Flex Duct… As Good As Sheet Metal?

– by Harry Boody

Harry Boody extolls the virtues of proper flex duct installation and of being willing to challenge your own observations. His conclusion? You CAN teach an old dog new tricks.

I’m told Flexible Duct (pipe) has been around for more than 150 years but the Flexible Air Ducts (as we know it today) came into its own around the 1930’s. It had its specific limited purpose where hard air duct piping was too difficult to install. The HVAC industry was predominantly all “Hard” sheet metal ducts. Then over the years (probably in the 1970’s) Flex Duct became more popular due in part to its cost (labor) cutting advantage. But there were unintended consequences; it promoted sloppy, ill-designed Air distribution installations.

Rather than going into the in-depth history and engineering justification calculations of the Flex Duct industry, I will tell you a real life story of redemption from the baring of false witness against the Flex Duct industry claiming it will destroy the HVAC Air distribution way of properly moving airflow. Let’s say, you can teach an old dog new tricks.

It was back in the late 70’s while researching ways to reduce energy consumption and improve comfort in homes that I discovered one of the culprits was the use of Flex Ducts which was being installed in a way that looked like spaghetti with ducts running over, under, in and out. It was an airflow nightmare.

That’s when I started preaching that all Flex Duct was evil, and hard sheet metal ducts must prevail for the good of the HVAC industry. Of course, all of our HVAC air distribution designs incorporated all hard sheet metal air ducts with the directive that NO Flex Duct shall ever grace any of my HVAC system designs. Soon the word was spreading around that my system designs was cutting energy consumption a lot and with enhanced comfort.

When I was asked back then (now in the 80’s) by the news media, technical magazines, Duke Energy and Piedmont Natural Gas, etc. to what I credited the success of my results, I said it’s because I do not use any Flex Duct; it’s a sinner in the HVAC industry. Well, they printed that preaching and shortly after that, I received a visit from the “white suits and ties guys” from the Flex Duct industry. That visit was a game changer.

I remember them being very professional, courteous and extremely concerned about the accusations I had been making which were gaining traction. They listened to all of my reasons why I thought Flex Duct was bad and when I was finished, one of them (who was an engineer) said to me, “Have you ever reviewed any of our Flex Duct engineering testing data?” Surprisingly, I realized I had not. I had to admit my findings were based on what I had witnessed by sight alone.

They agreed there was a serious problem in the industry that Flex Ducts were not being installed correctly but my accusations and blaming the Flex Duct products was misdirected, and the cause was bad installation practices. They made it clear to me that my statements over the years had caused them damage to their reputation but rather than making an example of crucifying me for defamation (which they surely had a case), they chose to challenge me to study all of their engineering testing data and to see if any of my negative claims lined up with their testing data. 

And, if the data showed that Flex Ducts (if installed properly) yielded results that met my standards, would I be willing to start designing Flex Ducts into my HVAC system designs? I monitor (track) every job I do because I guarantee to not exceed energy consumption or I will pay the difference.

It was obvious to me after reviewing the engineering data that there was not a technical reason why Flex Duct could not be incorporated into my HVAC designs. I also could see I was being given a get out of jail free card. So that’s what I did. I starting using “properly designed and installed” Flex Ducts and tracked the results, which was phenomenal. We found better energy performance, improved comfort and a tremendous reduction in labor costs. It was a win-win across the board.

Today, I find myself utilizing a lot of Flex Duct into my system designs and in cases, I have run Flex Duct lengths in excess of 100 feet without any problems. But here’s the caveat, Flex Duct must be sized and installed per the manufactures guidelines (which is a code requirement). Unfortunately, in many cases, it’s never done that way… so the “Beat goes On.”

Without being overly technical, below are some tips for installing Flex Ducts to stay out of trouble:

These principles are a good start for properly installing Flex Duct.

Remember you can’t judge a book by its cover and you can teach an old dog new tricks. Also, I’ll never forget the moment when I heard a TV evangelist say… “Perception can lead to Deception!” 

Principles for properly installing Flex Duct

  • Realize the same size round Flex Duct has a different pressure drop than hard sheet metal pipe.
  • The Flex Duct Liner MUST be pulled tight and NOT twisted to achieve proper results.
  • Any turn in the Flex Ducting exceeding 60 degrees should have a hard sheet metal elbow.
  • Flex Duct needs to be supported a maximum of every 32″ for up to 10″ round duct.
  • Flex Duct needs to be supported a maximum of every 24″ for 12″ round and up. 
  • Follow the manufacturer’s installation instructions for properly connecting to Trunkline A-collars.
  • Install the proper R-value Flex Duct per conditioned and unconditioned area.
  • Inspect the work that your HVAC installers are doing.
  • All vertical ducts over 24″ should be hard pipe.
Ductwork examples with Harry Boody

Harry Boody explains that properly designed and installed Flex Duct improves energy efficiency and adds to the longevity of the HVAC unit.

Examples of proper and improper installation of flexible ductwork

If the ductwork are arteries and the HVAC unit is the heart, then improperly installed Flex Duct is a heart attack waiting to happen.

Harry Boody

About the Author: Harry John Boody is founder and President of Energy Innovations by Harry Boody, Inc. He is a multi-International Technology Award Recipient for Outstanding Achievement in HVAC Design from the American Society of Heating, Refrigerating and Air Conditioning Engineers [ASHRAE]. He has been honored with three Governor’s Awards for Excellency in Energy Efficiency and the National Energy User News First Place Award for Excellence and Innovation in Energy Efficiency and Building Management. Harry Boody was the ASHRAE Regional First Place HVAC Technology Award winner for nine consecutive years. Since 1979, Boody has specialized in the Total Systems Design Approach to healthy, cost effective, energy efficient structures. Boody guarantees (for life) the HVAC energy usage and mold free designs. To date, he has completed thousands of HVAC Energy Efficient designs and has never failed to achieve the desired results.

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This article originally appeared in the April / May 2023 issue of Southern PHC print edition. To view this and other issues online, visit the DIGITAL ISSUES PAGE.

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