If a manual damper or automatic draft controller, is provided in the incinerator flue, it shall be capable of closing off not more than ninety-five per cent of the flue area, and the damper or controller shall be made of such materials and in such a manner so as to prevent warping, binding, cracking, corrosion, and distortion when exposed to operating temperatures. If an automatic draft controller is used, means shall be provided for temporary manual operation.