A ventilation device arranged for placement between a wall and a radiator arranged next to the wall, wherein the ventilation device is arranged to direct an outdoor air stream from a ventilation opening in the wall to the underside of the radiator, for heating the outdoor air stream before inflow to a room, the ventilation device comprising: two substantially vertical side pieces arranged at a distance from each other and arranged to prevent outdoor air, which is led through the ventilation device, from passing out of the ventilation device at the side pieces, wherein the side pieces in the direction between the wall and the radiator have a width, and the respective side piece further has an upper end and a lower end, at least one top piece which extends between the side pieces, preferably between its upper ends, wherein the top piece further is arranged to sealingly connect to the side pieces and arranged to prevent outdoor air, which is led through the ventilation device, from passing out of the ventilation device at the top piece, a first ejector part arranged to sealingly connect to the side pieces at its lower ends and further arranged to prevent outdoor air, which is led through the ventilation device, from passing out of the ventilation device at the first ejector part, and the first ejector part is arranged to direct an outdoor air stream in the direction away from the wall and in the direction of a lower part of the radiator by means of a first bottom part, wherein the respective side piece is arranged to connect to the wall and to a rear side of the radiator, and wherein the at least one top piece is further arranged to connect to the wall, and wherein the first ejector part is further arranged to connect to the wall, or wherein the ventilation device comprises a rear wall and a front wall which together with the side pieces, the top piece and the first ejector part form an air duct for guiding an outdoor air stream from an opening in the wall to the underside of the radiator, a second ejector part, which is connected to the first ejector part and arranged to prevent outdoor air, which is led through the ventilation device, from passing out of the ventilation device at the second ejector part, and the second ejector part is further arranged to direct an outdoor air stream upwards towards the underside of the radiator by means of a second bottom part, characterized in that the second ejector part is removably connected to the first ejector part, and in that at least one of the first bottom part of the first ejector part and the second bottom part of the second ejector part comprise an opening which allows the passage of radiator pipes through at least one of the first and the second bottom part (4a, 5a), which radiator pipes extend from a floor side in the direction up towards the radiator. two substantially vertical side pieces arranged at a distance from each other and arranged to prevent outdoor air, which is led through the ventilation device, from passing out of the ventilation device at the side pieces, wherein the side pieces in the direction between the wall and the radiator have a width, and the respective side piece further has an upper end and a lower end, at least one top piece which extends between the side pieces, preferably between its upper ends, wherein the top piece further is arranged to sealingly connect to the side pieces and arranged to prevent outdoor air, which is led through the ventilation device, from passing out of the ventilation device at the top piece, a first ejector part arranged to sealingly connect to the side pieces at its lower ends and further arranged to prevent outdoor air, which is led through the ventilation device, from passing out of the ventilation device at the first ejector part, and the first ejector part is arranged to direct an outdoor air stream in the direction away from the wall and in the direction of a lower part of the radiator by means of a first bottom part, wherein the respective side piece is arranged to connect to the wall and to a rear side of the radiator, and wherein the at least one top piece is further arranged to connect to the wall, and wherein the first ejector part is further arranged to connect to the wall, or wherein the ventilation device comprises a rear wall and a front wall which together with the side pieces, the top piece and the first ejector part form an air duct for guiding an outdoor air stream from an opening in the wall to the underside of the radiator, a second ejector part, which is connected to the first ejector part and arranged to prevent outdoor air, which is led through the ventilation device, from passing out of the ventilation device at the second ejector part, and the second ejector part is further arranged to direct an outdoor air stream upwards towards the underside of the radiator by means of a second bottom part, characterized in that the second ejector part is removably connected to the first ejector part, and in that at least one of the first bottom part of the first ejector part and the second bottom part of the second ejector part comprise an opening which allows the passage of radiator pipes through at least one of the first and the second bottom part (4a, 5a), which radiator pipes extend from a floor side in the direction up towards the radiator.