Computational Fluid Dynamics in Hybrid Passive–Active Heat Recovery Systems for High-Performance Buildings: A Critical Review

Hybrid passive–active heat recovery systems are increasingly used in high-performance buildings as a practical response to the growing pressure for lower energy use and more stable indoor environmental conditions. These systems combine passive heat transfer elements, such as heat pipes, with limited mechanical ventilation support in order to balance energy efficiency with operational controllability. As a result, they occupy a middle ground between fully passive solutions, which are often sensitive to boundary conditions, and fully active systems, which tend to impose higher energy and operational costs. Computational Fluid Dynamics (CFD) has become the main analytical approach for studying the behaviour of these hybrid systems. It is commonly used to investigate airflow patterns, heat transfer processes, pressure losses, and ventilation effectiveness under a wide range of operating conditions. At the same time, the existing literature shows substantial variation in how CFD models are constructed and applied. Differences in turbulence modelling, boundary condition definition, validation strategy, and performance metrics make it difficult to compare results across studies or draw general conclusions. This review critically examines published CFD-based studies on hybrid passive–active heat recovery systems in high-performance buildings. The focus is not on individual case studies, but on identifying recurring modelling practices, common simplifications, and their implications for predicted system performance. Particular attention is given to the treatment of mixed-mode ventilation, conjugate heat transfer, and the interaction between passive components and mechanically assisted airflow. The review also highlights areas where current CFD practice remains limited, including validation against field measurements and representation of climate-driven variability. By consolidating these findings, the paper aims to clarify the role of CFD as a supporting analytical tool for design and retrofit decisions, rather than as a stand-alone predictor of system performance.