types of duct layouts: (1) the INDIVIDUAL DUCT,
where each duct is connected directly to the furnace
plenum, and (2) the TRUNK AND BRANCH DUCT,
where the trunk duct connects to the furnace plenum and
then branches off to the outlets. These two types are
shown in figures 4-20 and 4-21.
Gravity-type furnaces are rated in leader area
capacity, the LEADERS being the warm-air pipes.
With respect to return ducts, the register-free area and
the return-air duct should not be less than 1 1/4 times
the area of the leader serving a given area. Gravity-type
installations, as shown in figure 4-21, use the
individual duct layout.
Forced warm-air systems usually have a register
temperature range of 150°F to 180°F. Ducts can be in
the form of a trunk with branches or with individual
leaders from a plenum chamber. Furnaces used with
forced-air installations must be equipped with
automatic firing devices. Velocities usually are in the
range of 750 to 900 fpm in trunks and approximately
600 fpm in branches. Outlet velocities at registers may
be as high as 350 fpm.
In this section, construction features, basic
components, gas burners, and controls of gas-fired
furnaces are discussed.
The various gas-fired furnaces available today
have similar basic components; however, there, are
variations in design with respect only to dimensions
and airflow. Unit features pertinent to dimensions and
airflow are important when selecting a furnace for a
particular space or application. A vertical counter-flow
unit, for example, is normally used where supply ducts
are located beneath the floor, because it has the return
in the top and the outlet in the bottom. The most
commonly used unit is the UPFLOW HIGHBOY
which, as a rule, draws air from the side or bottom and
discharges it from the top. It can be installed in small
spaces. In the HORIZONTAL UNIT, the air flows in
one side and out the other. This unit is suitable for
installation in crawl spaces, attics, and basements. In
another type, sometimes called a LOWBOY, both the
return and the outlet are at the top. It is a shorter and
wider version of the up-flow unit. The different
airflows are shown in figure 4-22.
Another type of furnace is the DUCT FURNACE.
It is designed for mounting in a duct system where air
circulation is provided by an external fan. It is
generally used with an air- conditioning system to
supply heat during the heating season by using the
same ductwork. This type can be installed as a
single unit or in batteries for larger requirements. A
Figure 4-20.Trunk and branch duct distribution systems.