The six types of furnaces are gas, oil, electric, propane, geothermal, and wood. Gas furnaces are the most common type of furnace and use natural gas to heat air. Oil furnaces use oil to heat air and are typically more expensive to operate than gas furnaces. Electric furnaces use electricity to heat air and are the most efficient type of furnace. Propane furnaces use propane to heat air and are typically more expensive to operate than gas furnaces. Geothermal furnaces use the earth’s natural heat to heat air and are the most efficient type of furnace. Wood furnaces use wood to heat air and are the least efficient type of furnace.