The ventilation systems used in Passive Houses provide unparalleled indoor air quality through the use of a high quality, F7 filter at the suction point (the unit must also be equipped with a drain). During heat recovery, the exhaust air must not mix with the supply air. Due to reasons of hygiene, a humidifier within the ventilation unit is not possible. It is important to remember that Passive Houses utilise ventilation systems, NOT air necessarily conditioning systems.