Upflow Highboy Furnace
A typical upflow highboy furnace (also referred to as an upflow furnace or a highboy furnace) is shown in Figure 10-1. These are com- pact heating units that stand no higher than 5 or 6 ft and occupy a floorspace of approximately 4 to 6 ft2 (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.
Upflow Lowboy Furnace
The upflow lowboy furnace (also referred to as an upflow furnace
or lowboy furnace) (Figure 10-2) is designed for low clearances and stands only about 4 to 41⁄2 ft high. Although shorter than either the upflow highboy or downflow types (see below), it is longer from front to back.
Both the return-air inlet and the warm-air discharge outlet are usually located on the top of the lowboy furnace. The lowboy furnace is found in heating installations where the ductwork is located above the furnace.
Downflow Furnace
Figure 10-3 shows an example of a downflow furnace (also referred to as a counterflow furnace or a downdraft furnace). It is very similar in size and shape to the upflow furnace, but it discharges its
warm air at the bottom rather than the top. The return-air intake is located at the top.
A downflow furnace is used primarily in heating installations where the duct system is either embedded in a poured concrete slab or suspended beneath the floor in a crawl space.