Abstract: Various heat transfer models are reported for vertical ground heat exchangers, and several basic analytical and numerical models of vertical heat exchangers are described and compared, and recent developments are discussed. To examine the effect of temperature rise in the soil surrounding a vertical ground heat exchanger on the performance of the ground heat pump, the heat transfer model that represents the temperature rise and heat flows outside the borehole is often coupled to the models inside the borehole via the borehole wall temperature. This temperature is an important factor that affects the heat delivery/removal strength of the system to/from the ground. In the current study, the results of a semi-analytical model that couples a model outside the borehole with one inside the borehole taking into account the transient borehole wall temperature is described. The results of this model for a constant borehole wall temperature are compared with those for a transient one with a numerical model. It is shown that transient borehole wall temperature results in more accurate temperatures for the circulating fluid flowing to the heat pump.