Downhole heat exchangers (DHE) eliminate the problem of geothermal fluid disposal, since only hear is taken from the well. For this reason, as well as their low cost and simple installation, they are frequently used in geothermal plants. In the last few years DHEs have been provided with a natural convection promoter to improve the heat and mass transfer of geothermal fluid between the aquifer and the well. Knowledge of the interaction between the fluid in the aquifer, in the well and in the promoter is necessary for DHE design. The authors experimentally verified the existence of a limit in the heat how obtained by the DHE, which is connected only to the aquifer-well-promoter interaction. This heat how limit is due to the short-circuit effect in the aquifer between cold and warm fluids, respectively leaving and entering the well, The authors propose a simplified model developed to determine the main lumped parameters characterizing the heat and mass transfer between aquifer, well and natural convection promoter.