The most popular large commercial building chilled water system today is the primary-secondary design. This system separates the generation zone (chiller/evaporator) of a chilled water system from the distribution zone (air handlers). The generation (primary) loop maintains a constant chilled water flow rate. The distribution (secondary) loop varies flow rate in response to cooling load fluctuations. This variable flow rate in the secondary loop permits energy savings over older, constant flow, single loop systems (Rishel, 1996). Recently, new chiller controls have been implemented that allow variable chilled water flow rate in chillers. As a result, a new chilled water system design has become available. This design is a single loop system with a single set of pumps. Like the primary-secondary system the single loop design has two-way valves in air handlers. This variable flow rate, primary system also eliminates the primary-secondary bypass loop. Existing primary-secondary systems can be converted to this design by simply valving off the bypass leg. The purpose of this study was to compare the performance characteristics of the primary-secondary system with the single loop, variable flow rate design. To accomplish this, individual system components were modeled. These component models were compared with manufacturer's data in order to validate their accuracy. The modeled components include; air handler, chiller, pump and cooling tower. These component models were combined to form primary-secondary and single loop system models. Comparisons of these two systems were made to determine relative system efficiency and cooling capacity. Different designs and cooling load conditions were applied to the primary-secondary and single loop systems.