# Effects of Night Ventilation on Indoor Air Quality in Educational Buildings--A Field Study
## 4 Discussion
The results showed that the daily TVOC level was higher than the morning level, highlighting the effects of activities in those indoor environments. Therefore, these results suggest concentrating on occupied ventilation time in improving wellbeing indoors. The existing pandemic has caused restrictions on space usage, which increased the demand for ventilation during the occupied hours. However, at night times, educational buildings are empty, and therefore night ventilation is useless in reducing the COVID-19 risk. Another hypothesis was that the indoor humidity cannot be extracted if the ventilation is off. The indoor humidity level is generally determined by the outside air and the internal humidity and airflow. The key issue is the ventilation, and in these public buildings, the ventilation is 2-4 1/h, which means that the premises can be flushed well. The results indicate that night-time air temperature and humidity conditions were rather constant in the measured buildings, and as a consequence, the greatest effect on thermal conditions was usually the sensible heat gain and humidity load generated by occupants during the indoor environment activities. The results reveal that in the morning, all the studied night ventilation strategies proved a similar level of indoor air quality. The continuous ventilation maintained the TVOC concentrations at a low level during nighttime. However, if the ventilation system is operating continuously, the energy consumption can increase unnecessarily related to guarantee healthy and comfortable indoor climate conditions. Therefore, continuous ventilation is not necessary. The ventilation efficiency is related to the air change efficiency [41]. This means that in night-time unoccupied conditions, the ventilation efficiency can be associated closely with the ventilation airflow rate, the supply air temperature, the room size, as well as the air distribution, and the pressure differences over the building envelope and between the spaces. In the spaces measured, the room heights were typical in education buildings (around 3 m), which means that the specific airflow rate described well the performance of the ventilation. In most cases, the air distribution was mixing ventilation from the corridor wall or the ceiling, and for example, displacement ventilation systems were not used in the buildings. The night purging efficiency will increase with the airflow rate. Additionally, the longer thrown patterns enhance mixing and diluting the pollutants more evenly over the total volume. However, the location of the extract is also important. Generally, a good result can be achieved if the short circuit between the supply and extract could be avoided. In larger room sizes, the number of supply and exhaust air terminal devices can be an essential matter. The spatial distribution of those devices may affect the local efficiency in the occupied zone because more devices can provide more uniform conditions by eliminating the differences and stagnant zones. The small airflows are difficult to manage in practice. The Finnish building code suggests that minimum ventilation is 0.15 L/s-m\({}^{2}\) for unoccupied periods. This is difficult to adjust because usually in educational buildings, the minimum airflow of the air handling unit is on the order of 30% from the maximum one, and as a consequence, 0.15 L/s-m\({}^{2}\) can be much lower than that. Moreover, the small airflows may not be distributed in ductworks as designed, because those have been sized to greater ventilation airflows. In addition, the control devices are not able to control the small airflows, because the minimum speed of measuring devices is on the order of 1 m/s and the minimum airflow may provide the speed well below that. Besides, ventilation efficiency is poor with minimum airflow providing low throw lengths in mixing ventilation systems. However, the mixing in the occupied zone may be improved by using a non-isothermal cool supply air (e.g., 18 \({}^{\circ}\)C). That is proposed to be used with minimum ventilation.