A corner seal device is disclosed for use in preventing unwanted air leaks in ductwork sections intended for directing conditioned air in heating, ventilating and air conditioning systems (i.e., HVAC systems). The corner seal device eliminates potential air leaks caused inter alia, by the flange members of the ductwork sections, and further, it avoids previously known costly expenses and loss of time to seal the unwanted air leaks created when the flange members of the ductwork sections are made and are prepared for assembly of the ductwork sections. A method of assembly of such ductwork sections incorporating the unique corner seal device of the invention, is also disclosed. A duct for directing conditioned air in heating, ventilating and air conditioning systems incorporating such ducts is also disclosed. A heating, ventilating and air conditioning system incorporating such unique corner seal device and related structures is also disclosed.