Many in our industry believe the duct material used in an installation determines how well the HVAC system moves air. Because of this mentality, flexible duct often gets a bad name. The issue isn’t the material type. Instead, it’s how we install the product.

When you test poorly performing systems that use flexible ducts, there are recurring installation problems that lower airflow and reduce comfort and efficiency. However, with some attention to detail, you can easily correct and prevent the most common mistakes. Let’s look at five tips that can help you can provide better flex duct installations to assure your systems work as intended.


No. 1: Avoid Sharp Turns

To increase the performance of your installations, avoid sharp flex duct turns at all costs. Systems work best when you route ducts as straight as possible. Because of many obstacles found in homes today, this isn’t always an option.

When a duct must make a turn, do your best to keep it to a minimum. Long, sweeping turns work best and allow an easier path for air to travel. Sharp 90° turns kink the flex duct internally and reduce delivered airflow. Since the sharp turn restricts airflow, static pressure in the system increases.

Some common places these restrictions occur are improper duct connections to takeoffs and boots. At the connection point, there is often an abrupt turn that destroys airflow. Solve this by providing plenty of support for the duct where it changes direction or use a sheet metal elbow.

Structural framing is another common obstacle you’ll find in many attics. To get around this, you may need to reroute ducts or a find another location to avoid sharp turns.


No. 2: Add Duct Support

Another common cause of poor airflow and comfort complaints is sags from inadequate duct support. Many installers only suspend their ducts every 5 to 6 feet, which results in severe drooping. This condition will worsen over the life of the duct system and continue reducing airflow. Ideally, flex duct should not sag over 1 inch in a 4-foot length.

Kinks and sags in the ducts require additional support. When you use narrow materials for hanging such as plumbing strap or wire, the duct becomes choked at that point. In severe situations, wire can cut into the duct, causing air leakage into unconditioned areas of the building.

When these defects exist, air gets choked off and slows down. To correct these problems, provide support at more frequent intervals such as every 3 feet instead of 5, 6, or 7 feet.

When you install more support, make wise choices in strapping materials to prevent unintentional restrictions. At a minimum, use 3-inch wide hanging straps or metal bands to cradle the duct. Duct Saddles are a premium product you could also use that works well for supporting flexible ducts.


No. 3: Remove Excess Core

Another common defect that results in low airflow occurs when the flexible duct inner core bunches up as it’s fastened to a boot or takeoff. This happens if you don’t pull the core tight and cut it to length. If you don’t do this, once you pull the insulation over the boot or collar, the bunching-up problem gets worse as the core compresses.

When renovating duct systems, we often remove up to 3 additional feet of core that a visual inspection would have missed. As a result, we’ve measured airflow increases of 30 to 40 cfm on a 6-inch duct.

So, remember to stretch ducts out as tight as possible. Once you connect the duct to the boot or takeoff, stretch it tight again from the opposite end to make sure you remove any excess core. Complete the connection by attaching to the other end and finishing the install.


No. 4: Avoid Remote Plenums

Remote plenums are rectangular boxes or triangles made from ductboard found in attic installations across the South. They connect one larger flexible duct into the plenum that feeds multiple smaller ducts that leave the plenum. The concept looks promising, but they have issues that you should be aware of.

These fittings have a large amount of pressure drop and lack direction for airflow as it attempts to leave the fitting. The air gets lost inside the plenum. This is mainly because of momentum loss in the fitting, as air from the duct feeding the fitting expands into a much larger space. Any air velocity drops there.

So, my suggestion is to avoid using these fittings. Instead, consider using extended plenum systems, lengthy transitions, or wyes. The cost to install these corrections will be slightly more than the remote plenum, but the improvement in airflow performance shows up immediately.


No. 5: Size Them Right

You can follow the previous steps to the letter and still have a duct system with poor performance if you size ducts according to old-school rules of thumb. When you size flexible ducts with methods intended for sheet metal duct, low airflow and high static pressure will result.

These duct materials have two different interiors. Sheet metal has a smooth surface, while flex has an uneven helix core. This difference often results in different airflow rates between the two products.

There’s only one person I know who can get flexible duct to perform like sheet metal duct and that’s Neil Comparetto of The Comfort Squad in Virginia. He uses some innovative installation techniques that allow his company to get similar performance from both duct materials.

Unless you can duplicate Neil’s install procedures, your systems will perform much better if you design your flexible ducts larger. Many like to use the .10 friction rate on a duct calculator and assume a 6-inch duct delivers 100 cfm. If this is your expectation, the results will disappoint you.

However, if you must use a metal duct calculator and default value, size your ducts at the .05 friction rate and follow the installation guidelines above. This gives you a greater chance of success and delivers systems that perform closer to intentions.


Trust and Verify

You can battle over duct design methods all day long, but until you take measurements and verify the installation delivers the required airflow, it’s strictly a guess. If you wondered how Neil knows he can get similar performance from flexible duct as metal, it’s because he measures.

Measured airflow values from a balancing hood are where the rubber meets the road with any flexible duct installation. Using the above-mentioned tips, you can show the increased airflow from these improvements to your installers. Help them see how their attention to detail makes a difference.

Share these tips with your installers and have the courage to install your duct systems the right way. Give your crews the flexibility to do the job right the first time. Your customers will appreciate it, and you will notice fewer callbacks.