Abstract: Mechanical ventilation could effectively manipulate the air quantity and quality that enter the room, while building air tightness is the key factor affecting its ventilation performance. This study utilized carbon dioxide as a tracer gas and investigated the removal effect of unidirectional and bidirectional ventilation systems on indoor CO₂ under different building air tightness conditions. The experimental results shou that room air tightness directly determines its ventilation effect for unidirectional ventilation system. Under a certain room air tightness, there is an optimal design ventilation volume, below which increasing the air volume has a greater impact on improving the ventilation effect. When the air volume is higher than this, the ventilation effect is less improved by continuing to increase the air volume. For bidirectional ventilation system, the ventilation effect mainly depends on the coupling of air tightness with input-outlet air volume parameters. By optimizing the air volume parameters, the same air exchange effect could be achieved in rooms with different air tightness. To sum up, the influence of building air tightness on the performance of different mechanical ventilation systems is studied, which provides theoretical guidance for the design and optimization of mechanical ventilation systems.

Key words: Building air tightness, Mechanical ventilation, Ventilation performance, Carbon dioxide