# Effects of Night Ventilation on Indoor Air Quality in Educational Buildings--A Field Study
## 4 Discussion
If the minimum ventilation is desired, there should be a separate system installed, but it seems rather unjustified according to this study. In this sense, the maximum intermittent ventilation can produce a slight advantage, although it may provide pressure differences while synchronizing different air handling units. The intermittent ventilation increased the night-time TVOC-concentrations at the highest level. This can be due to unbalance of supply and exhaust airflow rates. Generally, similar time schedules of air-handling units and exhaust fans are important. The airflow balances are recommended to be checked in the building commissioning phase and to ensure that the contaminants do not spread due to pressure differences between different zones. This means that the airflow measurements are suggested to be carried out in different control modes and paying special attention to the commissioning of VAV systems. The settled dust samples indicate that the microbe levels were rather typical and the systematic conclusions cannot be drawn between the ventilation strategies during unoccupied periods, and there was no statistically significant difference between different ventilation strategies. Analyzed microbes were selected to describe large microbial groups rather than individual species. These qPCR assays are generally used to measure microbes in indoor air studies. Parallel results that were collected from one room were very similar. Blank samples and positive and negative controls in each step in the laboratory were also used. Problems with the Gram-positive bacterial assay from outdoor samples have been found earlier. The reason is unclear, but these problems can be due to inhibiting material e.g., organic material or pollen in outdoor samples. Problems were not seen in indoor samples. The results showed that intermittent ventilation provided a slightly lower level of microbial concentrations than those other methods. However, the reliable reason for that was not recognized in the analysis. One possible reason could be that the indoor dust level can change during the 6-week measurements, or the more effective cleaning or lower activity in spaces can gradually decrease the amount of settled dust in indoor environments during abnormal conditions for space users in the awareness of measuring instruments. The bacterial content is highly affected by the occupants and their indoor activities whereas the fungal content is affected more by the outdoor fungal content. Therefore, most probably, the night ventilation strategy did not have a considerable effect on those results. Hypotheses about the use of night ventilation are useless when using effective pre-started morning ventilation. In schools and kindergartens, the energy use of ventilation can be the largest single factor, e.g., higher than the heat loss of the building envelope, and thus by optimizing the night use of ventilation, the building owners can keep energy consumption at a reasonable level. The energy consumption increases with ventilation operation time and airflow rates. It is energy efficient to use night ventilation only when necessary to ensure acceptable indoor air quality to individuals at the beginning of the space usage. However, healthy and comfortable indoor climate conditions should be guaranteed during the occupied hours because IAQ and thermal comfort are closely related to the wellbeing and performance of pupils and teachers in education buildings. Energy consumption can be reduced by operating night ventilation continuously with small airflow rates, but then the ventilation efficiency could be poor. Therefore, the pre-started ventilation with larger airflow rates could be a reasonable selection for night ventilation. This is supported by the knowledge that indoor air will typically change several times per hour. The recommendations on further studies are associated with the performance of advanced air distribution methods as well as the night ventilation effectiveness in diluting and removing contaminants.