Abstract: A portable ventilation system includes a portable unit (1) comprising a fan (11) for sucking-in fresh air from the outdoor atmosphere and blowing the fresh air into a room, and an extraction fan (12) for sucking foul air from said room and blowing the foul air into the outdoor atmosphere. The system includes a flexible air conduit (13) having two mutually separate passageways (131, 132) which are arranged so as to obtain a transfer of heat between the passageways, through the medium of a wall of one passageway (132). One end of one passageway (131) is connectable to the input side of the fresh-air suction fan (11) and one end of the remaining passageway (132) is connectable to the output side of the foul-air extraction fan (12). Remaining ends of the passageways are intended to be placed in connection with the outdoor atmosphere.