Class FB pipe is a manufacture and model specific chimney capable of handling the exhaust from appliances that burn cord wood, propane, & natural gas with temperatures up to 1000 degrees continuous and operating under a natural draft. They are commonly called double wall or triple wall. This is because manufactures use different engineering methods to achieve similar results. Regardless the outside shell of this pipe still gets hot and requires an air space. The metal used on the outside is galvanized steel and the metal used on the inside is 430 stainless steel. They all have different properties and you will need to align them with your objectives. Lengths range from 6″ to 48″ and diameters range from 8″ to 13″. We supply & install these products.