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 150F to 180F. 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.

GAS-FIRED FURNACES

In this section, construction features, basic components, gas burners, and controls of gas-fired furnaces are discussed.

Construction Features

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.