1. The water level of boiler drum decreases abnormally. 

2. The water supply flow rate is abnormal and is larger than the steam flow. 

3. The flue gas exhaust temperature increases and the flue gas oxygen content decreases. 

4. The positive pressure of combustion chamber increases, the flue gas sprays from the position of the boiler that is not well sealed， the current of the induced draft fan increases.

5. The combustion is not stable and the bed temperature at leakage side is low, in serious condition, the boiler fire goes out. 

6. When slight leakage occurs, from the external of the boiler wall, the sound of steam jetting can be heard. During blasting, significant noise occurs, water drops from the bottom of the air chamber or the bottom of the air chamber is wet. 

7. During serious leakage, the main steam temperature decreases and the steam pressure and feedwater pressure decreases too. 8. When the power of induced draft fan is sufficient, the explosion door of air combustion chamber is easily caused to blast.