The main objective of this work was to establish factsand tools, which could help us to obtain and predictpresent and future technical status of pre-insulatedbonded district heating pipes in operation. In order tosimulate ageing of polyurethane (PUR) insulation anaccelerated thermal ageing method was used.Accelerated ageing was performed by applying threedifferent elevated temperatures to the service pipes.The effect of the diffusion of oxygen through the casingwas examined by ageing district heating pipes with twodifferent thicknesses of the casing pipes.The evaluation of the technical status of the pipes afterartificial or natural ageing was done by measuring theshear strength (adhesion) between the PUR foam andthe steel service pipe. The tangential shear strengthtest method was mainly used for evaluation of thestatus of the pipes. The SP plug test method, which isa cheaper and more practical method in the field, wasalso used, and the results were compared with thosefrom the tangential shear strength test method.In the framework for improved maintenance strategies,the failure mechanism was considered as loss ofadhesion between polyurethane and the service pipe.The deterioration of the adhesion was assumed to be athermo-oxidative process governed by an Arrheniusrelationship. A model of how the development of faultsrelated to adhesion and costs of heat losses in a districtheating distribution network was sketched.