HVAC uses of flexible air duct have grown exponentially over the last two decades, all over North America. Contractors and installers say they like it, and some find it indispensable. It has changed the way architects design residential construction and how they engineer tall buildings, offices, stadiums, stores, factories, hospitals, schools, and shopping malls.

"Flex" has helped create the "technical home," whose appearance, roominess, and comfort - supported by technology - have changed traditional, centuries-old thinking about how we heat and cool dwellings and other buildings.

For instance, John Van Asdlan, product manager for Rite-Way Ventilating in Tucson, Ariz., is a firm believer in flex duct. He has been installing mechanical systems for the past 30 years.

"I did my share of large commercial jobs, as well as several large tract developments. We used flex in all of these projects," he said.

Now, he concentrates 99 percent of his time on residential new construction, mostly tract homes that continue to advance out into the desert terrain, both northwest and southeast of his city.

"The old, Santa Fe colonial-style house with its flat roof and seven-foot, fur-down, soffit construction has been replaced almost entirely," he said.

Builders, hoping to add some volume to their 1,600-square-foot to 2,000-square-foot entry-level homes in the southern Arizona desert, adopted 13-foot vaulted ceilings to create the effect of extra space that people moving from California and the East were used to.

"It used to be, in residential construction, that you built the house around the mechanical system, with its rectangular-shaped hard ducts in soffits. Now, the mechanical system has to be installed around the house," said Van Asdlan.

The Tucson contractor "likes the flexibility" of the flex air duct. It bends easily around fixed structures in an attic, and the builder/contractor is not constrained by the rectangular soffit shape of hard, sheet metal air duct.

Building insulation standards have changed, too.

"We used to work above the batt [ceiling insulation] in the attic. Now, we work below the batt [roof insulation]," he said.

That change cuts costly heat transfer and enhances the longevity of the flex duct by providing a more temperate operating environment for it, Van Asdlan explained. It also cuts the cost of energy for the homeowner, he said.

"You can pull flex through the various web truss openings, get inside of vaulted ceilings, and still handle more cubic feet of air."

Quality Improvements Are Recognized

Van Asdlan remembers the early days of flex when plastic jackets would deteriorate as oils in the plastic leached out.

"In the bad old days, flex sometimes fell apart, except for the insulation," he said.

Today, Van Asdlan believes the quality of flex duct has been greatly improved. Scrim foil backing is now impervious to heat, he said.

"Now, we also increase the R-value factor to R-6 or R-8 to reduce the cost of heating and cooling," he said. "This contributes to increased efficiency of the HVAC system, which is another way that homeowners measure quality.

"Labor savings, eliminating different sizes and fittings for hard ducting, ruling out certain custom-fabrication concerns, using specially-designed ‘T' and ‘Y' branches and tapered floor boxes - all of these contribute to the benefits of flex. You can accomplish the same results with flex that you can with hard ducting. If you want to keep up with the game, you've got to use it.

A crowded ceiling environment can make flex duct an attractive choice for HVAC contractors to install.

Easy To Install

Estimator Reggie Olney, of Greg's Heating and Air Conditioning in Orland, Calif., listed some of the reasons his firm uses flex in their HVAC projects.

"It's flexible versus rigid, which eliminates fabricating elbows. It's economical to install, and it has integral insulation in its design," said Olney. "You can estimate needing roughly half the time to install flex duct than hard pipe.

"You run it straight, and it's done. We teach our guys to do nice, straight, clean runs with no airflow restriction."

Olney's business that is 80 percent residential and 20 percent light commercial construction. He maintained there is "less labor" in using flex. "I've had no trouble with it in 14 years," he said.

Greg's Heating and Air Conditioning normally uses hard pipe for the main air handler and adds flex as the last seven feet to the air grilles in a commercial job. A local distance learning center, the College of Siskiyous Junior College, upgraded its air handler and replaced hard pipe with more flex (5- to 7-foot runs) to dampen noise. Teachers at the college said that it provided a quieter environment for sending their course work from studio classrooms to home computers, Olney reported.

Where Sound Is The Key Issue

Associate partner and professional engineer Wade Cleary, of Marmon Mok L.L.P. in San Antonio, often finds sound to be the major issue when designing large, commercial HVAC systems for blue-chip clients. One medical research office building owner, a client moving from leased space to the architectural firm's custom-designed suites, "never fails to mention" how quiet his new offices are, Cleary said.

"We have taught our associates to be aware of sound, even if it is not a critical issue for the building owner. We model the system as a whole, not just the sound at the air handler," he said. "That means taking into account sound attenuation qualities of floor coverings (carpet or tile), wall composition, ceiling insulation, furniture, if the room is glass or wall - all of the materials that go into a room. We model it as a whole and then work backwards."

