Question: I have coal stove that was installed approx. 10 years ago. Currently there is no liner in the chimney. The present liner is a 13 " square terra cotta. The stove is an "Old Mill" with a 8" opening. Is a smaller liner necessary? If it is- would the best choice be stainless steel or is there a better solution. I have been told that the coal ash will damage the liner within a few years. As you know the stainless steel liner is not cheap- so I would like to know that I am making the right decision Answer: It should definitely be lined for best performance with coal. The Old Mill was not the greatest coal stove- but it should work well with proper draft- coal and operation. Stainless will last quite awhile with coal as long as some precautions are taken. First- I'm assuming that you are using Anthracite (hard) Coal- which is low sulfur. Make certain you clean it at the end of each year- and if possible- brush some baking soda down the chimney after you have cleaned it to neutralize the acids. An 8" liner would match the size of the stove- and according to most codes- the chimney should not be downsized from the opening size. That said- coal usually works better with a smaller flue size- so a 6" liner may warm up faster. The only reason for the 8" flue on that stove is probably the large door opening- and the fact that they had a screen "open door" option for use with a wood fire.