1. The feed water flow is abnormal, it is greater than the steam flow,  in severe case, the steam drum water level drops. 

2. The flue gas temperature at economizer and air preheater drops or the temperature difference at both sides increases. 

3. The flue gas resistance magnifies and the current of induced draft fan increases. 

4. The leakage place of the economizer has abnormal noise and white gas emits from the chimney. 

5. The steam emits from the economizer flue that are not sealed well, in severe case, water leaks from the bottom of the flue.