Electricity producing device with fuel cell and method for starting up same

The device has an enclosure (2) including a fuel cell (3) i.e. proton membrane exchange type, and defining closed volume around the cell. The enclosure includes openings (4, 5) selectively sealable by a movable flap (14) mobile to control air circulation between an interior and outside of the enclosure. A selective reheating unit (6) is distinct from the cell and placed in the enclosure below a floor, where volume located under the floor communicates with enclosure volume located at top of the floor through a passage (8). The enclosure is a parallelepipedic cupboard shaped compartment. The floor is a parallelepipedic plate. An independent claim is also included for a method for starting an electricity producing device.