The most important substance in indoor air for human well-being is oxygen. However, the carbon dioxide (CO2) produced during respiration and other processes is measured. CO2 is what constitutes "used air": Oxygen that is "good" for humans has been converted into carbon dioxide that is unusable for humans. Too much carbon dioxide leads to tiredness and lack of concentration and can cause headaches in sensitive people. Fatal CO2 concentrations are not to be feared in living rooms, but the situation is different in fermentation cellars, for example. There, CO2 measurement is mandatory.
The decreasing performance with increasing CO2 concentration makes the topic particularly relevant for companies and schools. At the workplace, everyone should have a clear head, therefore the CO2 value must be below the MAK (maximum workplace concentration according to DIN 1946) of 1500 ppm. This value is already very high, 500-1000 ppm are recommended. For comparison: fresh air has 350-500 ppm.
The second group of substances that is interesting indoors are mixed gases (VOC, volatile organic compounds). On the one hand, mixed gases include harmful substances originating from adhesives, paints, building materials and cleaning agents. These occur in harmful concentrations especially in new buildings and conversions. In the ideal case, the problem is reduced from the outset by the choice of building materials.
In every building, the mixed gases are also recognizable as "stale air“. One does not want to have unpleasant odours of perfume, sweat, pets, food or tobacco in the room. In rural areas, additional odour pollution comes from agriculture, in the city from exhaust fumes.
In smart buildings, sensors measure the current concentration of CO2 or VOC. They should be installed in every room used for a longer period of time (living room, kitchen, bedroom, office...). Ideally, they should measure at head height to obtain the relevant concentration. Also, the sensor should not be placed in a draught, for example not at the door to the corridor, because there the values are falsified and usually better. It makes sense to measure directly where you are, for example in the corner with the couch.
An intelligent sensor, for example for the KNX system, also controls the ventilation. This can be so-called natural ventilation via windows, which are opened and closed by motors. Or a ventilation system that takes care of the ventilation. From a technical point of view, PI control is used here, which constantly determines how much readjustment is required (fresh air supply) in order to achieve the target state (setpoint).
Of course, opening the window only helps with carbon dioxide and mixed gas sources in the interior. Ventilation should be prevented if there are odours from outside. In polluted areas, an outdoor VOC sensor may even be worthwhile.
Ventilation also plays an important role in the air conditioning of indoor spaces, as outdoor air is usually cooler and drier than the air in a room used by people. Ventilation is therefore also necessary if the temperature and humidity are too high. A ventilation block is effective for the few cases where this is not the case, especially when the supply air is too warm in summer and too cold in winter. Too cold supply air can, for example, cause damage to house plants or lead to condensation on building parts. In the latter case, the dew point indicates when things become critical.
In summer, night recooling is often used, in which the building is cooled down considerably by night alone via windows, skylights and supply air flaps. The cool walls then continue to act as passive components long into the day and reduce the energy required for air conditioning.