Maintaining an acceptable indoor air quality of spaces by intentional natural ventilation or intermittent mechanical ventilation with minimum energy use

Ventilation is essential for maintaining good indoor air quality (IAQ). However, many buildings continue to suffer from inadequate ventilation due to a lack of either sufficient natural ventilation or proper operation of mechanical ventilation. During the pandemic of COVID-19, several Chinese guidelines have suggested that the household rooms for isolating infected persons and the classrooms that are not equipped with mechanical ventilation should open the windows for at least 30 min twice a day. This study first investigated whether this way of natural ventilation could ensure an acceptable IAQ. Then, the relationships between ventilation duration, ventilation frequency, and IAQ under different air change rates per hour (ACH) were examined to understand how to optimize ventilation design and operation in an efficient manner. When the ACH is ≥ 6 h−1, an acceptable IAQ can be achieved under the recommended ventilation frequency. An increase in ACH would improve the IAQ and reduce the IAQ difference caused by different CO2 generation rate per unit volume (Gr/V) values. Furthermore, simply increasing single ventilation duration while maintaining constant ventilation frequency is not an effective way to improve IAQ, as the improvement decreases as the ventilation duration is increased. Finally, when total ventilation duration is constant, increasing ventilation frequency can significantly improve the ventilation efficiency and thus IAQ, laying the groundwork for ventilation design that balances IAQ and energy use. The findings provide feasible and economical solutions for ventilation of buildings where conventional mechanical ventilation systems are not affordable.