When a wood stove is hooked into a thimble, (a round opening where a woodstove connects to a masonry chimney, usually made of terra cotta or metal) running directly into a terra cotta lined chimney, sometimes the flue is too big for the appliance to draft properly. NFPA (the National Fire Protection Association) standards require a flue area no bigger than twice the size of the flue collar on the appliance for an exterior chimney (built on an outside wall of the house, surrounded by the outside air on three sides), or three times the size of the flue collar for an interior chimney (built in the middle of the house, surrounded by the house on all four sides). The area of a six inch flue collar (the most common) is just over 28 square inches. The area of an eight inch flue collar is just over 50 square inches. Terra cotta tiles vary a lot in their thickness, so it is a good idea to measure the interior dimensions to get an accurate read. Typically an 8×8 flue tile has an area of 49 square inches, an 8×13 flue tile has a free area of 84 square inches, and a 12×12 flue tile has an area of 121 square inches.