Transite asbestos was a common product used to vent gas appliances such as furnaces, boilers, and water heaters in older homes in Minnesota. This material is no longer used because the interior of the vent can deteriorate and flake apart, collapsing in on itself. That, and it contains asbestos. Once the interior of the flue deteriorates it can get blocked, causing hazardous exhaust gases from the appliance to vent back into the home, rather than be carried to the exterior.