1. How much area that the heating surface at the rear is. If the heating surface at the rear is more, the exhaust fumetemperature will be reduced, but the too low fume exhaust temperature will cause metal corrosion at the rear and increase the consumption of metal. Generally the exhaust gas temperature is 110 ~ 160 ℃. 

2. The ash accumulating or scaling at hearting surface makes heat exchange become poor and cause the exhaust gas temperature rise. 

3. The coking in the hearth makes the flue-gas temperature from the hearth rise and causes the exhaust gas temperature to rise. 

4. The big air leakage from the boiler bottom makes the flame kernel raise; Air leakage from the flue also can make the exhaust gas temperature rise.