This European Standard for District Heating Twin Pipes, specifies requirements and test methods for straight lengths of prefabricated thermally insulated pipe-in-pipe assemblies for directly buried hot water networks, comprising two steel service pipes from DN 15 to DN 250, rigid polyurethane foam insulation and one cylindrical outer casing of polyethylene. The pipe assembly may also include the following additional elements: Measuring wires, spacers and diffusion barriers. This standard applies only to insulated twin pipe assemblies, for continuous operation with hot water at various temperatures up to 120 °C and occasionally with a peak temperature up to 140 °C. The estimation of expected thermal life with continuous operation at various temperatures is outlined in Annex B of EN 253:2003.