Abstract: When the load was down, the energy corresponding to output power of unit is equal to the sum of the heat created by combustion in furnace and the released cumulative heat. The output power at this condition may still be greater than the minimum load for steady combustion of certain coal grade, but its one part is produced by the boiler cumulative heat releasing, so the power produced by the combustion heat may be lees than the minimum load for steady combustion of the coal grade, and this will lead to extinguishment of boiler. According to analysis of concrete reason, the corresponding countermeasures were given.

Key words: load down, cumulative heat of boiler, minimum load for steady combustion, cumulative heat releasing