This paper presents a simulation model for evaluating the economic viability of a solar system for heating water in an apartment building. The system consists of solar collectors and a back-up oil burner. The model determines the optimal number of collectors needed and the optimal method for connecting the collectors-in series or in parallel. The simulation results show that the solar system for heating water is more economical as the life span of the system increases, as the prices of oil increase, and as the demand for hot water increases. Furthermore, connecting the collectors in series is more economical than connecting them in parallel.