System 636 concentric vent kits provide a complete two-pipe venting solution for high-efficiency condensing furnaces and boilers using a single roof or wall penetration. The concentric design routes combustion air in through the outer pipe and exhaust gases out through the inner pipe, simplifying installation and minimizing the number of roof penetrations. ULC S636 certified for use in Canada with approved high-efficiency appliances. Available in 2" x 16" and 3" x 20" configurations.