Cleary continued, "You have to be cognizant of all spaces. Our firm has created a chart based on the sound-deadening characteristics of the flexible air duct we specify. It shows various octave bands and how much we can knock the sound down. The flex air duct can be a real asset to us in decreasing the sound levels."

Sound-critical jobs have included conference rooms, a day-care center for a corporate entity, and a day-surgery operating room in a suburban medical center. "It had a very crowded ceiling," Cleary said of the operating room assignment.

Contained inside the elaborate, ceiling-mounted support system were a dense array of auxiliary ceiling lights; pipelines for medical gases; chill water and sewer lines; an emergency smoke removal system designed to evacuate smoky air in the event of a fire, offering time to safely move the patient. All were packed into the ceiling area where the air supply had to be positioned.

"We had to pump a lot of air into that hospital operating suite," Cleary recalled. "The number of air changes per hour is mandated by Texas state regulations.

"Flexible air duct helped in several ways with the surgicenter operating room," he said. "First, the insulation cannot be exposed to the airflow. The inner jacket of the flex meets state requirements to cover the fiberglass insulation, but sound waves pass easily through the jacket liner. Flex duct helps to attenuate noise while maintaining a barrier to biological growth in the HVAC system.

"Sound has a hard time getting through metal to use the sound-deadening qualities of surrounding insulation."

Using flexible air duct, the low-pressure run-out can connect easily and economically to the air diffuser, such as in this commercial installation.

High Productivity, Quality

Outside superintendent John Principe of Hill Mechanical Group in Chicago is used to big projects. He is accustomed to working with the best, including Turner Construction, Pepper Construction, Walsh, and Clayco.

Principe and his fellow superintendent, Rob Lancaster, oversee jobs like "a 52-story skyscraper coming out of the ground," at 111 South Wacker Drive in Chicago. Hill Mechanical was also involved in the renovation of Soldier Field, home to the Chicago Bears football team. Lancaster additionally boasts of a $4.5-million contract for Northern Illinois University's Convocation Center in DeKalb, Ill. - a glossy, new basketball and concert venue. All of these building projects include large quantities of flexible air duct.

"We use the best rating you can have of flex," stated Lancaster. "It is rated for Chicago code for plenum ceilings. We use the highest grade of flex in all kinds of buildings - duct and return, or plenum return."

Principe considers top-grade flex as "nice quality to work with" in large industrial and commercial projects. "The density of the material is good," he said. "It's good for its pressure class" - up to 6-inch standard pressure, he said.

"You pay more for the best grade," Principe explained, "but you get the best true-round shape. How easily it bends, how it keeps its shape without distortion - these are the things we look for in the highest grade of flexible air duct."

Applications can include main risers, medium pressure loops in floors, lobbies, and E-boxes.

"At Soldier Field, there were 3,000 registers in the project, each with 5 feet of flex," said Principe.

Lessons And Trends

Rudy Allahar, warehouse manager for Sterling Air Conditioning in Houston, said he has been using flex duct for at least 15 years.

"We send out 20 jobs per day from this location - all flex," he said. "A lot of our work is in tract homes, where the mechanical requirements are basically the same from house to house."

The company has two branches of the company serving Houston, and both are well versed in the benefits of flex duct.

"It sure has changed over the last 15 years," said Allahar. "As time went on, with technology, experimenting, the industry woke up, and eventually it came a long way. It's fantastic now. We used to use 1-inch thickness on the jacket insulation. Now we use R-6. There's better quality now. R-6 is now a city and state requirement for the state of Texas."

Using the architect's master plan, Sterling's engineering department determines the sizes and grades of flex duct that will go into each job.

"Compared to commercial installations, it's somewhat different in a house," he concluded. "There is no problem balancing an HVAC system in a house using flex duct."

Author Stephen Colvin is a freelance writer from Arlington Heights, Ill.

Sidebar: Air Diffusion Council Supplies Guidance

Not every homebuilder or HVAC contractor believes that you should string flex "like spaghetti." Poor installation can lead to problems, including air pressure drops, improper ventilation, and an unbalanced (or underperforming) HVAC system.

Eliminating these problems has been the job of the Air Diffusion Council (ADC), a trade association representing flexible air duct manufacturers across North America, headquartered in Schaumburg, Ill.

"We want to call attention to proper installation of flex," said the association's executive director, Jack Lagershausen. "Early in 2004, ADC will introduce its fourth edition Green Book. This up-to-date reference leads contractors and installers through proper installation techniques, step by step.

ADC's Green Book, which will be available in a printed version or on CD-ROM is designed to offer easy-to-follow illustrations on how to handle, hang, bend, splice, tape, and fasten flex to its terminal outlet.

For more information, contact ADC at www.flexibleduct.org.

Publication date: 12/01/2003