Abstract: Method for purging a furnace comprising a closable processing tube defining in its interior a closable, processing space, a purging gas being flowed along the outer surface of the processing tube separated from the processing gas flow inside the processing tube, wherein said processing gas flow comprises a gas being reactive with air at elevated temperature and said purging gas comprises an inert gas. A furnace assembly for realizing this method is proposed comprising a controller for controlling force connected in a conduit connected to a source of reactive and inert gas respectively. This controller is realized such that only after placing the closure of the processing chamber in the closing position the valve in the supply conduit for reactive gas can be opened and said closure can only be removed from said closing position after said valve in supply conduits for reactive gas has been closed.