Heating installation for a reactor. The reactor is provided with gas feed openings and gas discharge openings opening into a treatment chamber for accommodating a wafer floating therein. By means of such a treatment a wafer can be heated and cooled relatively rapidly. During the actual treatment it is important that the gas is heated sufficiently and to this end a heating installation is present. The latter consists of heating means, such as electrical heating means, arranged in a body which adjoins the reactor and in which channels have been made which connect into the gas feed openings and through which the treatment gas or other gas is fed.