The pipe for a district heating system has an outer pipe (22) of a plastics material, at least one inner pipe (10) of a polymer material with a base pipe (12) to carry the fluid, and a thermal insulation material (20) to embed the inner pipe(s) within the outer pipe. A diffusion barrier layer (14) is at the outer and/or inner surface of the inner base pipe (12). The diffusion barrier layer (14) is applied by plasma deposition, or in a dissolved form for spraying, spreading and/or immersion, using a coating material with a base of SiO or metal oxide.
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