Water flow window has been demonstrated an energy-efficient product that benefits both air conditioning and hot water systems. Its thermal performance was found affected much by the design geometry and system configuration. For those based on buoyant-driven flow, liquid water is at very slow movement; the magnitude depends on the overall flow-path friction loss and hence the connecting-pipe arrangement. In this study, the impacts of such recirculating pipe-work have been examined extensively. Firstly, experiment tests were executed to evaluate the system performances with and without distribution headers. Then their thermal and flow characteristics were compared using CFD analysis. Further on, the impacts of connecting-pipe dimensions were examined through the use of a validated FORTRAN program. The optimal pipe size was determined accordingly. It was found that without headers, more uniform temperature distribution close to the glazing surfaces could be achieved. The system thermal efficiency could be improved by around 5%. On the other hand, the variation in room heat gain was found around 1%, as a result of the self-regulating nature of the buoyant-driven flow. For the given window system, an optimal pipe size of 20-25 mm is recommended. (C) 2019 Elsevier Ltd. All rights reserved.