The behaviour of conventional pile groups (e.g., closely spaced) that are subjected to mechanical loads has been shown to be different than the behaviour of single isolated piles. The so-called group effects are responsible for this behaviour and must be considered for an optimal design of pile foundations. In recent years, energy piles have shown potential to work as both structural supports and geothermal heat exchangers and thus are subjected to both mechanical and thermal loads. An increasing amount of research has investigated the previously unexplored impact of thermal loads on the thermo-mechanical behaviour of energy piles. However, no field data over typical time-scales of practical geothermal applications have been available to analyse the development and impact of thermally induced group effects between energy piles (e.g., closely spaced) on their thermo-mechanical behaviour. To investigate this problem, a full-scale in-situ test of a group of energy piles and coupled 3-D thermo-mechanical finite element analyses were performed and are presented in this paper. This work demonstrates that significant thermally induced group effects characterise closely spaced energy piles. Attention must be devoted to these effects throughout the design process (e.g., geotechnical, structural and energy) of energy piles because they play an important role in the serviceability performance of these foundations.