Environmental control system holds vital importance as it is responsible for passenger’s ventilation and comfort. This paper presents modelling and sizing of the parameterized model of environmental control systems. Knowledge based engineering application serves as the base for designing and methodology for the environmental control systems. Flexibility in the model enables user to control the size and positioning of the system and also sub-systems  associated with it. Number of passengers serves as the driving input for the environmental control system. A 3-d model gives the exact representation with respect to volume occupied and dependencies on the number of passengers. It also provides a faster method to alter the system to user needs with respect to number of air supply pipes, number of ducts and pipe length. Knowledge based engineering gives the freedom to visualize various options in the conceptual design process.