Common Flex Duct Issues and How to Solve Them

Today's focus is on addressing some of the most common flex duct issues and how to fix them. In Tulsa, North East Oklahoma, and nearly all of the surrounding regions, flexible ductwork is by far the most commonly used in residential applications. This guide is perfect for homeowners looking to improve their comfort while saving money. With the right steps, you can identify and fix problem areas in your attic, basement, or crawl space—without needing a professional.

Before we dive into specific problems and solutions, let's quickly review the basics of how a forced-air heating and air conditioning system works. At the heart of this system is the air handler, which is responsible for heating or cooling the air. The air handler also contains the blower, which is the fan that circulates the air throughout the system.

In a forced-air HVAC system, the air is pulled in on one side and pushed out on the other. The ducts that carry the air to the system are known as return ducts, while the ducts that distribute the air are called supply ducts. The large duct connected to the air handler’s return side is called the return plenum, and similarly, the supply plenum connects to the supply ducts.

Now that we’ve got the basics covered, let's take a closer look at some common ductwork issues and how to address them.

1. Improper Sealing in Unconditioned Spaces
One of the most important factors in maintaining an efficient duct system is ensuring proper sealing, especially in unconditioned spaces like attics or crawlspaces. Unfortunately, even though ductwork sealing is widely recognized as essential, flex duct systems are often improperly installed in these areas.

Tip: Fully Extend the Flex Duct
When installing flexible ductwork, it’s critical to fully extend the duct before installation. Some experts recommend pulling a 25-foot section of flex duct for at least one minute before you begin the installation. This step helps reduce the compression and depth of the internal accordion-like corrugation, which can cause air resistance if left unaddressed. In fact, the Air Duct Council has found that 30% compression can increase air resistance by four times! So, making sure to fully extend your flex duct is an easy yet often overlooked step in ensuring efficient airflow.

Additionally, cutting off the 2 to 6 feet of excess flex at the end of the duct—rather than just using the whole length—can make a significant difference in the performance of your duct system.

2. Duct Sagging
Sagging ducts are another common issue, especially in older systems or those that have been retrofitted. Even small amounts of sag can create airflow problems by adding resistance and making the system less efficient. A study by Jack Rise revealed that the static pressure in a duct system increased by 0.2 inches of water column (wc) simply by fixing a sagging duct and properly strapping it.

Tip: Support Your Ductwork Properly
In retrofit situations, many technicians focus primarily on sealing duct connections. While this is important, failing to address sagging ducts can be just as problematic. The general rule is that ducts should sag no more than ½ inch for every 4 feet of duct length. It’s essential not to rely solely on the code-required strapping in your area—just because a system passes inspection doesn’t mean it’s been installed correctly.

3. Avoiding Excessive Curves
Curves in ductwork can have a significant impact on system performance. Each bend or angle in the duct increases the Total Effective Length (TEL), which can raise the Total External Static Pressure (TESP) in the system. A higher TESP can disrupt air balance and reduce system efficiency.

Tip: Keep Curves to a Minimum
To avoid these issues, it’s important to design and install ductwork with as few curves as possible. Each curve introduces resistance, so careful planning and routing of the ducts are key. Ensure that your ducts are extended, properly supported, and routed according to design specifications for optimal airflow and performance.

4. Sealing Duct Connections
Properly sealing duct connections is crucial for preventing air leaks, which can reduce the efficiency of your system and lead to higher energy bills. When working with flex ducts, mastic is typically used to seal the connections. It's essential to seal the inner liner first and allow it to dry before pulling insulation over the connection.

Tip: Focus on Common Leak Points
One area where leaks often occur is where the boot or can meets the ceiling or floor. It’s a good idea to seal this connection from both the inside and outside before installing the grilles. This extra attention to detail can go a long way in improving the performance of your duct system.

Conclusion
By addressing common ductwork issues like improper sealing, sagging ducts, excessive curves, and poor connections, you can improve both the comfort and efficiency of your heating and cooling system. With some basic knowledge and a few simple tools, these problems are often easy to fix yourself. Keep in mind that proper installation and regular maintenance are key to ensuring your duct system works at its best for years to come.