2–5% on boilers, far more on high-temp furnaces — regenerative burner retrofits commonly cut 15–30% of furnace fuel.
From our guide: Combustion air preheating — 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: