Franklin Furnace is a census-designated place (CDP) in Scioto County, Ohio, United States, along the Ohio River. The population was 1,537 at the 2000 census. Franklin Furnace was the name of an early iron ore furnace built in eastern Scioto County in 1826 The community that grew in that area is named after the furnace. Franklin Furnace's post office has the ZIP code of 45629.