Reducing heat losses in networks is a key factor in improving the energy efficiency of heat supply systems. The article investigates the problem of significant heat losses in underground heating networks with polyurethane foam (PUF) insulation. In particular, a comprehensive approach to the inspection and diagnostics of such networks is considered to reduce heat losses and accurately identify defects. Replacing old pipelines with modern PU foam pipelines is an effective way to reduce them, but even new pipelines need regular inspection due to possible damage and defects. In order to improve the inspection process and localize defects, a new comprehensive methodology combining energy audit and technical diagnostics is proposed. The methodology consists of two stages: the first is an energy audit to identify areas with the highest heat losses, and the second is a detailed technical diagnosis of these areas to accurately determine the location and type of defects. The paper presents the results of the inspection of several sections of heating networks where excessive heat losses were detected and their technical diagnosis was carried out. These results confirm the effectiveness of the proposed approach, which allows reducing the number of emergency repairs and reducing the time for conducting inspections. The proposed methodology is a promising tool for improving the efficiency of heating networks and ensuring their reliability. It can be recommended for implementation in the practice of energy monitoring and diagnostics of underground heating networks.