Combustion Fundamentals2 / 10
Combustion Efficiency
Complete vs incomplete combustion, and measuring flue-gas losses.
10 min read · Jacob Willis, Net Zero Lead · Last reviewed July 2026
Boiler efficiency is about how much of the fuel energy ends up heating water, not lost up the flue or through the casing. There are two ways to think about it: the energy lost and the efficiency number.
In this lesson
- 01The efficiency gap: input to output
- 02How boiler efficiency is measured
- 03Why boilers lose efficiency in the field
- 04Testing boiler efficiency on-site
- 05Flue-gas temperature and the baseline
- 06The economics of efficiency
- 07Sources and further reading