M&G DuraVent has the industry’s first UL-listed relining system for factory-built chimneys for use in the United States. Safely relining factory-built chimney systems has been a long-term industry unknown. Many factory-built chimneys were designed and installed as a system with a solid-fuel–burning appliance—often, a zero-clearance fireplace. Now, you can reline your inefficient factory-built fireplace and chimney with a wood-burning insert that requires a 2,100-degree UL 103 HT system. .