Impact of night-time ventilation on indoor air quality in kindergartens and schools

In Finland, the public sector employs about 30% of the total employment and the building users include just under a million children and students. Therefore, a good indoor climate in public buildings and the proper use of ventilation systems are important. Night ventilation is commonly used to improve indoor air quality in educational buildings before the premises are used. A typical use has been to turn off the ventilation after using the facilities and restart it about 2 hours before reusing those facilities. Another option is to keep night ventilation running at minimum ventilation. The third option is to use night ventilation intermittently. Although current technology allows ventilation systems to be monitored and controlled using air temperature, carbon dioxide, and presence sensors, it is very common to keep ventilation units running continuously, even if it significantly increases the energy consumption of ventilation. In this study, the pre-started, continuous, and intermittent ventilation strategies were compared by assessing indoor air quality in field measurements during different seasons in 2019 and 2020. The daytime ventilation was kept as usual. Each test period lasted for 2 weeks. Indoor air quality was assessed by measuring TVOC with the metal oxide semiconductor method and microbes by using the quantitative PCR method. Also, CO2, pressure over building envelope, and thermal conditions were measured. The results indicate that the average TVOC concentrations were similar during mornings with all the ventilation strategies. TVOC concentrations were higher during the day than at night. This indicates that the use of the facilities had the greatest effect on TVOC concentrations. The microbial concentration was usually only a few percent of the corresponding outdoor air concentration. The used strategy of night ventilation did not have a systematic effect on indoor microbial concentrations. In general, the natural variation of the measured physical quantities was greater during the test periods than could be observed with different night ventilation operating strategies. The working conditions at the measured buildings were at normal levels. The results show that 2 hours of ventilation before the premises are used is sufficient, and thus continuous ventilation at night is not necessary.