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Airflow Systems: Ductwork Structure & Uses

Airflow Systems: Chapter 5


This module will explain the different types of ducts and accessories. You will understand what the advantages and disadvantages for each are. Skip to quiz!

Setting Up Ductwork

Zoning helps identify which rooms need to be cooled and which do not. It also helps

determine the temperature needed by each room.

Zoning is the first step in designing a ductwork system. From here, we can determine which ducts to use and where.

Ductwork Structure

The main components of ductwork included the following:

  • Plenum or Main Trunk,

  • Trunk Duct,

  • Takeoff, and

  • Air Terminal Devices

A plenum in ductwork is a box that connects to the HVAC system. It’s function is to bring in,

distribute, and remove air. It goes directly from the air handler to the trunk duct.

A trunk is a duct that is split into more than one duct. Think of it like the trunk of a tree that splits into multiple branches. The trunk duct splits into multiple branches of duct work as well!

The fitting that attaches the branches of ductwork to the main duct are called takeoffs. The takeoff pushes the air to move into the branches.

The air terminal devices are where air is supplied to a room. This is where the air leaves a ductwork system through either grilles or diffusers. Grilles release air at a controlled rate. Diffusers release are in all directions.

Different Types of Ductwork

Here are the most common types of ductwork:

  • Radial ductwork,

  • Extended plenum,

  • Reducing extended plenum, and

  • Perimeter loop ductwork

Let’s take a look at each.

Radial duct work is designed so that all ducts come together at one point. Think about the spokes of a wheel. Each spoke joins together at the center of the wheel. Radial ductwork works very similar to this.

Radial ductwork is good because it maximizes airflow. They are generally used in small houses.

The extended plenum ductwork uses a large trunk duct across the whole building. The purpose of this is to have large amounts of air reach different places. This trunk is considered an extension of the plenum.

This large trunk stays the same size across the whole building. It has many branches coming out of it throughout its length. This is popular in homes with basements. However, it can result in branches not having enough airflow.

This system is similar to the extended plenum ductwork, except the trunk decreases in size. This allows the air to travel faster through the duct system and saves money.

One benefit of this system is that the airflow through all branches is the same. They save material costs and work better than normal extended plenums!

In perimeter loop ductwork, one trunk goes around the outside of the building. This trunk has branches that point in towards the center of the building.

Each branch of this ductwork provides equal air supply. These systems are best used in cold climates.

Ductwork Additions

A damper is a movable plate located in the ductwork that regulates airflow. It redirects it to specific areas of the home. The plates can block the branches of certain ducts so that air cannot flow through.

They decide how much heated or cooled air sent into different areas of your ductwork. If a

room is too hot, they will block the heated air ductwork and open the cooled air open. They are used in the zoning process of ductwork.

Recall that ducts cannot turn or bend without the help of fittings. Fittings are important because they get the air from the blower to the diffusers and grilles. We learned about elbows, reducers and end caps but there are many more!

A wye fitting is used to split one large duct into two smaller ducts. Recall that reducers change the size of a circular duct. Similarly, a transition changes the size of a rectangular duct. A boot is used to transition from a rectangular duct to a round duct.

Flex ducts are used to transport air from the HVAC system into the building. A flex duct is very convenient for attaching air outlets to rigid ductwork. Flexible ducts can eliminate the need for fittings like elbows.

Flex ducts are attached with either cable wire or metal clamps. Flex ducts can be cut to size with a classic utility knife. Slide the flex duct onto the metal duct end. It is important to seal this boundary so there is no air leakage.

Mastic is a waterproof putty that can be used to seal joints. Apply mastic to the the flex duct - ductwork boundary. Then secure the attachment with a simple zip tie. Repeat this process at the other end of the flex duct.

Upflow and Downflow

Upflow and downflow are the ways heated or cooled air is sent into a building. The climate around the system determines which is best to use.

In an upflow system, air comes into the furnace from the bottom. It gets heated and then flows upward into the home's ductwork. Upflow systems are usually located in a basement.

Recall that hot air rises and cold air sinks. Because upflow system are in the basement, the hot air rises more efficiently. This is why upflow system are installed where there are cold winters.

A downflow system is the opposite of an upflow system. Downflow system are in the highest part of a building, such as the attic.

These systems can efficiently push cool air into the home because cool air sinks. Downflow systems are usually seen in climates with very hot summers.

Reading Airflow

Anemometers are devices that read and measure airflow. Some of the common anemometers are rotating vane, hot wire, and swing vane. There are also velometers and capture hoods. Let’s see how they work.

We can also take the temperature of the airflow. First, poke a hole in an air duct and then push a digital thermometer through it. The temperature reading on the thermometer is accurate for the airflow.

Ductwork is a fundamental part of airflow systems. It provides the pathway for the air to travel through to reach all parts of the building.


Question #1: Which process comes first in airflow system design?

  1. Building

  2. Testing

  3. Zoning

Scroll down for the answer...

Answer: Zoning

Zoning is always the first step in the design of air systems.

Question #2: Which ductwork part has smaller branches of duct coming off of it?

  1. Plenum

  2. Trunk Duct

  3. Take Off

  4. Air terminal devices

Scroll down for the answer...

Answer: Trunk Duct

The trunk duct is the central duct. It connects to the other branches of ductwork that travel throughout the building.

Question #3: Which building would be best for radial ductwork?

  1. An Office Building

  2. A Single Family Home

  3. A Large Museum

  4. An Apartment Complex

Scroll down for the answer...

Answer: A Single Family Home

Radial ductwork is best suited for small buildings like single family homes and other small houses.

Question #4: Reducing Extended Plenum Systems are _______ cost effective than Extended Plenum Systems.

  1. More

  2. Less

  3. Equally

Scroll down for the answer...

Answer: Less

Reducing extended plenum systems save material costs and work better than normal extended plenums.

Question #5: Which fitting would you use to change from a circular duct to a rectangular duct.

  1. Elbow

  2. Wye Fitting

  3. Transition

  4. Boot

Scroll down for the answer...

Answer: Boot

Boots can change from one duct shape to another.

Question #6: What ductwork component can be replaced by flex ducts?

  1. Vent fans

  2. Elbows

  3. Grilles

  4. Mastic

Scroll down for the answer...

Answer: Elbows

Because flex ducts can bend and change direction, there is no need for elbow joints.

Question #7: Where in a house would a downflow system be located?

  1. Basement

  2. Garage

  3. Living Room

  4. Attic

Scroll down for the answer...

Answer: Attic

Downflow systems are supposed to be located in the highest point in the house. This makes attics the best choice.


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