2-Ply 3003 aluminum liner can be used to vent Category I 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 oversized flues. This practice may cause serious condensation and performance problems. By relining the chimney with National Chimney M-FLEX aluminum liner you can reduce the condensation problem and improve the performance of the appliances.