An investigation was conducted on a locally made natural circulation domestic hot water system to show the important variables, how they are related, and how they affect the performance of the solar system. For that purpose, several measurements were made, which included the temperature distribution along the fin between the absorber tubes and in the flow direction and the thermosyphonic mass flow rate. Using these data, the main parameters of the solar collector were obtained. A comparison between the experimental and theoretical temperature distributions, in the flow direction of the absorber, is also included.