Geothermal energy can be used directly, through heat exchangers, to provide a clean renewable form of heating or in a highly efficient way to produce cooling through Ground Source Heat Pumps (GSHP). It can also be utilized as the renewable energy source needed in Organic Rankine Cycle (ORC) plants to produce electricity. One of the key parameters in designing geothermal plants, be it direct heat-to-heat utilization, GSHPs, or for the production of electricity, is the energy available at the exit of the geothermal well. This can be measured, for example, in terms of temperature, pressure, and mass flow rate of the working fluid exiting the well. Coaxial heat exchangers (tube in tube) are recommended for use in extracting geothermal energy due to their simple geometry and the fact that the working fluid does not interfere in any way with the sub-surface structures. This paper presents results from a study using water as the working fluid, entering the outer annulus, and exiting through the inner pipe. Results are presented on the effect of the heat exchanger parameters, namely the inner diameter of the heat exchanger pipes and the insulation material and thickness.