Let's face it: many in our industry don't believe in using flexible duct. The reason: They think the material used in an installation determines how well the HVAC system moves air. The facts: the material type doesn't matter. Instead, it's how we design and install the product.
Testing poorly performing systems that use flexible ducts, you may find 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. Here are five things you can do to provide better flex duct installations and ensure your systems work as intended.
1. Avoid Sharp Turns
First, avoid sharp flex duct turns at all costs to increase the performance of installations. Systems work best with ducts running as straight as possible. This isn't always possible because of the many obstacles in homes today.
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 air travel path. Sharp, 90-degree turns kink the flex duct internally, reducing delivered airflow. Since the sharp turn restricts airflow, system static pressure increases.
One common place where this restriction happens is improper duct connections to takeoffs and boots. At the connection point, an abrupt turn often 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 framing issues, you may need to reroute ducts or find another location to avoid sharp turns.
2. Add Duct Support
Second, poor airflow and comfort complaints often occur because of sagging from inadequate duct support. Many installers only suspend ducts every five to six feet, which results in severe drooping. This condition worsens over the life of the duct system and will continue reducing airflow. Ideally, the flex duct should not sag over one inch in a four-foot length.
Kinks and sags in the ducts require additional support. When you use narrow materials for hanging, such as plumbing straps 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. You can correct these problems by providing support at more frequent intervals, such as every three feet instead of five, six, or seven feet.
When you install more support, make wise choices in strapping materials to prevent unintentional restrictions. At a minimum, use three-inch wide hanging straps or metal bands to cradle the duct. Duct Saddles are a premium product that works well for supporting flexible ducts.
3. Remove Excess Core
Third, watch for this common defect that results in low airflow: the flexible duct inner core bunches up as it's fastened to a boot or takeoff. Bunching 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 three 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 six-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 remove any excess core. Complete the connection by attaching it to the other end and finishing the installation.
4. Avoid Remote Plenums
The fourth tip involves remote plenums. These are rectangular boxes or triangles made from ductboard often 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 there are issues you should know about.
These fittings have a large amount of pressure drop and lack direction for airflow as air attempts to leave the fitting. That air gets lost inside the plenum mainly because of momentum loss in the fitting. In other words, as air enters the plenum from the duct feeds, 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 higher than the remote plenum, but the improvement in airflow performance will show up immediately.
5. Size Them Right
Finally, my fifth tip is to size those ducts correctly. You can follow the previous steps to the letter and still have a flexible duct system with poor performance if you size ducts using old-school rules of thumb. When you size flexible ducts using methods intended for sheet metal ducts, 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.
Your systems will perform much better if you design using larger flexible ducts. Many like to use the .10 friction rate on a duct calculator and assume a six-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. Doing this gives you a greater chance of success and delivers systems that perform closer to intentions.
Trust but Verify
You can battle over duct design methods all day long, but until you take measurements and verify that the installation delivers the required airflow, it's strictly a guess.
Measured airflow values from a balancing hood are where the rubber meets the road with any flexible duct installation. Using the tips above, you can show your installers the increased airflow from these improvements. Help them see how their attention to detail makes a difference.
Share these tips with your installers, and dare to install your duct systems correctly. Give your crews the flexibility to do the job right the first time. Your customers will appreciate it, and you will notice fewer callbacks.
David Richardson serves the HVAC industry as vice president of training for National Comfort Institute, Inc. (NCI). NCI specializes in training focusing on improving, measuring, and verifying HVAC and building performance.
If you're an HVAC contractor or technician interested in improving your installations, contact David at ncilink.com/ContactMe. NCI's website, www.nationalcomfortinstitute.com, is full of free information to help you improve your professionalism and strengthen your company.