Currently space heating accounts for a large portion of the world's energy demand. The majority of this heat is produced from carbon intensive fossil fuels. Ground Source Heat Pumps (GSHPs) are a technology which can be utilized to offer low carbon emissions heating/cooling and hot water supply. However, currently GSHPs are often a more expensive alternative to conventional fossil-fuel heating systems. Moreover, improperly designed GSHPs can increase operational costs and often do not provide the expected reduction in emissions. Due to the many uncertainties involved in the design of GSHP systems, designers prefer to use unnecessarily high factors of safety when choosing the size of a system, which in turn leads to high installation costs. Hence it is important to reduce these uncertainties and improve design procedures of GSHP systems in order to maximize the advantages of this technology and make it more competitive in the market. This paper presents a case study of a 1.5 MW (426 ton) capacity GSHP system installed in the One New Change retail development in London, UK. The system includes 192 thermal-piles underneath the building foundations and two open-well heat exchangers. The GSHP system was simulated using the TRNSYS energy simulation platform. A conventional borehole model based on the 'Duct Ground Heat Storage Model' was used to model the thermal-piles. Results of the model matched to within +/-11% of actual performance data collected from the system over a period of eight months. It was therefore concluded that the borehole model can be used to simulate the performance of thermal-piles, provided that the piles are of an equal length and are spaced within a uniform grid. The model was then used to evaluate the energy usage, total operational cost and CO₂ emissions of the GSHP system over a lifetime of 30 years. Results show that although the GSHP system significantly reduces energy consumption, with the current gas and electricity prices in the UK, operational costs for the GSHP system are higher than those of a conventional heating and cooling system.