A door assembly for a premixed burner boiler comprises a door (1) having a substantially flat and annular body (5) whose central bore (7) is connected to an admission duct (6) for feeding a mix of air and gas, a burner arranged in correspondence with said bore (7) and at least one usual electrode for igniting the flame of the burner being associated with a first face (8) of such body (5), on the same face (8) an annular insulating element (3) being present and covering the portion of said face delimiting such central bore (7), such insulating element (3) being fixed to such face (8). There are present elastically deformable elements (21) suitable for being elastically coupled with said first face (8) of said body (5) and for coupling themselves with portions (36) of said insulating element (3) so as to hold it bound to such face (8).