Ducts and Duct Systems:Extended Plenum Systems

Extended Plenum Systems

In the extended plenum system (see Figure 7-3), a large rectangular duct extends straight out from the furnace plenum (hence the term extended plenum) and generally in a straight line down the center of the basement, attic, or ceiling. Round or rectangular supply ducts extend as branches from the plenum extension to the warm- air supply outlets. The large extension to the plenum permits a better airflow rate with reduced resistance because of its large duct diameter. The branching ducts are usually located between joists and can be easily covered with a ceiling.

Crawl-Space Plenum Systems

It is possible to incorporate the entire crawl space into a heating system if the crawl-space walls are tight and well insulated. The heated air is forced down into the crawl space and enters the rooms through perimeter outlets, usually located beneath windows.

This type of duct arrangement may be referred to as a crawl- space plenum system and represents a modification of the extended plenum system. Because the entire crawl space is filled with warm air, this system provides relatively uniform temperatures through- out the structure.

Duct Materials

Ducts are manufactured from a variety of different materials. The material selected will depend on the use for which it is intended. It is very important that the duct material be taken into consideration when designing a duct system because not every material is suitable for all conditions in which ducts are used.

It is possible to purchase ducts manufactured from the following materials:

• Steel

• Galvanized sheet steel

• Aluminum

• Copper

• Glass fiber

• Paper fiber

• Vitrified clay tile

Plain steel and galvanized sheet steel ducts are available in thick- nesses ranging from 0.0163 to 0.1419 inch. Ducts manufactured from this material are preferred for use in warm-air gravity and forced-circulation warm-air heating systems. Table 7-1 indicates the thicknesses, gauges, and weights in which plain steel and galvanized sheet steel ducts are available.

Aluminum ducts are available in thicknesses ranging from 0.012 to 0.064 inch (see Table 7-2) and are used in the same types of heat- ing systems as steel ducts. Although aluminum ducts are lighter than steel ones, they generally cost more. Aluminum ducts are frequently used in duct systems located on the outside of buildings.

Copper ducts are available in sizes and gauges matching the aluminum ones and are frequently used in outside ductwork.

Round glass-fiber ducts can be purchased in a number of different sizes ranging up to 14 inches in diameter with duct walls up to 1 inch in thickness. Square or rectangular glass-fiber ducts can also be made from flat glass-fiber board. Because of their composition, glass-fiber ducts dampen sound.

Paper-fiber ducts are laid in concrete and used in warm-air heating systems. They are accordingly not recommended for use in attics, basements, or other exposed areas.

Vitrified clay tile ducts represent another duct material suitable for installation under a concrete slab. These ducts range in outside diameter from 51⁄8 to 421⁄4 inches.

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