3–6 percentage points of boiler efficiency standard; condensing units more. On a 10 MW gas boiler that's ≈€100k+/yr at current prices.
From our guide: Boiler economizers — 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: