The heating pipelines are the most vulnerable element of district heating system. The average service life of heating pipelines which were built in the Soviet Union does not exceed 15 years. Every year in St.Petersburg 230 km of heating pipes in one pipe calculation goes up in the category of network that has reached the passport life (25 years). Reconstruction of old heating networks with installing new pipelines in polyurethane foam insulation will reduce heat loss significantly. In the article the economic efficiency of the reconstruction of district heat networks calculated. In accordance with the scheme of heat supply by 2030 planed to renovate 163900 m of heating pipelines (819500 m in the double-pipe calculation) dilapidated heating networks owned by the "St. Petersburg Heating" JSC.