This study explores the flexibility potential of HVAC systems in residential and office buildings through a comprehensive simulation-based approach. Utilizing detailed numerical models within the TRNSYS simulation environment, the research incorporates both building characteristics and HVAC system thermal inertia to quantify the load-shifting capabilities of heating, ventilation, cooling, and domestic hot water (DHW) systems across different months and seasons. The proposed methods involve iterative year-long simulations for various demand response (DR) events to identify distinct patterns in flexibility potential and rebound effects associated with each HVAC category. The findings reveal significant seasonal variations in HVAC system flexibility. A granular analysis of air handling units (AHU), heat pumps (HP), chillers, and other heating systems provides insights into their respective contributions to the overall flexibility of the buildings, as demonstrated in two case studies. This research contributes to the development of tools and methods aimed at evaluating and optimizing the deployment of flexibility services. By enhancing the understanding of HVAC system behavior in response to demand response strategies, the study supports efforts to integrate and manage energy flexibility within diverse building environments, ultimately contributing to the advancement of energy efficiency and grid responsiveness in the building sector.