Low delta-T syndrome widely exists in the existing HVAC systems and results in increased energy consumption. This paper presents a model-based method to evaluate the energy impact of the low delta-T syndrome in a complex chilled water distribution system. Simplified models are employed to establish the reference benchmarks of total energy consumption of chilled water pumps under various cooling load condition. When the low delta-T syndrome occurs, the energy impact upon the chilled water pumps is determined by comparing the measured total pump power with the reference benchmark. The proposed energy impact evaluation method was validated in a dynamic simulation platform representing a complex HVAC system. The results show that the proposed method can accurately determine the energy impact of low delta-T syndrome upon the chilled water system.