## 5 Conclusions  
Pressure differences in the building envelope cause leaks, which can cause problems such as increased heating and cooling loads and moisture spread. To solve this problem, it is necessary to understand the leakage in the building. Typically, leakage in a building is calculated by obtaining a leakage function through the fan-added pressure method and substituting the pressure difference measured by a pressure sensor. However, it is difficult to install continuous pressure sensors in an operating building. Previous studies on building leakage prediction models based on confidential data still require pressure difference measurements. In addition, most studies have failed to implement real-time leakage monitoring and have relied on assessing or estimating the leakage at fixed pressure differences, which limits the ability to continuously and accurately characterize the leakage of a building and respond quickly to changes in the indoor and outdoor environment. Therefore, in this study, we proposed a method for predicting the change in the leakage airflow with a change in the supply and return airflow of the air conditioning system in an operating building. To verify this method through experiments, a laboratory at a university with a variable airflow system was selected as a test bed, and the airtightness performance was measured through the pressurization/decompression method using an air handler. To analyze the relationship between the supply and return airflow of the variable airflow system and the indoor-outdoor pressure difference of the test bed, the indoor-outdoor pressure difference was measured by changing the supply and return airflow. The experimental results revealed that the indoor-outdoor pressure difference  
Figure 15: Comparison of leakage airflow rate between measured and predicted values in series operation. was proportional to the difference in the supply and exhaust airflow, and the R\({}^{2}\) value was 0.9406, which is close to 1. This result confirms the correlation between the supply and return airflow differences of the air handler and the indoor-outdoor pressure difference, and the ability of the model to predict the pressure difference in the building through the supply airflow. Next, the relationship between the supply airflow and the pressure difference was substituted into the leakage function derived from the measurement results to derive the relationship between the supply airflow value and the leakage value. The cvRMSE value of the predicted and measured leakage was 11.05%, which is lower than the limit specified by the ASHRAE (\(\leq\)30%) or IPMVP (\(\leq\)20%) guidelines. The results of this study show that by knowing the supply and exhaust airflow of the test bed, it is possible to predict the pressure difference and leakage in the envelope without installing pressure sensors. This enables more accurate monitoring of the real-time leakage characteristics of the building and contributes to the effective control of the supply and return air volumes within the building to manage the indoor-outdoor pressure difference. In particular, this method makes full use of the existing system to enable real-time leakage monitoring, which increases the efficiency of the operational phase of the building. However, there are some limitations to this study. Firstly, the experiments were only conducted under certain conditions, and further validation is required in buildings with varying airtightness performance. Also, the experimental equations do not require the installation of pressure sensors, which is necessary for the experimental process. This can be overcome with further research covering a range of conditions and building types. Future research should explore how to operate an air conditioning system that can control the pressure in a building by considering the leakage airflow predicted using the supply and return airflow based on these experimental results. Conceptualization, H.-J.K. and Y.-H.C.; formal analysis, K.-W.K. ; investigation, K.-W.K.