A ventilation system including a ventilator; and an outdoor unit, wherein the ventilator includes a housing including a first intake, a second intake, a first outlet and a second outlet, a plurality of dampers to change flow paths of air flowing through the housing, first and second air blowers, a heat exchanger in the housing, and a processor to control the dampers and first and second air blowers to form a first flow path in which outside air flows through the heat exchanger, or form a second flow path in which room air flows through the heat exchanger, to thereby form a flow path in the ventilator, while air flows through the flow path, control the outdoor unit to form frost on the heat exchanger, and control the outdoor unit to stop operating based on a preset frosting termination condition so that the frost melts to clean the heat exchanger.