Variable Air Volume (VAV) unit system is one of the commonly used forced-air heating and cooling systems in office buildings. It controls the airflow with the discharge air temperature fixed for cooling, while the discharge air temperature is adjusted with the airflow fixed at the minimum for heating. This study presents the simulation of conventional VAV unit system performance applied in the typical office building prototype. Each VAV unit has the minimum airflow setting for the ventilation purpose and its impact on the energy consumption is significant. The aim of this study is to evaluate the energy impact of the minimum airflow setting of VAV box using EnergyPlus ver. 6.0 software platform. The energy result breakdown of three model cases is discussed: minimum airflow fraction setting of 10%, 20% and 30%. As a result, the minimum airflow setting has significant impact on the reheat energy and thus the annual boiler gas consumption shows significant difference among each simulation case. Monthly heating energy profile is also discussed for the better understanding of the impact of different minimum fractions on the heating energy.