This 2 Ply 3003 M-Flex Aluminum Chimney Liner can be used to vent Category1 gas appliances with A.F.U.E. of 83% or lower. In many cases, especially when only water heaters are vented into masonry flues, the gas appliances are vented into over sized flues. This practice may cause serious condensation and performance problem. By relining the chimney with M-FLEX aluminum liner, you will reduce the condensation problems and improve the performance of the appliance. Manufactured in the U.S.A.