Class A Chimney Pipe, also called All-Fuel pipe, is designed to vent appliances fueled by wood, oil, and coal, though some brands are also approved for use with gas-fueled devices. Chimney systems created with Class A pipe are almost always UL Listed, allowing them to be compatible with a greater variety of fireplaces, wood stoves, inserts, and furnaces than other kinds of vent pipe.It is extremely important to have the correct size chimney pipe. To find the diameter of pipe you need, measure the flue opening of the appliance you plan to install. In most instances, the diameter of this opening should equal the inner diameter of the chimney pipe you use. Some types of installation, however, may affect the size of pipe you need, so it is always best to consult your appliances instruction manual prior to ordering. It is also important to note that while the diameter of your chimney can be larger than your appliances flue, it should never be smaller.