Thermal environment prediction in a large open office space with hybrid HVAC system using dynamic coupled BES–CFD

The building sector accounts for approximately 40% of primary energy consumption, with over half attributed to heating, ventilation, and air conditioning (HVAC) systems. Therefore, improving the energy efficiency of HVAC systems is critical for reducing energy consumption and greenhouse gas emissions. In parallel, environmental standards such as Net-Zero-Energy Building (ZEB) and WELL require HVAC systems that are multifunctional, responsive to dynamic conditions, and capable of delivering high thermal comfort. Hybrid HVAC systems that integrate different conditioning principles, such as hydronic radiant heating and cooling (HRHC) systems, all-air systems, and dedicated outdoor air systems (DOASs), have gained attention because of their potential to achieve both energy efficiency and localized comfort control. However, the thermal behavior of such systems, particularly in open or large spaces, remains insufficiently understood owing to the complex interactions among convective, radiative, and latent processes. Conventional steady-state or simplified models are often inadequate for capturing these dynamics. To address this issue, this study developed a comprehensive numerical analysis method for evaluating the hygrothermal environment of hybrid HVAC systems, based on a coupling framework that integrates a Building Energy Simulation (BES) tool with Computational Fluid Dynamics (CFD). The proposed framework incorporates detailed mathematical models for DOAS and HRHC systems, allowing for the accurate prediction of thermal inertia, time-varying surface temperatures, and coupled moisture transfer. The BES and CFD models were dynamically linked through boundary-condition exchange, including time-dependent convective heat transfer coefficients and airflow between zones. Moreover, the framework was validated using a reference house equipped with a hybrid HVAC system, and a case study was conducted for cooling season conditions. The results showed that the system maintained stable hygrothermal conditions throughout the day, largely facilitated by the thermal storage effect of the HRHC-equipped slab. CFD analysis confirmed that thermal comfort was achieved within the occupied zone. This integrated approach provides a robust tool for the design and operation of hybrid HVAC systems. Future work will extend the analysis to long-term and seasonal performance evaluations, including heating operations, to support the development of optimized control strategies.