This paper presents a new numerical model of the vertical U-tube ground heat exchanger (GHE). Objective of creating the new model was to obtain more accurate, detailed and reasonably fast simulation of the GHE system, which guarantees proper sizing of such a system during design process. Different approaches for modelling GHE are described with emphasis on the new thermal resistance model (TRM) of an U-tube GHE. The 1D model of the heat transfer between working fluid of a GHE and the borehole is applied. This model was coupled with a 3D model of heat transfer in the ground surrounding the GHE. The global model of a ground heat source system is obtained by connecting segments representing volume of ground influenced by each of the GHE. The paper describes a discretization scheme, coupling method, boundary conditions and limitation of the proposed model. The simulation results and the measured in-situ data from literature are compared, it differ by 3% taking into account the heat extracted from the ground. Simulation results of heat extracted from the ground, working fluid temperature and ground temperature distributions around GHE are presented. Conclusions concerning obtained results shows possible use for simulation of the operation of GHE. (c) 2020 Elsevier Ltd. All rights reserved.