| 摘要 |
PROBLEM TO BE SOLVED: To prevent abnormal combustion due to excessive amount of air when the temperature lowering of combustion air occurs. SOLUTION: When the load of a fan motor is increased, a control device increases the rate of power supply for the fan motor to maintain a predetermined number of revolution. When a loading point, operated from a relation between the rate of power supply and a power source voltage is not in a combustion area wherein good combustion is effected and it is decided that abnormal operation due to the excessive amount of air can be effected, the objective number of revolution of the fan motor is reduced to reduce the amount of combustion air.
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