Uncertainty analysis of various CO₂-Based tracer-gas methods for estimating seasonal ventilation rates in classrooms with different mechanical systems

Indoor air quality (IAQ) data of 220 classrooms in the Midwestern region of the US were measured during 2015–2017. During three seasons (fall, winter, and spring), indoor and outdoor CO₂ concentrations of each classroom were measured under both occupied and unoccupied conditions. To calculate ventilation rates from the collected CO₂ data, three main methods derived from the mass-balance equation were used: (1) steady-state; (2) decay rate; and (3) build-up. Since the uncertainty of the individual measurement parameter affects the accuracy or reliability of the calculated, an uncertainty analysis was performed for all three methods. The uncertainty of the estimated ventilation rates in relation to the volume of the classroom, indoor and outdoor CO₂ levels, and estimated student CO₂ emission rates were determined. The study shows that the steady-state method has the least uncertainty in ventilation rate calculations, while the decay and build-up methods had the lowest and highest values for ventilation rates, respectively. The results also show the estimated ventilation has a larger variance in single-zone systems than in multiple-zone systems. In field measurements, CO₂ readings contribute to the largest portion of uncertainty for all three methods. Therefore, with CO₂ as the tracer gas, improving the accuracy of CO₂ measurement should receive the highest priority for study.