Estimating ventilation rates in school classrooms from time series CO2 measurements

This study evaluates the applicability of two analytical (steady-state) and one numerical (transient) method for estimating ventilation airflow rates based on time-series CO₂ measurements. Over the course of a year, CO2 concentrations were monitored in two Austrian school classrooms - one with natural ventilation and one with a decentralised mechanical ventilation system. The results show that in mechanically ventilated environments with stable CO₂ levels and continuous ventilation, all three methods produce comparable results. However, in cases with intermittent ventilation events, such as naturally ventilated classrooms or mechanically ventilated class-rooms supplemented by window ventilation during warmer weather, signif-icant differences were observed between methods. The steady-state method using daily mean CO₂ concentrations yields ventilation rates up to 35% higher than the steady-state method based on the 95th percentile. The transi-ent model produces similar estimates to the steady-state method using daily mean concentrations but captures short-term fluctuations more effectively. The findings indicate that steady-state methods based on the 95th percentile effectively estimate baseline ventilation rates provided by air handling units but fail to reflect dynamic air exchange variations. In contrast, transient and steady-state methods using daily mean CO₂ concentrations offer a more ac-curate assessment of user-driven ventilation behaviour. Further work is needed to improve the numerical limitations in the transient solution in-cluding the identification of transient occupancy changes.