Abstract: The air supply system for feeding air to a firebox within a building comprises a fresh air intake box (3) in the outside building wall (19) and, in the immediate vicinity of the firebox, a distributor box (7), connected with several pipes (5a, 5b, 5c, 5d). The fresh air intake box (3) has an extensible tube (25) for adaptation to varying wall thicknesses of the outside building wall (19). The prefabricated parts can be readily installed on the building site. By the use of several pipes (5a, 5b, 5c, 5d), the individual pipe cross section can be kept at a small value so that no static impairment of the building ceiling (11), wherein the pipes (5a, 5b, 5c, 5d) are extended, occurs. Advantageously, a fan can be installed in the extensible and readily removable tube (25) in order to increase the flow velocity of the fresh air; servicing of the fan is possible without any problems.