Defines efficiency of boiler plant, notably combustion efficiency and boiler efficiency at 1) full load, rated output, continuous operation, 2) part load, continuous operation, 3) part load, cyclic operation. Summarises in a table typical boiler casing losses as derived in laboratory tests. Treats improving boiler efficiency by more effective insulation, reducing flue losses, improving the load factor and applying energy conservation measures. Provides a guide to the sort of improvements which can be expected from these measures.