Learning Objectives: Understand the basic types of ductwork systems and the components of those systems for distribution of conditioned air.

Distributed air must be clean, provide the proper amount of ventilation, and absorb enough heat to cool the conditioned spaces. To deliver air to the conditioned space, air carriers are required, which are called ducts. Ducts work on the principle of air pressure difference. If a pressure difference exists, air will flow from an area of high pressure to an area of low pressure. The larger this difference, the faster the air will flow to the low-pressure area.


There are three common classifications of ducts—conditioned air ducts, recirculating-air ducts, and fresh-air ducts. Conditioned air ducts carry conditioned air from the air conditioner and distribute it to the conditioned area. Recirculating air ducts take air from the conditioned space and distribute it back into the air conditioner system. Fresh air ducts bring fresh air into the air-conditioning system from outside the conditioned space.

Ducts commonly used for carrying air are of a round, square, or rectangular shape. The most efficient duct is a round duct, based on the volume of air handled per perimeter distance. In other words, less material is needed for the same capacity as a square or rectangular duct.

Square or rectangular duct fits better to building construction. It fits above ceilings and into walls and is much easier to install between joists and studs.


There are several types of supply duct systems (fig. 7-42) that deliver air to room(s) and then return the air from the room(s) to the cooling (evaporator) system. These supply systems can be grouped into four types:

  1. Individual round pipe system
  2. Extended plenum system
  3. Reducing trunk system
  4. Combination (of two or more systems)

Return air systems are normally of three types—single return, multiple return (fig. 7-42), or combination of the two systems.

Figure 7-42.—Supply duct systems:
A. Individual round pipe; B. Extended plenum;
C. Reducing trunk; D. Multiple return air system.


Ducts may be made of metal, wood, ceramic, and plastic. Most commonly used is sheet steel coated with zinc (galvanized steel). Sheet metal brakes and forming machines are used in fabricating ducts. Elbows and other connections, such as branches, are designed using geometric principles. Some types of duct connections used in constructing duct systems are shown in figure 7-43.

Sheet metal ducts expand and contract as they heat and cool. Fabric joints are often used to absorb this movement. Fabric joints should also be used where the duct connects to the air conditioner. Many ducts are insulated to lower noise and reduce heat transfer. The insulation can be on the inside or the outside of the duct. Adhesives or metal clips are commonly used to fasten the insulation to the duct. As we are only briefly discussing construction here, you can find construc- tion and fabrication methods in the Steelworker, volume 2. It details design and fabrication of steel ductwork.

Figure 7-43.—Typical duct connections:
A. Elbow; B. Tee: C. Reducing tee; D. Cross: E. Lateral.


To enable a duct system to circulate air at the proper velocity and volume to the proper conditioned areas, you can use different components within the duct system, such as diffusers, grilles, and dampers.

Diffusers, Grilles, and Registers

Room openings to ducts have several devices that control the airflow and keep large objects out of the duct. These devices are called diffusers, grilles, and registers. Diffusers deliver fan-shaped airflow into a room. Duct air mixes with some room air in certain types of diffusers.

Grilles control the distance, height, spread of air-throw, and amount of air. Grilles cause some resistance to airflow. Grille cross-section pieces block about 30 percent of the air. Because of this reason and to reduce noise, cross sections are usually enlarged at the grille. Grilles have many different designs, such as fixed vanes which force air in one direction, or adjustable to force air in different directions.

Registers are used to deliver a concentrated air stream into a room, and many have one-way or two-way adjustable air stream deflectors.


One way of getting even air distribution is through the use of duct dampers. Dampers balance airflow or can shut off or open certain ducts for zone control. Some are located in the grille, and some are in the duct itself. There are three types of dampers used in air-conditioning ductwork—butterfly, multiple blade, and split damper (fig. 7-44). When installing a damper, always draw a line on temperature control.

Figure 7-44.—Three types of duct dampers.

Fire Dampers

Automatic fire dampers should be installed in all vertical ducts. Ducts, especially vertical ducts, will carry fumes and flames from fires. Fire dampers must be inspected and tested at least once a year to be sure they are in proper working order. There are two types of fire dampers, which are fail-safe units— spring-loaded to close and weight-loaded to close. Fire dampers are usually held open by a fusible link. Heat will melt the link and the damper will close by either gravity, weights, or springs (fig. 7-45).

Figure 7-45.—Fire damper in OPEN position.


Air movement is usually produced by some type of forced airflow. Fans are normally located in the inlet of the air conditioner. Air is moved by creating either a positive pressure or negative pressure in the ductwork. The two most popular types of fans are the axial flow (propeller) or radial flow (squirrel cage) (fig. 7-46).

The axial-flow fan is usually direct-driven by mounting the fan blades on the motor shaft. The radial-flow fan is normally belt-driven but can also be direct-driven.

Figure 7-46.—Principal types of fans:
A. Radial flow; B. Axial flow.


Balancing a system basically means sizing the ducts and adjusting the dampers to ensure each room receives the correct amount of air. To balance a system, follow these steps:

  1. Inspect the complete system; locate all ducts, openings, and dampers.
  2. Open all dampers in the ducts and at the grilles.
  3. Check the velocities at each outlet.
  4. Measure the "free" grille area.
  5. Calculate the volume at each outlet. Velocity x Area = Volume
  6. Area in square inches divided by 144 multiplied by feet per minute equals cubic feet/minute.
  7. Total the cubic feet/minute.
  8. Determine the floor areas of each room. Add to determine total area.
  9. Determine the cfm for each room. The area of the room divided by the total floor area multiplied by the total cfm equals cfm for the room.
  10. Adjust duct dampers and grille dampers to obtain these values.
  11. Recheck all outlet grilles.

In some cases, it may be necessary to overcome excess duct resistance by installing an air duct booster. These are fans used to increase airflow when a duct is too small, too long, or has too many elbows.

Questions for Lesson 5

  1. What are the three common types of ducts?
  2. What are the three types of return air systems?
  3. Sheet metal ducts expand and contract as they heat and cool. True /False
  4. What are the three types of dampers?
  5. Once you have checked the velocities at each outlet, what is the next step when balancing the duct system?