1. The temperature of flue gas at air preheater inlet  rises abnormally.
2. The primary air temperature and secondary air temperature at the preheater outlet rises abnormally.
3. The luminous word plate of “Fire alarm of air preheater” is emitted.
4. The shell of the air preheater burns or temperature radiates from the shell.
5. The current of the air preheater swings.
6. The exhaust gas temperature rises rapidly and the oxygen content in the flue gas decreases.