Center for the Built Environment

CO2–based demand controlled ventilation (DCV) can save energy while maintaining acceptable indoor air quality. CO2 concentration may vary within an occupied space and it is unclear how sensor location influences the ventilation and energy performances. The objective of the present study is to investigate the effect of CO2 sensor position on the performance of DCV systems under mixing and displacement ventilation. Experimentally validated computational fluid dynamics (CFD) models were simulated under representative indoor ventilation and occupancy conditions. The results show that the ventilation strategy, occupancy level, and air change rate have notable impacts on the CO2 sensing performance. Under mixing ventilation, CO2 sensors placed at the room exhaust can meet the requirements of sensor accuracy defined by ASTM E741 and California Title 24. However, the sensor errors associated with sensor location can be higher than the acceptable threshold under displacement ventilation, which exhibits vertical CO2 stratification with two separated zones (lower transition zone and upper uniform zone). The dividing height of the two zones is highly sensitive to the occupancy level. In such cases, exhaust sensors can overestimate the breathing zone concentration and result in additional energy consumptions for thermal conditioning as well as fan operation, especially for densely occupied buildings. The study findings suggest that for ensuring good performance of CO2–based displacement ventilation, it is necessary to develop quantitative relationships between CO2 concentrations at the breathing height and the room exhaust considering ventilation strategies.