The lifetime of pre-insulated district heating pipes (DHPs) is commonly evaluated using the method described in the normative European Standard EN 253. This lifetime is normally calculated using an Arrhenius equation, which makes use of test results from accelerated ageing tests at elevated temperatures. In this investigation, long-term accelerated ageing tests of DHPs at elevated temperatures were carried out. The ageing behaviour, especially at the interface between steel pipe and polyurethane (PUR) foam, showed several routes of degradation. It is clearly demonstrated using measurements of shear strength, thermal conductivity and alterations of chemical structure by Fourier Transform Infrared spectroscopy that the results of accelerated ageing at 170 and 150 °C significantly diverge from those obtained from the ageing test at 130 °C. It is therefore concluded that accelerated ageing at commonly used high temperatures does not create an acceleration of degradation processes at the steel/PUR interface relevant for the DHP application, but rather a significant alteration in mechanism. This finding is of crucial importance for the use of EN 253 and the development of future methods for lifetime prediction of DHPs.