controlling, by the processor, an alternator or an actuator of a switching door to reduce a cooling capacity provided by the HVAC system by determining whether a temperature of an engine coolant discharged from the internal combustion engine is higher than a first threshold temperature;

determining whether an accumulated cooling capacity provided by the HVAC system exceeds a reference accumulated cooling capacity by determining whether the temperature of an engine coolant discharged from the internal combustion engine is higher than a second threshold temperature after the cooling capacity is reduced;

determining a lower limit temperature of an evaporator of the HVAC system according to a temperature of an engine coolant when the accumulated cooling capacity exceeds the reference accumulated cooling capacity; and

repeatedly turning a clutch of a compressor of the HVAC system on or off by comparing a temperature of the evaporator and a lower limit temperature of the evaporator;

wherein the second threshold temperature is higher than the first threshold temperature; and

wherein the accumulated cooling capacity is the sum of cooling capacities accumulated after the operation of the HVAC system.