This study investigates the mechanical performance of three temperature-resistant 3D-printable polymer composites for turbine impellers used in district heating networks for pressure reduction. Using fused deposition modeling (FDM), tensile strength and deformation of ASA-X CF10, PA6-GF30, and ePAHT-CF15 were evaluated at temperatures representative of real operating conditions (60–130 °C). These polymer composites were systematically tested, with particular emphasis on annealed ePAHT-CF15. Results demonstrated that annealing significantly improved mechanical performance, yielding higher tensile strength, Young’s modulus, and reduced deformation. Structural analyses confirmed that ePAHT-CF15, particularly when annealed at 200 °C, exhibited superior thermal stability and rigidity, making it the optimal material choice for high-temperature turbine impeller applications. These findings support the design of 3D-printed composite impellers for pump-as-turbine applications in district heating systems, where high stiffness and heat resistance are required.