Assessment of Airflows in a School Building with Mechanical Ventilation Using Passive Tracer Gas Method

Abstract

The focus of this study is to assess the airflows in a school building built in 1963 in Gävle, Sweden, which is subject to energy conservation measures (ECMs) in a forthcoming renovation. Today, the school building is mainly ventilated by several mechanical ventilation systems, which are controlled by a constant air volume (CAV) strategy. Schedules and presence sensors impose a high operation mode during the day and a low operation mode at night, on weekends and on holidays. The homogeneous tracer gas emission method with passive sampling is used to measure the average local mean age of air (τ) during different operation modes. Temperature, relative humidity and CO₂ concentration are simultaneously measured. The calculated relative uncertainty for the average local mean age of air in every measured point is approx. ±20 %. The results during low operation mode show an average value of τ of approx. 8.51 h [corresponding to 0.12 air changes per hour (ACH)], where τ in various zones ranges between 2.55 and 16.37 h (indicating 0.06–0.39 ACH), which is related to the unintentional airflow in the school. The results during mixed operation mode show an average value of τ of approx. 4.60 h (0.22 ACH), where τ in various zones ranges between 2.00 and 8.98 h (0.11–0.50 ACH), which is related to both unintentional and intentional airflows in the school. Corridors, basement and attic rooms and entrances have lower τ compared to classrooms, offices and other rooms. High maximums of the CO₂ concentration in some rooms indicate an imbalance in the mechanical ventilation systems. During a regular school week of mixed operation, which includes both high and low operation modes, it is found that mainly the low operation modes show up in the results. The dynamics of the highly varying airflows in the building cannot be identified using the passive sampling technique.