A simplified model to simulate single U-tube ground heat exchangers is presented in this paper. The model is based on the electrical analogy to simulate heat transfer within the borehole and the use of thermal response factors (g-functions) to estimate injection-extraction heat flow to surrounding ground. The inclusion of two capacities for the heat carrier fluid and the grout material makes the model suitable for short time step simulations. The thermal resistance of the borehole has been changed considering different heat conduction paths. Several variants of the model have been validated through comparison with a refined computational fluid dynamic (CFD) reference model. Good results have been obtained for the different variables analysed (outlet temperature and surface borehole temperature). RMSE (Root Mean Squared Error) values were smaller than 1 degrees C while relative errors were under 5%. (C) 2013 Elsevier B.V. All rights reserved,