The low delta-T syndrome exists in many large primary-secondary chilled water systems, which results in the degradation of the system overall energy performance. This paper presents a method and a case study on diagnosing the low delta-T problem resulted from the deficit flow that frequently occurred in the chilled water system of a super high-rise building at its early operation stage. The history operation data during the days when deficit flow and low delta-T syndrome occurred are analyzed. The improper set-point of outlet water temperature on the secondary side of heat exchangers is finally diagnosed as the fault that resulted in the deficit flow and low delta-T syndrome. Diagnosis of this fault was also validated in the in situ experimental tests. The deficit flow could be eliminated if temperature set-point was reset higher. Compared with the original set-point of outlet water temperature on the secondary side of heat exchangers, 87.67 kW (72.37%) of the total power of pumps on primary and secondary sides of heat exchangers could be saved in the test cases when higher set-points were used. (c) 2012 Elsevier Ltd. All rights reserved.