A thermosyphon loop is analyzed in this study by use of numerical and experimental techniques. A rectangular loop was constructed using copper pipe and the sections which were heated or cooled were designed as concentric tube heat exchangers. Hot or cold water was circulated outside of these sections, and both the surface temperatures and heat transferred to and from the loop were measured. Within the numerical study both the momentum and energy equations were also solved using a SIMPLEX algorithm. Numerical results were obtained for laminar flow within the circuit when there was uniform wall temperature in the vertically heated and cooled sections of the thermosyphon. The two-dimensional numerical model provided results which agree with those found experimentally from the thermosyphon loop. In addition, a simulation model was constructed using a correlation included both the Grashof and Prandtl numbers to evaluate heat transfer through a thermosyphon loop.