The Thermal analysis of flat plate solar water heater is a complex task, because of a large numbers of parameters affecting its performance. In the last years, a lot of studies have been developed, using numerical approaches to describe and analyze the behavior and thermal mechanism of the thermal solar collector. In this paper, we develop a numerical model of the flat plate collector in forced circulation regime. We describe the basis of the model and show the comparisons with other bibliographic model. The main goal of this work is to simulate the efficiencies of the solar collector using an "4n-node model". The results are giving a very good agreement with the literature. The model is used to examine the impact of different parameters, which may affect the performance of the solar collector with single glass cover.