Made from two separate strips of stainless steel (904 inner – 316 outer) this 5" 904 flexible flue is the best on the market. As most other liners use one strip of steel and fold it to create the inner and outer layer. It’s this folding that weakens the steel. Dura Flue is made from two strips of steel (an inner and an outer layer), it’s this unique way that Dura Flue constructed that makes it the strongest and most reliable liner on the market. You’ll require a 6 inch flue liner if you wish to burn coal, you can only use a 5 inch liner with a Defra Approved Wood Burning Stove.