## 3 Results  
### Ventilation Rate  
#### 3.1.1 Controlled Tests  
Several controlled tests were performed, two on the 10 th of September and three on the 15 th of October 2021. The monitors were positioned along a diagonal of the room, 0.7 m above the ground. The selected measurement points are described in Figure 2. Monitors AQ1, AQ2, AC1 and AC2 were set in positions P1, P2, P3 and P4 respectively, except for tests 1 and 2, in which monitors AQ1 and AQ2 swapped positions. Table 1 shows the ventilation configuration for each test. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Test Configurations** & \\ \hline
**Test** & **Open to Exterior** & **Open to Corridor** \\ \hline
1 & V2 & — \\
2 & V2 & V5 \\
3 & V1 + V4 & V5 \\
4 & V1 & V5 + door \\
5 & V2 & V5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Controlled tests configurations. Windows are labelled as described in Figure 2.  
The CO\({}_{2}\) decay series registered at each point were adjusted to the model given in Equation (5). A non-linear least squares method was used, with the following restrictions  
\[\left\{ \begin{aligned} 0\text{ h}^{-1}&\leq \text{ACH}&\leq\qquad 30\text{ h}^{-1}\\ 350\text{ ppm}&\leq\quad C_{\text{eq}}& \leq\qquad 500\text{ ppm}\\ C_{1}-20\text{ ppm}&\leq\quad C_{\text{o}}& \leq C_{1}+20\text{ ppm}\end{aligned} \right. \tag{13}\]  
where \(C_{1}\) is the first data point in the adjustment window. The restrictions imposed to \(\text{ACH}\) were observed not to constrain the results. The restrictions imposed to \(C_{\text{eq}}\) are reasonable as there were no occupants during the decay tests. Finally, the restriction imposed to \(C_{\text{o}}\) was selected to relax the initial condition value, which otherwise could have been just selected to be \(C_{1}\). The parameters adjusted are shown in Table 2, as well as the RMSE of the adjustment. Furthermore, the mean (\(\overline{\text{ACH}}\)) and standard deviation (\(s_{\text{ACH}}\)) of \(\text{ACH}\) for each test are included. Additionally, Table 3 provides the mean values for indoor air temperature (\(T_{\text{in}}\)) and relative humidity (\(\text{RH}_{\text{in}}\)) measured by the AC1 and AC2 sensors during the controlled tests. We also provide the mean values for outdoor air temperature (\(T_{\text{out}}\)), relative humidity (\(\text{RH}_{\text{out}}\)), wind speed (\(U\)) and wind direction (\(\varphi\)) registered by a nearby (approximately 800 m) environmental station (Aeroqual AQM10 Environmental Monitor). These values are provided in order to further characterise the analysed scenarios, but evaluating their impact on the ventilation rate is beyond the scope of the research. The results show that the classroom can attain very high ventilation rates, well above ASHRAE's requirement and WHO's recommendation. Furthermore, there is a significant difference between the ventilation rates attained with unilateral (test 1) and crossed ventilation (tests 2-5). Ventilation rates in a crossed scheme are an order of magnitude higher and comply with the ASHRAE requirements and WHO's recommendation. It should be noted that during the controlled tests a relatively small portion of the total window area was actually open. Opening only half of an exterior window and the awning window attained more than 10 \(\text{ACH}\) (tests 2 and 5). Figure 2: Floor plan of the classroom. The sensors positions P1 to P4 are identified and windows are labelled. Windows V1 to V4 connect to the outdoor playground and window V5 connects to an interior corridor. Figure 3 illustrates the difference found between unilateral and crossed ventilation. It shows the measured CO\({}_{2}\) concentration series and the adjusted box models curves for tests 1 (unilateral) and 2 (crossed ventilation). The concentration decay speed with crossed ventilation is approximately four times larger than with unilateral ventilation.