The ash accumulation on boiler heating surface can be reflected on the instrument, the flue differential pressure of the heating surface of ash accumulation increases, because the heat absorption capacity decreases after the ash severely accumulates on the heating surface, in this way the outlet temperature of the working medium of part of heating surface is reduced, the flue gas outlet temperature rises. The heat surface of boiler with most severe ash accumulation usually is air preheater, because the hot air temperature drops, the exhaust gas temperature rises, the current of induced draft fan rises, the air suction quantity is insufficient, in severe case, the fan can only be run with output reduced.