## 3 Experiment Setup  
### Test Bed  
To analyze the relationship between the supply and return airflow and pressure difference, we selected a university laboratory located in Gyeongsan, Gyeongsangbukdo, Republic of Korea, which is air-conditioned by an air handler that can calculate the supply and return airflow. The laboratory was chosen as the test bed because the supply and return airflow of the VAV system can be precisely controlled through the building automation control system, enabling various experimental conditions to be realized. In addition, it provides an environment similar to that of a real building, increasing the reliability of the research results. In addition, pressure sensors installed on the exterior facing side facilitate pressure monitoring to collect data for accurate leakage estimation. To analyze the effect of air handler-controlled airflow on the indoor-outdoor pressure difference, the experiment was conducted under conditions where the pressure difference due to temperature variations was minimized. During the testing period, the outdoor temperature ranged from 15 to 16 degrees Celsius with humidity levels between 47% and 54%, while the indoor environment maintained a temperature of approximately 20 degrees Celsius and 30% humidity. This setup ensured that the pressure difference caused by indoor-outdoor temperature and humidity disparities remained below 0.2 Pa, providing a controlled environment for consistent data collection and analysis. The test bed is composed of two zones as shown in Figure 5, but the door of the room was opened during the experiment, and it was considered as one zone. The test bed is air conditioned through a variable airflow system, and the specifications of the air handler system are shown in Table 1. The test bed is equipped with four supply air diffusers and four exhaust air diffusers, as shown in Figure 5.  
To determine the airflow supplied and ventilated to the zone and the resulting pressure difference, the airflow and pressure difference should be measured. The change in airflow was measured by setting the fan speed and opening rate using the building automation system (BAS). The configuration screen of the BAS that can control the target air conditioning system is shown in Figure 6. A pressure difference sensor that measures the  
Figure 4: Schematic diagram of leakage prediction method based on supply and return airflow. indoor-outdoor pressure difference was installed on the wall facing the outside of the test bed, and the specifications of the pressure difference sensor are shown in Table 2. The air handler used in the experiment is a variable airflow single-duct type. The variable airflow single-duct system consists of a central air handler, ductwork, and VAV terminal unit to maintain a predetermined temperature by adjusting the supply airflow with changes in the heat load of the test bed. Utilizing measuring equipment, supply and return airflow rates and pressure differences were measured for five minutes for each experimental case. Data was recorded \begin{table}
\begin{tabular}{c c c c} \hline \hline  & & Airflow (CMH) & 12,000 \\  & Supply fan & Static Pressure (mm Aq) & 92 \\  & & Power (kW) & 5.5 \\ \cline{2-4} AHU & & Airflow (CMH) & 9600 \\  & Return fan & Static Pressure (mm Aq) & 35 \\  & & Power (kW) & 3.7 \\ \hline VAV Terminal unit & & Rated airflow rate (CMH) & 1360 \\ \cline{2-4}  & & Capacity of reheating coil (kcal) & 4000 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: HVAC system specifications in test-bed. Figure 5: The floor plan of the test bed and the sensor locations. Figure 6: The building automation system (BAS) screen of the VAV system. at one-minute intervals to capture transient fluctuations and ensure data stability. The average value of these measurements was then calculated to provide a representative data point for each case.