Sizing: Central Boiler sizes chimneys by their inside diameter. A 6" chimney has a 6" inside diameter and an 8" outside diameter. An 8" chimney has an 8" inside diameter and a 10" outside diameter. A 10" chimney has a 10" inside diameter and a 12" outside diameter. When ordering a chimney part for a Central Boiler furnace, be sure to use the Central Boiler designated chimney size when ordering rather than the actual measurement. For example, when ordering a chimney cap for a Classic 6048, even though the actual outside diameter is 10”, order the 8” chimney cap. If you do not know the proper size for chimney parts, please call first. All returns for incorrectly ordered parts drop shipped from Central Boiler will incur return shipping and a 20% restocking fee.