This M-Flex Chimney liner has the greatest acid fighting capability! Our unique manufacturing system used to make M-Flex, utilizes a continuous strip of stainless steel, interlocked and diagonally crimped to produce a gas and watertight seam of superior strength & durability. Its corrugated construction allows for expansion and contraction during the heat up and cool down periods, removing any stress on the system. AL29-4C liner is used to vent gas appliances that burn 84% effcient or higher.