Implementing Decentralised Mechanical Ventilation Systems in Existing School Classrooms: A CFD-Based Performance Assessment

The COVID-19 pandemic has renewed focus on the essential role of ventilation in maintaining good indoor air quality and low airborne transmission risks in school buildings, thereby supporting occupant well-being. However, many European classrooms still depend on natural ventilation, which often proves insufficient, especially during the heating season. Consequently, the integration of Mechanical Ventilation Systems (MVSs) with heat recovery in existing classrooms has become increasingly adopted, and decentralised MVS solutions, favoured for their minimal installation impact, have gained particular traction. Yet, despite their widespread implementation, a notable gap remains in the investigations into their air distribution efficiency and overall ventilation performance. To address this gap in the literature, this study offers a systematic assessment of the ventilation effectiveness for air distribution schemes related to decentralised MVSs. Computational Fluid Dynamics simulations were performed in a standard classroom, while statistical methods were used to established significant differences among the schemes. The optimal performances were observed with a single air inlet/outlet positioned on the long side of the room, providing CO2 ≈ 1088 ppm and a mean local air change efficiency of 1.23. Ceiling-standing units installed in the room show CO2 levels around 1200 ppm, with mean local air change efficiency between 1.02 and 1.07, whereas the floor-standing unit yields ventilation effectiveness lower than that of ideal mixing conditions. The results deliver an actionable ranking and design trade-offs to guide school retrofits.