Different gas furnace efficiency requirements and labels are used for different regions of the country. Certified gas furnaces in the southern half of the U.S., where .Furnace definition, a structure or apparatus in which heat may be generated, as for heating houses, smelting ores, or producing steam. See more..Lo.ng for a high quality, reliable furnace? Check out our line of oil and gas furnaces that Lennox Residential has to offer..A furnace is a device used for high temperature heating. The name derives from Greek word fornax, which means oven. The heat energy to fuel a furnace may be supplied .