TABLE 13-2. - Materials for Ductwork

finally. insulated ducts made from materials, such as asbestos and fiberboard, were developed. Passageways, formed by studs or joists, are sometimes used for return air when a fire hazard does not exist.

Ducts made of asbestos are no longer legal. If discovered, asbestos in any form must be removed and disposed of according to the laws and regulations discussed in chapter 16 of this manual.

The material used for the construction of ductwork depends on the application of the duct. Use table 13-2 as a guide in the selection of duct material. The thickness of the material depends primarily on the pressure developed within the duct, the length of the individual sections, and the cross-sectional area of the duct. The developed length of a section for a particular gauge can be increased by installing angle bracing around the duct. It is beyond the scope of this manual to include the technical details necessary for the selection of proper metal thickness and section length for different pressures and for different cross- sectional areas of duct material. However when repairs are made, the same thickness of metal that was originally included in the system must be installed. Where the original ductwork was destroyed by pressure, repairs may include increasing metal thickness or adding of angle bracing.

Ducts are either round or rectangular in cross section. Although rectangular ducts usually have the advantage of saving room space and being easier to install in walls, round ducts provide less resistance to air flow and should be used whenever possible.

Additionally, round ducts require less material to construct; thus, by using round ducts, you can save both money and material during installation. Initially, an air-handling duct is usually sized for round ducts. Then, if rectangular ducts are wanted or required, duct sizes can be selected to provide flow rates equivalent to those of the round ducts originally selected.

Table 13-3 is a ready reference to determine the size of a rectangular duct that equals the carrying capacity of a predetermined round duct. To use this chart, convert a rectangular duct with sides of 17 inches by 16 inches, respectively. First, come down the left-hand column until you reach 17 inches; then trace the line horizontally across the columns until you reach the column headed by 16 inches. At the center of these intersecting lines is 18.0 inches. This is the round duct size equivalent. In the second example, following the same procedure, it is clearly shown that a 22-inch by 17-inch rectangular duct has a 21-inch round duct equivalent.

TYPES OF DUCT SYSTEMS

In this section, the advantages and disadvantages of a double-duct system are discussed. Since there are many possibilities for an adequate duct system, one such system is modified to fit the needs of two different residential configurations.