flue gas duct element for transitioning from single-wall elements to double-wall insulated elements for condensate flue assembly. The internal contour is made of 0.5 mm thick AISI 430 stainless steel. No thermal insulation. Cold-formed socket joint with ideal geometry and gas tightness. Laser butt welding ensures a thin and durable seam, significantly increasing service life. Operating temperature up to 450°C.