Water flow window has been well proven as an efficient way for both solar energy utilization and cooling energy saving for air-conditioned buildings, especially in the cooling-dominant regions. However, investigations on its heating application performance in cold winter season, when the direct solar thermal energy penetration is not sufficient to maintain a comfort room temperature, are quite few. In response, this paper suggests to supply warm water to the window gap and act as heat radiator, in order to help maintaining a suitable indoor thermal environment in winter. In this way, window is no longer the weak point of heat insulation, but a combination of both solar collector and heat radiator. Parametric optimization was completed numerically with a validated FORTRAN program to determine the proper operation parameters. The impacts of water circulation mode, supply water flow rate, warm water temperature and supply period were investigated. With open circulation, the energy saving potential was found to be superior compared to close circulation design with less room heat exchange and improved thermal efficiency. Supply water flow rate of 400 ml/min was recommended for the cases under investigation. The proper temperature of supply warm water varied with the climate, especially the solar intensity. A higher temperature was required for cities with weaker solar radiation and lower ambient temperature. Warm water supply was an effective measure to provide assisted heat source during heating season and the proper supply period was from November to March.