Abstract: A structure placed in the fire-box of a furnace to improve combustion comprises a plurality of stainless steel strips which are supported on a frame in position to be heated to incandescence by the flame of the burner. At least some of the strips are twisted in corkscrew manner. In one embodiment, twisted strips alternating with untwisted strips are hung on a cross piece of the frame on the side facing the burner. A perforated metal sheet is mounted on the back side of the frame. In another embodiment horizontally extending twisted strips are supported between spaced inner and outer arched supports which extend up from runners resting on the floor of the fire-box. A perforated metal sheet is applied over the outside of the outer supports.