Fans are the basic elements of mechanical ventilation. The fans discharge polluted air from inside the room. If we want to maintain high quality of air in the room, choosing a fan can be a key issue. The fan, when working, forces the specified amount of air to flow in a specific direction with a certain efficiency in a unit of time. A fan is a kind of compressor which, by increasing the kinetic energy of the air, gives it speed and causes the air to move in a specific direction. The fan acquires energy from the electric motor, therefore it must meet the standards corresponding to electrical devices. The fans can be divided due to their design and use. Axial fans, often called screw fans, are fans designed to convey low pressure air. It is used wherever the air has a short path to go and flowing through the fan does not change direction flowing along the axis of the engine. Centrifugal or centrifugal fans are intended for pumping a large amount of high pressure air, which is why they are best suited as duct fans. The impeller blades scatter the air and direct it to the outlet. The second division of fans refers to their place of use, location and place of installation. Here we start with roof fans. Roof fans are installed at the outlets of ventilation ducts on the roofs. On the one hand, roof fans extract air from the duct and, on the other hand, they use outside conditions to increase the efficiency of ventilation. Most often, roof fan casings are made of galvanized sheet steel and fans of plastics such as polyesters or polyamide reinforced with fiberglass. The fan should be installed as far away as possible from windows, doors and ventilators and positioned as high as possible above the floor. This solution ensures that the fan pushes out the used air outside, not the freshly wound air. Wall-ceiling fans can improve the unsatisfactory operation of gravity ventilation without the need to rebuild the entire system. They are small fans installed directly on the wall or in the ceiling and can be directly powered. The duct fans are mounted in round ducts. They are very efficient fans that can replace natural ventilation. They are built in such a way that if they are turned off, the efficiency of the gravitational air is practically not reduced. Ventilation in the kitchen and bathroom requires specialized fans. Cooker hoods are used to remove steam created during cooking. The hood is a fan when it discharges the air outside the building by a separate ventilation duct. The air collected in the eaves goes to the fat filter and then is removed from the building through the ventilation duct. Bathroom fans should have an IP5 protection degree. IP5 is a protection against getting a stream of water. They must be mounted at a distance of 1.25 m from the reach of a bathing person. Good bathroom fans ensure five times the air exchange per hour. To increase the comfort of using the bathroom, the fans are equipped with a return flap to prevent air blows; and more and more often in the functional automation, which allows you to control the fan so that it does not work continuously.