Installation: Installer equals poor airflow performance of flexible ducts. Great installation equals great airflow performance from flexible ducts. You decide how your product will work. (courtesy of David Richardson)
Many in our industry believe that the duct material used in an installation determines the ability of an HVAC system to move air. Because of this mindset, flexible ducting often gets a bad rap. The problem is not the type of material. Instead, we install the product.
When you test inefficient systems that use flexible ducting, you will encounter recurring installation problems that reduce airflow and reduce comfort and efficiency. However, by paying attention to details, you can easily correct and prevent the most common mistakes. Let’s take a look at five tips to help you better install flexible ducting to keep your system working properly.
To improve the quality of the installation, avoid sharp turns of the bent pipe at all costs. The system works best when you lay the pipes as straight as possible. With so many obstacles in modern homes, this isn’t always an option.
When the pipe has to make turns, try to keep them to a minimum. Long, wide turns work best and allow air to pass through more easily. Sharp 90° bends the flexible tube inside and reduces the airflow supplied. As sharp turns restrict airflow, the static pressure in the system increases.
Some common places where these restrictions occur is when plumbing is improperly connected to take-offs and boots. Joints often have tight turns that disrupt the airflow. Correct this by giving the duct enough support to change direction or by using sheet metal elbows.
Structural framing is another common problem you’ll find in many attics. To fix this, you may need to reroute the pipe or find another location to avoid the sharp turn.
Another common cause of poor ventilation and comfort complaints is sagging due to insufficient piping support. Many installers hang the pipes only every 5-6 feet, which can cause a lot of sagging in the pipe. This condition worsens over the life of the duct and continues to reduce airflow. Ideally, flexible pipe should not sag more than 1 inch over a 4 foot length.
Bends and sagging pipes require additional support. When you use a narrow hanging material such as adhesive tape or wire, the duct may become clogged at this point. In severe cases, wires can cut into ducts, causing air to leak into unconditioned areas of the building.
When these imperfections are present, the air is blocked and slowed down. To eliminate these problems, install supports at more frequent intervals, such as every 3 feet instead of 5, 6, or 7 feet.
As you install more supports, choose your strapping material wisely to prevent unintentional restraint. Use at least 3-inch clamps or metal clamps to support the pipe. Pipe saddles are a quality product that can also be used to securely support flexible pipes.
Another common defect that causes poor airflow occurs when the flexible core of the duct gets knocked off when attached to the boot or when removed. This can happen if you don’t stretch the core and cut it to length. If you don’t do this, the sticking problem will be exacerbated by compressing the core as soon as you pull the insulation over the boot or collar.
When repairing ductwork, we typically remove up to 3 feet of extra core that may be missed on visual inspection. As a result, we measured an airflow increase of 30 to 40 cfm compared to a 6″ duct.
So be sure to pull the pipe as tight as possible. After attaching the pipe to the boot or removing it, tighten it again from the other end to remove the excess core. End the connection by connecting to the other end and completing the installation.
The remote plenum chambers are rectangular boxes or triangles made from ductwork in south attic installations. They connected a large flexible pipe to the chamber, which feeds several smaller pipes that exit the chamber. The concept looks promising, but they have issues that you should be aware of.
These fittings have a high pressure drop and lack of airflow direction as the airflow tries to leave the fitting. Air is lost in the plenum. This is mainly due to the loss of momentum in the fitting when the air supplied from the pipe to the fitting expands into a larger space. Any air speed will drop there.
So my advice is to avoid these accessories. Instead, consider an extended boost system, a long jump, or a star. The cost of installing these equalizers will be slightly higher than installing a remote plenum, but the improvement in airflow performance will be noticeable immediately.
If you size duct according to old fashioned rules of thumb, you can do the same thing as before and your duct system will still perform poorly. When you use the same methods that work for sheet metal piping to size flexible piping, it results in low airflow and high static pressure.
These piping materials have two different internal structures. Sheet metal has a smooth surface, while flexible metal has an uneven spiral core. This difference often results in different airflow rates between the two products.
The only person I know who can make flexible ducting like sheet metal is Neil Comparetto of The Comfort Squad in Virginia. He uses some innovative installation methods that allow his company to achieve the same pipe performance from both materials.
If you can’t reproduce Neal’s installer, your system will work better if you design a larger flex pipe. Many people like to use a friction factor of 0.10 in their pipe calculators and assume that 6 inches of pipe will provide a flow of 100 cfm. If these are your expectations, then the result will disappoint you.
However, if you must use the Metal Pipe Calculator and the default values, select a pipe size with a friction coefficient of 0.05 and follow the installation instructions above. This gives you a better chance of success and a system that is closer to the point.
You can argue all day about duct design methods, but until you take measurements and make sure the installation delivers the airflow you need, it’s all guesswork. If you’re wondering how Neil knew he could get the metallic properties of coiled tubing, it’s because he measured it.
The measured airflow value from the balancing dome is where the rubber meets the road for any flexible duct installation. Using the tips above, you can show your installer the increased airflow these improvements bring. Help them see how their attention to detail matters.
Share these tips with your installer and find the courage to properly install your plumbing system. Give your employees the opportunity to get the job done right the first time. Your customers will appreciate it and you will be less likely to call back.
David Richardson is a Curriculum Developer and HVAC Industry Instructor at the National Comfort Institute, Inc. (NCI). NCI specializes in training to improve, measure and verify the performance of HVAC and buildings.
If you are an HVAC contractor or technician and would like to learn more about high precision pressure measurement, please contact Richardson at davidr@ncihvac.com. The NCI website, www.nationalcomfortinstitute.com, offers many free technical articles and downloads to help you grow professionally and strengthen your company.
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