Energy performance of buildings has been continuously and systematically improved in Europe with next step of transition to nearly zero energy buildings (NZEB) in 2019-2021. Well insulated and airtight NZEB provide challenges or opportunities – depending on point of view – for ventilation systems. Heat recovery ventilation may be expected to be major ventilation solution because in Continental and Nordic climates, it is simply impossible to build nearly zero energy buildings without heat recovery. In warmer climates, co-benefits of heat recovery ventilation units as filtration of pollen and carcinogenic particulate matter (IARC 2014) as well as sound insulation make it very strong alternative to conventional airing solutions, however this existing evidence is not yet widely accepted and understood (JRC 2016). The role of ventilation is twofold, in addition to energy saving contribution the main task of ventilation is to provide fresh air so that indoor air quality (IAQ) and thermal comfort are ensured. In well insulated and airtight buildings, the importance of controlled ventilation is stressed, because air infiltration through building envelope is practically missing and opening of windows during heating season will waste a big amount of energy. In addition to supplying proper air flow rates, balanced operation and compensation of cooker hood, fireplace or central vacuum cleaner have new meaning in airtight buildings, that can be called as a security issue: if not properly designed for instance children are not able to open doors because of high pressure differences.