Low delta-T syndrome is recognized as a typical technical problem in many chilled water systems. This syndrome typically increases the energy consumption of chilled water pumps, decreases system overall operating efficiency, and undermines occupant comfort. Although low delta-T syndrome has been widely discussed in theoretical and practical studies, these studies were not focused on systematic summaries that describe underlying reasons for the syndrome. In this study, causes of the syndrome in buildings are investigated comprehensively, through both qualitative and quantitative analyses. Firstly, a qualitative research is conducted on all issues that may cause low delta-T syndrome from three aspects, which are causes related to terminals, hydronic system design and construction, and local controls. Subsequently, a simulation platform that can simulate the dynamic features of the chilled water system is built to quantitatively analyze the effects of these causes. Finally, different causes of low delta-T syndrome are introduced into the simulation platform and the effects are analyzed. The simulation results indicate that the issues summarized all have effects on the appearance of low delta-T syndrome and increase the pump energy consumption to varying degrees. The poor valve authority and control failure of the valves are major causes for severe low delta-T syndrome.