Since the appearance of building regulations relative to HVAC Installations in the different countries is necessary to control the ventilation depending on the building use and occupation thereof, for example, fitness centers, theaters, cinemas, malls, restaurants ...But in practice has shown that not only in this type of premises and buildings need ventilation control for reasons of energy saving in air conditioning and ventilation systems but for health reasons in office buildings and hotels by what is known as sick building syndrome. Due to the architecture of certain buildings, for example glazing facades not practicable, it is necessary an optimum ventilation system for the right internal conditions of temperature, humidity and CO2 in different areas. Carbon dioxide gas is difficult to detect but when their concentration is high, for example due to its expiry for influx or concentration of people in a room, causes feelings of fatigue and discomfort when these high concentrations are prolonged. 

Usually the ventilation requirements of a building are designed with a maximum occupancy pattern, resulting in most cases during the operation of the facility to a waste of energy where the ventilation is constantly working at maximum flow. This is aggravated by having different patterns of occupation during the operation of the building, by not considering the HVAC system-plant the occupation thereof or zoning places... It is therefore the use of CO2 sensors which measure gives us the occupancy of a room, and depending on the needs of this ventilation and air renewal, it is necessary both in terms of energy savings and health of indoor air quality. So the cost of heating and cooling fresh air can become the most significant cost of HVAC system operation. Extended and flexible work hours, and increased use of schools and public buildings beyond traditional business hours add additional cost to HVAC system operation.

http://www.enocean-alliance.org/en/enocean-co2-sensor-iaq/
 

CO2 MODBUS Analogue sensor datasheet

co2_enocean_868_24Vdc_datasheet

co2_enocean_868_230Vac_datasheet