The aim of this project was to assess the thermal conductivity of polyethylene (PE) filled with carbon black (CB), specifically for geothermal pipes. The project explored the potential modification of PE's thermal conductivity by incorporating recycled textile fibers. Different types of shredded recycled fibers were tested, including two types of polyamide fibers with varying contaminations and one type of polyester fiber. Following several preparation steps, various composite materials were manufactured and compared to bulk PE using various testing methods: Differential Scanning Calorimetry analysis (DSC), mechanical testing (flexural and tensile), and laser flash analysis (LFA). The results revealed alterations in the mechanical properties of the composite materials in comparison to PE filled with CB. The LFA tests demonstrated the effectiveness in reducing polymer thermal diffusivity at higher temperatures, particularly when the material was loaded with recycled polyester fillers.