The impeller has radial blades so that the flow of air is not obstructed when the motor is not running. The exhausting power is then the same as it is for the motor less ventilator 3 to 12 m3/hr at a wind force of 1 to 4 m/sec. The ventilator should be placed in the roof, on a surface that is as even and horizontal as possible. If the ventilator is positioned on a sloping surface, it should be rotated so that one of the three drainage-openings points downwards. An inclination of up to 20° is acceptable without reducing the ventilation when the motor is not running. Note! In every installation, there must be the required inlet for replacement air. The opening should be at least 60 cm2.