What Size Twin Wall Flue do I need? It is easy to get confused when talking about the diameter of sizes of flue pipe. When it's just single wall flue like stove pipe such as vitreous enamel pipe then there is just one diameter that you refer to as it is a single skin of pipe. Typically the spigot of a stoves is 6 inch 150mm or 5 inch 125mm / 130mm. The manufacturer will give guidance on the flue size and typically this is the size of the spigot. You can size up your flue system i.e increase in size going up but you cannot decrease the flue size going upwards in the flue system as this goes against HETAS guidelines and will limit the draw and effectiveness of the stove and flue system. When we refer to twin wall flue size we are referring to it by the inner diameter (the inner skin of the two metals) and you will shop by this size on our website. Please note that the outer diameter will be bigger in size as it holds compacted insulation material between the two skins of stainless steel. The outer diameter will differ between brands of flue as all are different. This is why different brands of flue do not all just go together. For example we sell two brands of flue and both of these are different - an inner diameter of Convesor KC or TW Pro 5inch 125mm has an outer diameter of 175mm whereas with Dinak DW it is 5 inch 130mm inner and an outer of 190mm. The Conversor KC or TW pro 6 inch is 150mm inner and 200mm outer, whereas the Dinak DW twin wall 6 inch is 150mm inner and 210mm outer. So make sure you are shopping by the inner diameter of your twin wall flue requirements. Ensure that you buy the same brand of flue and always by an Adapter and a Terminal with every order to ensure it is covered by the 10 and 12 year Warranty.