Instructions / Assembly

Heating and Cooling Essentials
sufficient and uniform air circulation for maximum
comfort.
Figure 28-1. Installing an air duct system may be a part of
your job in either new construction or remodeling work.
28.1.2 Perimeter System
The perimeter system uses forced air and locates the
supply air outlets in the baseboard along outside walls, or
in floors or ceilings near outer walls Figure 28-2. The
supply air "blankets" the wall against drafts (heating
mode), or radiant heat during the cooling mode. This
air circulates inward to a return air grille located in an
inside wall where it is drawn off and returned to the
furnace for retreatment. The perimeter system provides
28.2 Furnace Classifications
All forced air furnaces are classified according to the
direction of the airflow for Ihe supply air. The upflow and
downflow furnaces are most common, but horizontal-
flow models are necessary for some applications.
28.2.1 Upflow Furnace
The upflow furnace is normally located in a base-
ment or closet, Figure 28-3. The supply air duct system
is attached to the furnace plenum (sometimes called
the bonnet), a sheet metal chamber anchored to the
top of the furnace with sheet metal screws. The air
conditioning evaporator coil is mounted on top of the
furnace, but inside the plenum. The supply air duct is
often insulated to help maintain the temperature of the
treated air. Return air enters the bottom of the furnace.
The blower serves for both heating and air conditioning
mode, but airflow direction remains the same.
28.2.2 Downflow Furnace
A downflow furnace is usually used in a building
having a crawl space under the first floor, or in a mobile
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Figure 28-2. The perimeter system uses outlets located along outside walls.