Question: What is the difference between a Class A and a Class M chimney and how could one tell them apart? Answer: Class A is an all-fuel chimney, which is good for wood, coal, gas or oil. I have never heard of a "Class M" and though it may exist, it is not commonly used or labeled. Class B is for gas only. Class L is for Pellet and Oil. Perhaps "M" is the new direct vent gas pipe.