Typically 10–20% between plant and customer; modern dense networks reach 6–8%. Losses are fuel, money and CO2 in direct proportion.
From our guide: Decarbonizing District heating — full context, charts and sources there.
Boilers, kilns, heat exchangers, valves and steam lines lose energy continuously. Inzonex makes patented (UK GB2508992.1) removable modular insulation — snap-fastened covers engineered per temperature tier, not generic off-the-shelf jackets: