleaned with a brush or with a vacuum cleaner. Sometimes it's possible to take out the grid and wash this with a mix of water and detergent.
Is the ventilation system kept clean and well maintained, but it still doesn't work? Please file a repair request