District heating networks in Estonia are mostly old and in bad condition. The state of the district heating networks of Tallinn is typical for the rest of Estonian DH networks. The paper includes an analysis of the Tallinn district heating networks. Valid data about damages in district heating systems received for the last 12 years were used for an analysis of the network damages. Different types of network damages are analysed: external corrosion, internal corrosion, defect of installation, factory defects, defect of construction and other reasons. The scale of damages for the different elements of networks is compared in the paper: armature, compensator, construction and pipes. The main factors which influence damages in district heating networks are the age of networks, the quality of construction works and the network operation conditions. The damage quantity dependence on the age of networks is also defined and analysed in the paper. The scale of damages can be diminished by reducing the average age of the networks. This is possible by replacing old pipelines and other network system elements. The pipes average age changes for a 20 year period are simulated according to different intensities of renovation works.