The thermal performance of a horizontal slinky ground heat exchanger has been investigated using a validated transient 3D model for different trench separations, initial soil temperature profiles and soil properties. The effect of trench separation on the thermal interference was analysed for a range of centre-to-centre distances between parallel trenches from 1.5 m to 11 m. The initial soil temperature was found to have a significant effect on the predicted thermal performance. The predicted heat extraction using a varying and more realistic initial soil temperature for a heating season would decrease with the increasing depth of installation whereas using a uniform initial temperature would lead to increasing heat extraction with installation depth. It has also been found that soil with a high thermal conductivity would exacerbate the thermal interference between trenches with a small separation as a results of heat depletion in the ground.