This 8 inch (200mm) 904 grade flue liner (904 inner – 316 outer) is one of the strongest and most reliable flue liners on the market. It comes with a 30 year guarantee compared to most other liners that only offer 20 years. Dura Flue is different as it is constructed from two individual strips of stainless steel, where as most other liners are made from only one strip of steel which is folded in half to create the inner and outer layers. When the steel is folded it weakens it. As Dura Flue is made from two strips of steel this weakening doesn’t happen meaning it’s a stronger and more reliable flexible flue liner.