An Environmental Control System (ECS) in an aircraft ensures the correct air temperature and pressure for passengers and crew. The ECS development process requires the simulation of the ECS system functionality in order to find an optimum system configuration and to support sizing of components. FLECS, the "Functional Model Library of the Environmental Control System", is programmed to support this development process. It offers a library of generic models of ECS components and the aircraft cabin. The library is based on the commercial standard simulation software MATLAB/Simulink. FLECS allows the simulation of a network topology of a system, combining heat and mass flows. Fundamental to the simulation are generalized volume and generalized resistance elements – mass flow resistances as well as heat flow resistances for conduction, convention and radiation. The mass flow of predominantly dry air may contain water vapor and CO2. A highly dynamic sample simulation from FLECS components was compared with results from another verified simulation program: Both results were in good agreement. Various Graphical User Interfaces (GUI) in FLECS support everyday use in the industrial environment.