Boiler Losses can add up in a big hurry. The data is an average range for losses. Fuel to steam efficiency is BTUs burned compared to BTUs delivered to your facility.

Combustion Efficiency 75-86%
Radiation and Convection Losses 0.3%-6%
Boiler Design and Load Management 2-7%
Scale Buildup in Boiler 7-11%
Excess Boiler Blowdown 0.1-1%
Excess Air 0-7.5%
Range of Efficiency 42.5-76.6%

(Source ASHRAE Journal September 1994)