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The Importance of Ductwork Design

The ducting system that distributes heated and cooled air to your home has a profound impact on your comfort, energy consumption and health. HVAC contractors should follow best practices when sizing, building and installing ductwork.

These include ensuring that equipment is properly sized using ACCA’s Manual S protocol, and that duct sizes are correctly determined through a balancing procedure known as Manual D.


The length of ductwork is important because it affects the flow of air through your home. If your ductwork is too long, it can cause pressure drops and friction that reduces the velocity of the air flow. This means that you won’t feel the air blowing as strongly from your vents. The duct size is also very important, as it determines how much air your HVAC system has to work to keep your house cool or warm.

Ducts come in a variety of shapes and sizes, but they are typically made of sheet metal and cut to length from coils or flat stock. Round ducts are most common and cost less to fabricate then rectangular ones. Sheet metal fabrication shops can make any size duct required, but it is often easier to start with pre-fabricated duct sections that are close in length to the final duct size needed.

As air moves through a duct, its velocity varies across its cross-section, and the velocity is proportional to the square of the cross-section area. This variation in velocity is known as a’velocity profile’ and must be considered when designing a system. For this reason, standard ducts are measured with multiple positions along their length, and ‘pitot traverses’ are used for commissioning purposes to measure the average velocity of a system.


Even the best ductwork design can be compromised by poorly sealed and insulated sections. In addition to wasting energy, this can lead to problems such as hot or cold spots in rooms and extra wear and tear on air conditioning equipment due to the need to work harder to compensate for poor flow.

The insulating requirements of the duct system depend on its location and climate. Sheet metal ducts in attics, for example, may deteriorate faster than those installed in the ceiling or in a conditioned space. In general, ducts should be insulated with rigid foam board or other insulation to prevent thermal transfer and to keep conditioned air at the temperature intended.

It’s important to have all ductwork properly sized for the building, and an HVAC professional can help with this by using published industry standards to size the system. A correctly sized system will deliver conditioned air to all areas of the space without causing excessive wear or consuming excess energy.

The best ducts route indoor air to and from living spaces through registers or diffusers rather than through structural voids in walls or floors. When possible, ducts should be located in the interior ceiling and supply and return registers should be placed on each floor of the building, so that every room has access to conditioned air. It’s also important to seal and insulate all duct sections to prevent leaks, which waste energy and can cause odors, drafts and other air quality problems.


One of the biggest problems with ductwork is poor sealing, especially at the connections between duct runs. A good contractor will use mastic duct sealant or metal foil tape to secure these joints.

Leaks in ducts can allow conditioned air to escape from the home, or draw in contaminated outside air. When these leaks occur, the HVAC system has to work harder to heat and cool the house. The extra energy used to compensate for lost conditioned air causes the system to overheat, and this shortens its lifespan.

Another problem with leaky ducts is that they can introduce moisture into the home. This can lead to mold and mildew problems in attics, crawl spaces, and basements. Leaks in return ducts can also introduce outdoor pollutants and hazardous vapors from paint, cleaning supplies, gasoline, and automotive products into the home.

Duct sealing can eliminate these leaks and reduce energy consumption in the home. This means less pollution for the environment, a smaller carbon footprint, and more money in the homeowner’s wallet. Contact Warner Service to schedule an appointment for a professional to examine your ductwork and recommend suggestions to improve the flow of air through your system. We will test static pressure in the ducts and perform airflow tests to ensure your heating and cooling system is operating at maximum productivity.


The air that a duct system transports from the HVAC equipment to points throughout your building requires a path of least resistance. Straight and properly insulated ducts can minimize resistance, increasing energy efficiency. However, the configuration and layout of ductwork also has an impact on your system’s performance.

To maximize performance, the ducts should be routed within the conditioned envelope of the building whenever possible. Ducts that run in unconditioned attics, crawl spaces or basements suffer thermal losses that reduce energy efficiency and increase utility bills. Additionally, ducts routed inside exterior walls create condensation in wall voids that can lead to rot and mold.

Once the ducts are routed they must be properly sized to accommodate the amount of conditioned air your system needs to move through them. An HVAC design professional can determine the best duct size for your system by using a duct sizing software program or calculator.

One common problem that leads to poor ductwork design is not providing enough return vents for the system to pull air back into the supply ducts. This can cause hot and cold spots in a room, excessive wear on the equipment and unhealthy indoor air quality.