Coupled simulation of building energy systems (BES) and computation fluid dynamics (CFD) often focuses on the integration of air handlers with indoor environment, and does not incorporate vapor compression systems into the analysis, yielding inaccurate prediction of building energy consumption. This paper presents a coupled simulation to explore the pull-down performance of a room air conditioning system. The dynamic models of the air-conditioner are constructed in Modelica, whereas the indoor environment is simulated in OpenFOAM. Dynamic characteristics will be compared with different vane angles and airflow modes. Numerical simulations demonstrate that both vane angle and airflow mode exhibit pronounced impact on the pull-down time. Meanwhile, the well-mixed assumption that most of building energy simulation programs are built upon exhibits drastically different dynamic characteristics compared to the detailed CFD model, suggesting that neglecting non-uniform air flow and temperature distributions in buildings might lead to significant errors in control design.