Does Flex Duct Reduce Airflow?

There is no questioning of the flexible duct that is revolutionized the HVAC industry. The arrival of flex duct means that contractors now can dramatically reduce the amount of time and skill it takes to install a duct system. However, the key to its optimum performance relies on the proper installation, and too often, flex ducts are not installed correctly.

Does Flex duct reduce airflow? Yes, the flex duct is known to slightly reduce the airflow compared to a smooth duct on the same diameter. In larger duct sizes, the volume of the air in corrugated ducting at the same pressure drop is up to 20% less.

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On the other hand, the ribbed constructions makes more friction than a smooth wall, hence the less flow of air. In this short read, you will understand more the real issue with the flex duct and why it is vital to know the primary causes of reduced airflow.

The Problem With Flex Duct

With the popularity of flexible ducts in the HVAC world, it would seem that most people should know how to install it properly. There are plenty of reasons and combinations of things that play a role in mishandling the installation of the duct system. People look to shave cost, and they are pushed for time, and as a result, they rush the job instead of taking time to do a good job. Some professional services are not taking the time to get properly trained when it comes to the installation process.

One of the most common installation errors that contractors are guilty of is installing ducts that are too small. For instance, contractors put a 6-inch duct where it should be an 8-inch duct. Another issue is that they are not properly joined together and might result in leakage.

Causes Of Reduced Air Flow In Ducts

When it comes to reduced airflow in ducts, two things are responsible for giving blower in your furnace or air handler. As a result, they make the blower push more pressure, and this leads to reduced airflow or an increase in your energy use. Here are the two causes of this phenomena:


The first cause of reduced airflow is none other than friction. When air moves through a duct rubs against the inner surface, it can lose its energy. It will eventually slow down, and the pressure will drop. The more rubbing movement, the more of those things can happen. It is like walking down a busy street with your shoulder rubbing against other people.


The other cause of reduced airflow is turbulence. It is a kind of friction of the air rubbing against itself. The main cause of the turbulence within ducts is turning the air. When the air is sent through a 90-degree turn, the suitable type you use to do so makes all the difference.


Overall, it is a known fact that the flex duct can work well if it is sized and installed correctly. It is a very versatile product that is both a blessing and a curse. There are plenty of times that flex ducts are done wrong, and this is why problems arise. When you design and install ducts, you need to know first the science behind it as friction and turbulence can play a significant role in your duct system.

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