The width of a twin wall flue pipe is determined by the diameter of the inner pipe, with the outer pipe being larger due to the insulation layer. For a 6-inch twin wall flue pipe, the inner diameter is 6 inches (150mm), and the outer diameter is typically around 8 inches (200mm). This additional width is necessary to accommodate the insulating layer between the inner and outer pipes, which ensures the safe and efficient operation of the flue system. Always ensure that your installation allows sufficient space for the outer diameter of the twin wall flue pipe.