This paper presents the evaluation of the ground source heat pump (GSHP) system implemented in a net-zero energy office building through detailed computer simulations. The building and system are currently undergoing final commissioning. The system studied is a hybrid system which includes two identical GSHPs and one air source heat pump. The ground loop system consists of three vertical borehole heat exchangers and twelve horizontal linear heat exchangers, which can either operate in parallel or in series. In this study, a detailed simulation system representing the real hybrid GSHP system has been developed based on the TRNSYS software platform. A parametric study has been performed for both horizontal linear heat exchangers and vertical borehole heat exchangers to understand the effects of different design variables on system performance. The performance of the GSHPs under different operating combinations of ground heat exchangers (i.e. parallel and series) has been evaluated through comparison of the annual load balance within the vertical borehole heat exchangers and energy consumption of the GSHPs. Potential solutions that could be used to solve the issue of potential load imbalance in the vertical borehole heat exchangers over the course of a year are also discussed.