To reduce the cost of ground source heat pump systems (GSHPs), the use of foundation piles as ground heat exchangers is a promising alternative. Unlike conventional systems, no additional drilling costs are incurred, greatly reducing the initial cost. In this study, the influence of varying pile sizes and grout backfill material thermal conductivity on the performance of a foundation pile ground heat exchanger is numerically modelled and optimized. Piles with diameters between 0.15 m and 2 m with a depth of 30 m, and backfill material with thermal conductivities of 2.0 W/mK and 3.6 W/mK have been considered. The study was accomplished using a thoroughly validated finite element model that determines the ground temperature distribution and energy transfer rates accurately. First, steady state performance was determined for different inlet temperatures in the range 0 to 45oC, then, using an actual heating and cooling load profile for a balanced building, the performance of the heat pump system was determined over a 4-year period. From the study, only the 2000 mm pile gives the most stable performance for the load used with COPs between 3.1 and 6.2 for the 2.0 W/mK backfill material. Smaller piles are shown to be unsuitable for the load, leading to malfunctioning of the heat pump in the cooling season. For a 1200 mm pile, the COP is in the range 0 – 6.0, but does not handle the cooling loads in the summer. Improving the thermal conductivity from 2.0 W/mK to 3.6 W/mK for the 1200 mm significantly improves the cooling performance and helps avoid failure of the heat pump. The cooling COP increases to about 2.0 from below zero values when the thermal conductivity is 2.0 W/mK for the 1200 mm pile.