## 4 Experiment Result and Discussion  
### Relationship Between Airflow Rate and Pressure Difference  
The indoor and outdoor pressure difference owing to the change in the supply and return airflow of the variable airflow system was obtained through experiments in the settings shown in Table 3 (Case 1-18). The airflow and pressure difference data collected through experiments are shown in Figure 10, which shows the change in the indoor-outdoor pressure difference with a change in the supply airflow based on the presence or absence of ventilation airflow. When the ventilation fan speed was operated at a constant 20 Hz, the ventilation airflow was approximately 1900 CMH, and as the supply airflow was varied from approximately 100 to 2800 CMH, the indoor-outdoor pressure difference varied from \(-10\) to 8 Pa. When the supply airflow increased from approximately 100 to 2800 CMH with the ventilation fan off, the indoor-outdoor pressure difference was approximately 1-24 Pa. When the supply airflow was the same, the indoor-outdoor pressure difference differed by an average of 11 Pa depending on the return airflow rates. This indicates that the pressure difference generated in the building envelope should be considered as a combination of the supply and return airflow rates. Therefore, the graph of the indoor-outdoor pressure difference as a function of the air supply and exhaust airflow of the air handler was plotted (Figure 11). As the indoor supply and return airflow difference increased, the indoor-outdoor pressure difference also increased, exhibiting a linear and proportional relationship. To further understand and quantify this relationship, linear regression analysis was used, which is particularly well-suited for identifying direct, proportional relationships between variables. By applying linear regression, the correlation between the supply and return airflow difference and the indoor-outdoor pressure difference was analyzed, and the coefficient of determination (R\({}^{2}\)) was calculated, yielding a value of 0.9406. This high R\({}^{2}\) value, which approaches 1, indicates that the model effectively captures and describes the actual relationship between the variables. Using linear regression in this context allows for a clear interpretation of the strength and nature of the relationship, confirming that as the supply and return airflow difference increases, so does the indoor-outdoor pressure difference in a predictable, linear manner. This suggests that there is a correlation between the supply and return airflow differences of the air handler and the indoor-outdoor pressure difference. The relationship equation derived from the regression analysis can be used to predict the indoor-outdoor pressure difference when the supply and exhaust airflow of the air handler have been determined. Figure 12 shows the difference between the indoor and outdoor pressure difference predicted by the derived relationship when the air conditioning system is automatically operated and the value measured by the pressure difference sensor installed in the test bed. The error between the data collected by the pressure difference sensor from 10 am  
Figure 10: Comparison of the pressure difference between supply and return airflow rates. to 4 pm and the indoor-outdoor pressure difference predicted by the supply and return airflow of the air handler at the same time was analyzed. The error values of the pressure difference sensor and the predicted values were similar, ranging from 0.002 to 3.97 Pa, with an average error of 1.18 Pa. Next, the cvRMSE, an important metric for evaluating the performance of a prediction model, was calculated. The cvRMSE of the model was 18.97%, which is lower than the limits specified by the ASHRAE (\(\leq\)30%) or IPMVP (\(\leq\)20%) guidelines, indicating that it meets the conditions specified in the guidelines [39; 40].