To date, no field data referring to various operating energy piles over time-scales of practical applications had been made available to investigate the thermally induced “group action”. This paper presents the results of a full-scale field test and a three-dimensional thermo-mechanical finite element analysis of four operating energy piles over 12 months. When the number of operating energy piles increases, greater thermally induced vertical strain and lower stress develop along the piles for the same average temperature change. Opposite stress variations to those that may be expected based on the type of applied thermal load can develop in piles.