Abstract: We develop, analyse and validate a multizone model that describes the mechanical ventilation and mixing within an instrumented classroom computer laboratory housing 70 workstations that are used heterogeneously in time and space. The model predicts CO_2 concentration by accounting for air exchange between the classroom and its ceiling plenum by parametrising irreversible mixing. A combined analysis of the model and observation data reveals a flow response that is characterised by a large separation of time scales, which permits a simplified description of the system’s behaviour on the time scales that are relevant to the room’s occupation. The existence of such regimes suggests that the forced exchange of volume between the room and the plenum is `overdriven’ in this instance and that a more efficient operation could be achieved without compromising air quality. Our observations further show pronounced diurnal and seasonal influences on the temperature structure and mixing in the room, highlighting the coupling with contaminant transport.