HVAC duct systems provide efficient air delivery, improve indoor air quality, conserve energy, enhance occupant comfort and are quiet and cost-effective. Below is a comprehensive guide to HVAC duct systems to give you a better understanding of how they work and the different types.

Purpose of an HVAC Duct System

The purpose of any HVAC duct system is to distribute reliable and efficient heating and cooling throughout a residential home or commercial building. For the purpose of this guide, we will discuss HVAC duct systems from a residential perspective.

As with many other construction activities, duct fabrication consists of multiple, complex steps. Contractors should review all relevant regulations impacting HVAC duct systems before installing one in a home. These regulations typically outline minimum requirements in regard to energy efficiency, duct leakages, system testing and air pressurization.

How Does a Duct System Heat and Cool a Home?

The simplest way to understand how a duct system works is to take a look at each of its components.

Pipes and Trunks

Flexible aluminum pipes and steel trunks operate as air distribution channels in a ductwork system. A trunk connects directly to the HVAC system, with the pipes branching out from it into various rooms throughout the home.

Duct Transitions

The trunk and pipes connect to one another via specialized attachments known as duct transitions. Securely connecting the trunk and pipes via the transitions is vital to preventing air loss. Installers try to avoid making sharp turns when attaching ductwork via the attachments because this can limit airflow and decrease heating and cooling efficiency.

Some of the more common attachments used to secure pipes and trunks are:

  • Directional takeoffs
  • Rectangular takeoffs
  • Round takeoffs
  • High-efficiency takeoffs
  • Vanes
  • Cleats


All HVAC duct systems need a plenum, which is a large container used for storing air that directly attaches to the trunk lines. Many HVAC systems require two plenums, with one being a return plenum and the other a supply plenum. The supply plenum stores the heated and cooled air before it’s released through the duct system. Any returned air goes to the return plenum and then makes its way outside of the house through a flue and then back through the system once it goes through an air filter.

Air Handler

An HVAC system’s air handler contains the blower fan and filter. The purpose of the blower fan is to blow heated or cooled air through the ductwork. Depending on the exact system type, the air handler may consist of heating and cooling coils, too.

Flues and Exhaust Outlets

Heating and cooling air can, sometimes, produce toxic gases, such as carbon monoxide. Duct systems push these toxic gases outside of the house through exhaust outlets and flues in order to avoid carbon monoxide poisoning.

Drain Pans and Drain Lines

In addition to heating and cooling air, HVAC systems pull moisture out of the air through a dehumidification process. It’s not uncommon for condensation to build up inside the system during this process, which can rust the system and harm your health. To avoid these issues, duct systems release condensation buildup into a drain pan. To keep the pan from overflowing, drain lines release the moisture buildup outside of the house.

Refrigerant Lines

HVAC duct systems heat and cool air through a process that uses liquid refrigerant. The refrigerant makes its way into the system’s compressor via a refrigerant line.


Once heated or cooled air fills your home’s ductwork system, it gets released into individual rooms and spaces via air vents. It’s crucial to keep these vents clean because dirty vents impede airflow, cause unnecessary strain on the HVAC system, and lead to poor indoor air quality.

Air Filter

An HVAC system’s air filter captures harmful airborne particles before heated or cooled air gets released into the ductwork and makes its way into your home. Changing the filter on a regular basis is crucial to indoor air quality and the longevity of the HVAC system.

Electrical Components

All HVAC systems attach to a circuit breaker for power and use several electronic components to operate. A thermostat regulates interior temperatures according to your preferred settings, while electronic sensors and switches help maintain optimal humidity levels.

Three Types of Duct Systems

Duct systems come in three primary types: rigid ductwork systems, semi-rigid ductwork systems and flexible ductwork systems.

1. Rigid Ductwork

Rigid ductwork comes in all sorts of sizes and materials, and it is often insulated. Many homeowners opt for rigid ductwork systems because of their durability and reliability. This type of ductwork consists of rectangular or spiral ducts to create the ductwork system.

Rigid duct systems come in three common material types:

Rigid duct systems made of sheet metal are the types you see in action movies, where the characters crawl through a building’s or home’s ductwork to make a great escape. Made of non-porous aluminum or galvanized steel, sheet metal ducts are lightweight, simple to install and less prone to mold and mildew buildup.

Fiberglass-lined ducts are made of sheet metal with an interior lining made of fiberglass. Although more commonly used in commercial settings, fiberglass-lined ducts work well in homes in need of noise reduction. However, because of their high susceptibility to deterioration and potential harm to occupant health, it’s best to avoid installing fiberglass-lined ducts in residential homes and commercial buildings that don’t have state-of-the-art air purification systems.

Fiberboard ducts are made of fiberglass strands that are compressed with resin and then sealed with a sheet of foil laminate for moisture-protection purposes. Rigid duct systems consisting of fiberboard ducts present several health concerns because they are breeding grounds for mold and mildew, especially when installed in humid climates. Fiberboard ducts have rough surfaces as well, which hinders airflow and overall heating and cooling efficiency.

2. Semi-Rigid Ductwork

Known for being the most effective type of duct system, semi-rigid systems provide optimal ventilation and zero air leakage. Semi-rigid systems are of particular value in homes where installers must switch between spiral and rectangular ducting because they allow you to do so without compromising system performance or losing hydraulic pressure. The only downside to semi-rigid systems stems from an increased risk of crushability.

3. Flexible Ductwork

Most flexible ductwork systems are crafted from insulated, spiral-shaped plastic ducts that easily bend. Flexible ducts perform well in homes where ductwork needs to attach to an air supply in hard-to-access spaces. When installing flexible ductwork, installers must securely fasten the entire system to eliminate potential sagging. An installer will also strive to minimize any turns and kinks in the duct network so as to avoid airflow restrictions and inefficiencies.

Contact Us to Learn More About Duct Systems

Now that you have a better understanding of how duct systems work, you can make a more informed decision about installing one in your home. We specialize in a wide range of heating and cooling services and offer maintenance and repair for plumbing systems as well. Contact HB Home Service Team now to schedule an HVAC installation consultation and to learn more about your duct system options.

HB Home Service Team

company icon