As a modified building-integrated solar thermal system, building-integrated dual-function solar collector here proposed is able to provide passive space heating in cold winter, and water heating in warm seasons. In this study, evaluations were made on this modified collector system for the warm period operation under the water heating mode with natural circulation of flow. A dynamic numerical model has been developed and validated by experimental data. Based on practical air-conditioned room design conditions, numerical analysis was performed to study the water heating performance, as well as to compare the solar transmission through building facade in different seasons with or without its presence. The results show that when working in the water heating mode, the system performs well in providing services hot water in the warm seasons without bringing in summer overheating problem. (C) 2010 Elsevier Ltd. All rights reserved.