The design of borehole fields for ground coupled-heat pump systems is often based on so-called g-functions, a pre-calculated dimensionless temperature response to a step heat load. The g-function is specific to the borehole field geometry. For a long-term analysis of a particular borehole field, the average borehole wall temperature is obtained from the g-function and a temporal superposition of its thermal load steps. There are two accepted provisions for the g-function regarding the boundary condition used at (and along) the borehole wall: a constant heat flux at every instant of time, or a uniform temperature at constant total heat flow to the borehole field, respectively. In this paper, a numerical model is built up according to the geometrical characteristics and ground thermal properties of a 2x3 borehole demo site at the Universitat Politecnica de València, Spain. The model is separately studied with regard to the two boundary conditions. The models are first compared in terms of their g-function, which are verified against reference solutions. Then, the daily fluid temperatures are obtained from each of these models with measured daily loads during a six year period. The results are compared with measured daily fluid temperatures for the sixth year of operation. The simulated values present, in general, a good agreement with the measured data. The results show that there are no significant differences with regard to the boundary conditions at the borehole wall, which for this specific case is due to the fact that the system is thermally balanced. The simulated temperatures are more accurate during cooling periods.