Solar thermal water heating systems reduce household energy bills by using the free solar radiation provided by the sun to heat water for residential needs. In order to eliminate the need for electricity to run a pump to drive the fluid circulation in these systems, fluid buoyancy effects can be employed to move the fluid from lower elevations to higher elevations. There are several operational challenges with conventional "thermosyphon" systems, such as reversing flow and overheating, which can all be addressed by using a geyser pump mechanism. Although the solar thermal geyser pump water heating system has been on the market, little research has been done on the optimization of the system to improve its efficiency. Engineering Equation Solver (EES) and the Transient System Simulation Tool (TRNSYS) were used to implement a mathematical model for the geyser pump system operation under transient weather conditions. The model allows for parametric studies of the design attributes to investigate optimum efficiency conditions for the thermally driven pump.