How Ducts Are Made

How many times have you watched movies or TV episodes that feature characters crawling through ductwork? In the film world, fictional characters seem to believe without question that ducts offer stealthy entrance into hard-to-access rooms.

In reality, though, most ducts in homes and buildings are too small for humans to crawl through. Although it’s impossible to get to know the duct system at your home or workplace from the inside out, you can appreciate ductwork more by learning how it’s made.

Duct Materials

Builders and HVAC specialists use various materials to create ductwork. Here's an overview of the most common duct materials.

  • Metal. This is the most common base material for ductwork. Galvanized steel and aluminum are the most common metals used to create ducts. Many metal duct manufacturers add fibreglass liners to insulate the duct and reduce air leaks.
  • Fibreglass. This material is an alternative to metal ductwork. Fibreglass duct board is lightweight and has built-in heat and sound insulation. However, it’s also more expensive than traditional metal ducts.
  • Plastic. Ducts made from plastic are notable for their flexibility. A wire coil in the plastic helps the duct retain its desired shape. Plastic ducts sometimes attach to more rigid metal or fibreglass duct systems.
  • Asbestos. Older homes and buildings sometimes had ducts made from asbestos. This material is hazardous, so call in an HVAC specialist if you suspect you have asbestos ducts.


Because metal is the most common duct material, the next sections of this blog focus on metal duct systems.

Duct System Components

Metal duct systems have two main branches. The supply branch sends conditioned air from the furnace or air conditioner to the rooms. The return branch takes air from the rooms back to the heating and cooling units. The return branch is also where fresh outside air enters the system.

The individual branches consist of rectangular or round metal duct lines. These duct lines are not continuous. Instead, they come in smaller sections because this makes them easier to transport and install.

Not all rectangular and round metal ducts are straight. Many bend to create elbows and guide air from one straight duct to another. Other connector pieces, known as transitions, allow air to flow from large ducts into smaller ones or vice versa. Some connector pieces form X, Y, or Z shapes to join several pipes. In addition, many duct pieces create the opening where air enters or leaves a room.

Our team of HVAC installers would mix and match all these pieces to create your complete duct system. It's like assembling a giant puzzle that bends as it passes through walls, floors, and ceilings.

Duct-Making Machines

In order to create an HVAC system, the installer needs to design the system and assemble the various components. Luckily, automated machines perform the precise grooving, cutting, and folding required to fabricate new duct sections.

Let’s examine how machines turn flat sheet metal into straight rectangular or round duct lines.

Rectangular Duct Lines

Your house’s ductwork probably consists mostly of rectangular duct lines. Rectangular ducts fit well underneath floors, above ceilings, and inside walls.

A rectangular duct starts as a flat piece of sheet metal. This metal sheet passes through a coil machine, which puts grooves into the metal. The grooves act as guides later in fabrication and installation. A shearing machine or plasma cutter cuts the sheet metal to the appropriate size.

Next, other machines bend or cut the edges of the sheet. Then a mechanical arm bends the sheet metal once into an L shape or three times to create a tube shape. A worker joins two L-shaped duct pieces or seals the tube shape. That process creates one section of rectangular duct. Rectangular ducts come in standard sizes, but machines can make them smaller or larger as needed.

Round Duct Lines

These ducts are also known as spiral pipe. A spiral helix machine makes these duct lines.

The process starts when a spool of sheet metal feeds into the machine’s head. Inside the head, the sheet metal coils like a screw. Wherever the metal edges meet, the machine fuses the edges together, creating air-tight seams. This creates the distinct spiral appearance of a round duct line. A separate part of the machine cuts the pipe when it’s the desired length.

Spiral pipes come in standard lengths, such as 10 feet, but spiral helix machines can cut them to shorter or smaller sizes. They can also create different pipe diameters. These qualities make spiral pipe ducts easy to customize and fit into small and large spaces. They’re used more often in commercial and industrial buildings.

Now that you know more about how ducts are made, you’ll appreciate the intricate design and careful construction of HVAC systems in every building. That’s true whether you work in a warehouse with exposed ductwork or never see the ducts concealed behind your home's walls.

Keep checking our blog for more HVAC facts and tips, or contact our helpful and experienced staff at East Side Ventilation. We also offer comprehensive duct cleaning services in Winnipg to keep your ducts functioning.