The objective of a system simulation model is to duplicate the performance of a real system at all operating conditions of weather and load; giving temperatures, flows, cooling loads, kW demand of all equipment, and total site kW as weather and operational conditions change. The simulation model of this paper consists of a set of simultaneous equations that obey the laws of thermodynamics and includes the non linear performance characteristics of the plant equipment and air side equipment, and estimates the performance characteristics of the buildings in the system. The set of equations always arrives at equipment energy equilibrium after a change in the steady state condition, just as a real system would respond. The purpose of this paper is to demonstrate system simulation modeling as a tool for system designs and system energy control.