The temperature of water in large water reservoirs remains essentially constant at approximately 60° F throughout the year and therefore is attractive for space-conditioning applications. The energy exchange with the water reservoir is accomplished through a heat exchanger. In the present work, two heat-recovery heat exchangers were designed using a simplified computer model. Design-1, which used smooth tubes, was tested at a flooded abandoned mine vein at Scranton, Pennsyluania. The subsequent design, which used the spirally fluted tubes, was tested at the Ohio State University ( OSU) and at a lake in Lackawanna Stale Park in Pennsylvania. A more detailed thermal hydraulic model was developed for the analysis of these heat-recovery heat exchangers. The data obtained at OSU and Lackawanna were analyzed using the thermal hydraulic model to develop a correlation for natural convection. Finally, using this correlation, it was found that the design with spirally fluted tube clearly offers advantages in terms of reduced size of the heat exchanger for a given heat duty.