Demand-controlled ventilation of classrooms is the basis of the concept of ‘Frisse scholen’ (literally ‘Fresh schools’). Each classroom has a CO2 absorber which ensures a higher supply of fresh air when the CO2 concentration in the room increases. The ventilation capacity of the air treatment cabinet is attuned to the occupancy of the room. For each pupil, 30m3/h of air is refreshed with a maximum occupancy. Less pupils mean less ventilation and therefore less energy consumption for the heating or cooling of the ventilation air. In this way, there is never too much ventilation and the air quality remains guaranteed.