Although EnergyPlus is widely used by researchers and engineers for whole building energy simulations, it is not capable of modeling variable airflow fan-powered terminal units (FPTUs) using build-in modules. Energy Management System (EMS) and user-defined modules in EnergyPlus provide flexibility in modeling customized equipment and systems. In this paper, the performance model of variable airflow series FPTUs was implemented using EMS and user-defined modules. The performance model was also integrated with the rest of HVAC system to investigate the impact of terminal unit fan operation on system energy consumption. A single story, five-zone office building model was created in EnergyPlus as the testbed. Simulations were performed with four different climates and two terminal unit fan operation modes. Compared with the constant airflow series FPTUs, simulation results revealed that the variable airflow series FPTUs can save up to 85% of terminal unit fan energy consumption. It is also shown that the variable airflow series FPTUs can save central supply fan and cooling coil energy consumption by 4% and 6%, respectively, due to decreases in motor heat gains from terminal unit fans and primary air reduction at a given zone cooling load. Although the heating energy is increased due to the reduced heat gain from terminal unit fan motors, the use of variable airflow series FPTUs can lead to 15-35% savings in the annual energy consumption of HVAC system.