The operation and performance of forced-air ventilation systems with the aid of a dynamic modeling and simulation computer program are presented. The functions and features of GEMS (Generalized Engineering Modeling and Simulation), a dynamic modeling and simulation software tool, are briefly described. Using GEMS, the effects of different ventilation airflow rates and sensible and moisture efficiencies on the thermal comfort environment within the conditioned space were analyzed. The impact of these ventilator parameters on the indoor air humidity levels and annual energy requirements for two cities-one cooling dominated and one heating dominated-was studied. Simulation results from the cooling-dominated region are presented.