If the ventilation unit is to ventilate a dwelling that is used to varying degrees, it can be a good idea to control ventilation via a CO2 sensor.
Without a CO2 sensor in the dwelling or the rooms, the unit will ventilate at a low level. If the CO2 level increases in the dwelling/rooms because more people are gathered, candles are lit or other factors impact negatively on the indoor climate, the CO2 level in the air will cause the unit to increase ventilation.
That way you do not have to increase the ventilation level manually when a higher degree of ventilation is required. Also, you do not have to remember to lower ventilation again later, as the control system of the unit will do this automatically.
The CO2 sensor is retrofitted in the ventilation unit.