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longevity of HVAC flex duct

longevity of HVAC flex duct

What is the functional lifespan of a flexible duct system? A search of industry articles, blogs, and chat rooms indicates it lasts from 10 to 25 years. Most flexible duct manufacturers warranty their products for about 10 years. Let's take a look at the longevity of flexible duct systems and the opportunity they offer.

First, let's address the elephant in the room. Do you love or hate flex duct? What influences your opinions about flex duct? Company culture, region of the county, training, local codes, or even family traditions may affect your opinion. 

I would ask you to consider that my unique perspective comes from decades of testing duct systems. Generally speaking, whether a duct system is metal, flex, or duct board, makes little difference. All that matters is the duct system delivers proper airflow and temperatures from the equipment to the conditioned space. If the duct system can fulfill these requirements, the material has little effect on a system's performance.  

There are a variety of duct applications and specific needs for each job to consider. But if you have disdain for flex, I'd invite you to lighten up and increase your flexibility to its virtues.

Let's assume the flex duct has a functional life of 20 to 25 years. Perhaps you are unaware, but each day across the country, duct systems of all material types are rebuilt and replaced. Many systems less than five years old perform poorly and will benefit from redesign and upgrades.  

Many factors affect the life of a flex duct system. These include:

chantanee/iStock/Getty ImagesInferior duct systems reduce equipment performance by 40% or more as the equipment operates outside of published specifications. Most likely, these ducts needed upgrades since the day they were born.    

Whatever the magic number of years flex lasts, once you become aware of duct deterioration, the occasion is yours to identify new opportunities for you and your customers.  

An overwhelming percentage of your customers need your expertise to resolve duct system issues. If you don't resolve these issues, chances are your customers will unknowingly suffer from substandard comfort and efficiency long into the future. I say, if they do, it's your fault.  

Sizing up the Need for Flex System Improvements

Data supporting the number and age of flexible duct systems are not readily available. But here are a few facts to help you size up the opportunity for yourself.

The U.S. Energy Information Administration documents about 77% of the 140 million homes in the United States have central heating and cooling systems.   

Flexible HVAC duct was patented late in 1965. That means we have been using flex for about 55 years. According to the 2019 U.S. Census Bureau, American Housing Survey, roughly 54% of existing homes were built from 1965 to 2000. 

Here's the bottom line. In most areas of the country, approximately four of 10 homes have flexible duct systems beyond their useful life that need repair or replacement.   

What's most important is that each home's duct system is unique. Your assessment of each duct system confirms the duct system is good to go or needs an upgrade or replacement. 

You can assess the duct system's functional life and performance using several inspections and test methods. This assessment should be quick and conclusive. Consider one or two of the following methods and try them on the systems you work with this next week.

Duct Inspections – Can you stick your finger through the flex? This may sound simple, but many 30+-year-old flex systems don't pass this test. A visual inspection and a couple of pictures with your phone may reveal the obvious need for flex replacement. 

Is the flex poorly installed? Sagging duct, failed duct connections, excessive bends and pinches, and long twisted duct runs are evidence the duct system will not function correctly. This week I used the term "Air don't do that" while inspecting a duct system with a homeowner. He immediately understood it was time to upgrade his 33-year-old duct system.

Duct Sizing – Yes, the original duct system is undersized.  Flex and metal duct system need different sizing methods. Here at NCI, we developed duct sizing tables based on thousands of field airflow measurements from across the country. Thousands of HVAC professionals use these tables to assess duct sizes of all types. Send me an email if you'd care to receive a copy and a procedure on how to use it. 

Pressure Testing – You can take the supply or return duct pressure and interpret how restrictive a duct system is in just a few minutes. You will need a manometer with a pressure hose, static pressure tip, a couple of test port plugs, and a drill with a 3/8" bit.

Install a test port in the duct or plenum a few inches before the air enters and exits the equipment. Measure and record a single pressure for the supply and another for the return duct system.

Find the maximum rated total external static pressure on the air-moving equipment's nameplate. Multiply this number by 20%. For example. If the fan is rated at .50" w.c. x .20 = a maximum duct static pressure of .10". If either of the duct pressures exceeds that pressure, duct resistance is excessive and needs work to improve airflow performance.

One of the best old duct systems I ever tested was made from rejected printed sheets of Folger's Coffee cans. It was built during World War II. As I said before, the duct material often doesn't matter. 

What DOES matter is that duct systems are overlooked thousands of times each day by service techs and salespeople. The result is that customers continue to suffer from poor comfort and excessive utility bills. 

The duct system controls HVAC system efficiency and comfort. Ideally, you'll add some level of duct assessment to most equipment replacements and begin to discover this opportunity during maintenance visits.

Rob "Doc" Falke serves the industry as president of National Comfort Institute, Inc., an HVAC-based training company and membership organization. If you're an HVAC professional interested in a free duct sizing table, contact Doc at ncilink.com/ContactMe or call him at 800-633-7058. Go to NCI's website at nationalcomfortinstitute.com for free information, articles, downloads, and current training opportunities