In this paper, a solar water heating system (SWHS) is designed to substitute for a gas geyser system (GGS). First, a mathematical model for the SWHS is built according to the thermodynamic and hydrodynamic theories; then the transient performances of the system are analyzed using the TRNSYS (Transient System Simulation) program; and finally, the techno-economics of the SWHS are evaluated. The annually mean solar fraction of the SWHS is 0.56. TRNSYS simulation results show that the water temperatures at the solar collector outlet and in the tanks are much higher in summer than in winter because of the solar radiation and ambient temperature effects, and auxiliary heat is frequently required in winter. Comparing with the original GGS, the SWHS is cost-effective and its payback period is 7.4years. The redesign work is of high value for northwest China, considering the abundant solar radiation, the underdeveloping economics, and the environmental conservation in such regions.