A typical upflow highboy furnace (also referred to as an upflow furnace or a highboy furnace). These are compact heating units that stand no higher than 5 or 6 feet and occupy a floor space of approximately 4 to 6 sq. ft. (2 ft. x 2 ft. or 2 ft. x 3 ft.). The heated air is discharged through the top of the upflow furnace (hence the name) and the return air enters the furnace through air intakes in the bottom or sides. Cooling coils can be easily added to the top of the furnace or in the duct system.