A fan unit includes a housing having an air inlet and an air outlet; a first fan and second fan mounted in the air inlet and the air outlet respectively and arranged in tandem so as to generate an air flow; a first valve mounted adjacent to the air inlet for allowing air to flow into the housing and preventing air from flowing out of the housing; and a second valve mounted adjacent to the air outlet for allowing air to flow out of the housing and preventing air from flowing into the housing.