A ventilating device (1) comprises one or more impellers (2) extending in a longitudinal direction around an axis of rotation. Each of the impellers (2) is adapted to suck air from an ambient atmosphere in a transverse direction over a substantial part of the impeller's length. A coupling for transmitting a driving force from one impeller to another is disclosed. The ventilating device may be comprised in a unit with a cover (6) and a chassis, and it may be mounted to or around a heating or cooling pipe (3; 4) in a room of a building. A holding member (5) which may be connected to the cover (6) holds the pipe or pipes (3; 4) and/or one or more impellers (2). The device is easy to mount and may have improved energy efficiency. A building with a ventilating device, a method of operating the ventilating device, a system comprising a plurality of ventilating devices, and a mounting member for one or more pipes and/or one or more impellers are further provided.