Leak testing of borehole heat exchangers according to SIA 384/6 is critical for operational safety and environmental protection. However, field experience shows significant pressure drops during the resting phase depending on depth and construction type, deviating from the normative target values. Using the H-EP testing device, backfilling and testing processes were comprehensively documented, and several thousand datasets were analyzed in cooperation with drilling companies. The analyses highlight the impact of geology, backfill materials, temperature, and time on test performance. The results provide a solid foundation for optimizing testing procedures and material selection and offer valuable insights for the further development of existing norms and standards.