Vertical ground coupled heat pump (GCHP) systems used in buildings in China can help to increase the share of renewable energy and improve the energy efficiency. A building energy simulation (BES) tool including a vertical GCHP model is beneficial for analysing the system performance under dynamic building loads. This paper presents a large-scale vertical ground heat exchanger model integrated to a heat pump model in the BES tool COMFIE, in a Matlab-Delphi co-environment. This model was applied to a residential building block with a 26 × 26 borehole field in Wuhan, China. The heat pump performance improvement was investigated for different design parameters. For half-hourly simulation of 1 year, the specific calculation time of the proposed GCHP model is 4 seconds out of 8 minutes for the whole building. The proposed model can be used as a design aid to optimise the system performance.