A device for heating supply air for the ventilation of a room, which is supplied with supply air from a glazed balcony located adjacent to the room, has an outer balcony parapet (5), which is permeable to the heat radiation of the sun, and a heat-absorbing inner balcony parapet (12) located inside the outer balcony parapet (5) and adapted to be heated by the solar heat radiation that penetrates the outer parapet. The outer and inner balcony parapets (5, 12) define between them a substantially vertical air gap (13). A fan arrangement (16) is adapted to force air present in the glazed balcony through the air gap (13) from the lower part thereof to the upper part thereof for heating of said air. Inner outlets (19) are arranged in the upper part of the air gap (13), through which outlets the heated air is allowed to flow out into the glazed balcony before being supplied to the room as supply air.