Central furnaces are intended for installation in family housing or nonfamily housing structures primarily to supply heated air through a sheet metal duct system. Unit heaters are intended primarily for installation in such nonfamily structures as warehouses, industrial shops, auditoriums, hangars, and gymnasiums where warm air will be directly diffused into the space in which the heater is located. Downflow furnaces can be used to an advantage with under-floor duct systems in structures having no basements for central-furnace location. Downflow furnaces are also referred to as counterflow units since the heated-air path runs in a direction counter to the flue gas passage.