B-vent Chimney Liner Kit 4"x35' Flexible Aluminum Chimney Liners & Connectors Flexible Chimney Liners Many thousands of highly efficient gas-burning appliances are being vented into existing masonry chimneys. If vented into a potentially oversized and unlined masonry chimney, the rapidly cooled exhaust gases are likely to condense on the inside walls of the chimney. The acidic gases will, in time, cause deterioration of the masonry chimney -- causing health hazards and costly repairs. When a properly sized flexible aluminum chimney liner is installed, the warm flue gases exit the chimney faster. Most of the water vapor in the flue gases is safely vented up and out of the chimney. Any remaining water is contained within the watertight conduit created by the flexible aluminum liner. Metal-Fab's listing with UL allows the liner to be used all the way from the termination, out the masonry wall, and through open (unenclosed) space to the appliance. Brochure Installation Instructions