Ducts may be commodities that just seem to be a part of the HVAC equation. But they are vital components in heating and cooling. And duct design and maintenance are complex and ever-changing aspects of comfort systems.

The NEWS contacted more than 30 manufacturers and associations involved in the creation of ductwork and related products to ask them what are the top topics contractors should be aware of.

Feedback stated galvanized sheet steel may have been the long-time standard for air conditioning ductwork, but there is a wide range of newer materials such as fiberglass, aluminum, various forms of flexible duct, and one that is especially catching on in Europe and is expected to make a strong impact in North America - spiral metal duct.

Leaks continue to be a challenge but that aspect is being addressed by some newer and often factory-created solutions such as the use of plastic and an adjustable boot frame.

The No. 1 bugaboo - mold - is being addressed with both improved insulation methods and new innovations in duct cleaning.


"If trends in Western Europe are any indication, there may be a shift here towards spiral duct based on the energy- and labor-savings inherent in the duct's design," said Dave Pest, vice president of operations for Lindab Inc., which makes spiral duct systems.

"In Denmark, Norway, Sweden, and Finland, the percentage of spiral duct has gone from 5 percent (market share) in 1960 to 85 percent today. The percentage of the total ductwork market in Germany, France, and Great Britain has grown in a similar fashion."

Larry Jeffus, a Texas contractor and industry consultant, credited much of the technology's growth to improved manufacturing methods including those done on-site. "Spiral duct is made from long strips of narrow metal and fabricated with spiral seams," he said. "Machines are available for making ducts on the job to fit required diameters and lengths."

Pest said that spiral duct comes in round, oval, and flat oval configurations and "exhibits a high level of air tightness."

Another approach is to use flexible duct. Said Jeffus, "Flexible duct, without the insulation, comes compressed in a box. When opened, it expands lengthwise into ducts. It is easy to route around corners."

Gary Richards, HVAC sales manager for Clevaflex, said his company makes solid core flexible duct either in aluminum or stainless steel. "This kind of duct is used primarily as connections to VAV boxes, diffusers, etc. or any place where there may be turns or angles and would eliminate the cost of transition pieces.

"This type of flexible duct is not the everyday type because of the cost, but it is used in special applications - such as hospitals and universities - where the need for solid core flex duct must be specified." Jeffus tells contractors that special care must be exercised "during handling and installation so as to not flatten the duct and reduce the internal area."

An approach gaining favor because of rising steel prices is fiberglass duct board, said Robin Bectel of the North American Insulation Manufacturers Association (NAIMA), a trade association of manufacturers of fiberglass, rock wool, and slag wool insulation products. "As part of this trend, the sheet metal unions are offering apprenticeship programs that include duct board training. And NAIMA has set up train-the-trainer programs in more than 30 technical colleges in the Southeast and Mid-Atlantic to address the need for more training professionals in this market," Bectel said.

Jeffus said, "Fiberglass duct has the advantage of being insulated. This reduces the duct losses and provides sound absorbing qualities. It is particularly useful for duct running through cold areas such as an unheated attic space."

The improvements in duct design at the factory-level is causing many in the industry to encourage contractors to opt for "manifolded or factory assembled configurations" rather than stock components of uncut pipe and loose fittings. D. J. Nunamaker, communications manager for SEMCO Duct & Acoustical Products, a spiral duct manufacturer, said, "In some situations, a manifolded project may cost slightly more in initial cost, but will significantly save money in labor, thus making the project more profitable for the contractor.

"To get the contract, everyone is geared to be low at bid time. But the bottom line is that the lowest project cost does not equate to the lowest overall installed costs. Overall value should not be overlooked as all duct systems are not created equal."

He echoed the view of all manufacturers who responded to The NEWS' solicitation for comment in saying that contractors should consult with manufacturers as well as local code authorities before making a decision on what type of duct technology to use.


"The old axiom, ‘out of sight, out of mind,' applies to most aspects of a home especially the duct system," according to a white paper from DuraTite. "But what you don't see could be costing a lot of money each year."

The reference was to duct leaks, which the report said could drain energy efficiency and cause some systems to be oversized as a means to overcome problems with inefficient delivery.

The solution, the report said, is to better control leaks, and new technology is available to do that. For example, DuraTite noted it invented, manufactures, and markets nonmetallic plastic connectors (take offs) that the company said are dent proof, lightweight, nonconductive, and rust proof. "A new Clip and Zipâ„¢ fastening system secures the take-off in seconds. An installer can save three to five minutes per take-off which translates to a minimum of 30 minutes of labor savings in the average house."

According to Steve Rosen, vice president of sales and marketing for DuraTite, "This is intended for discrete parts in open plenums." He further noted that the product now has Underwriters Laboratory codes UL94HB and UL2043.

Another approach comes from Paramount Concepts Ltd. and involves an adjustable boot frame. Michael Pratt, president of Paramount said, "By using sheet metal screws to attach the boot to the frame, the assembly will not come apart, ensuring the airflow is delivered to the roof, not the wall cavity. The boot is installed flush with the bottom of the frame as opposed to sitting in the metal channel, ensuring there is no airflow loss. Two self-threading mounts provide a secure connection for the diffuser."

Closed-cell, elastomeric foam is one method being used to combat mold. (Courtesy of Armacell.)


"There are more than 100,000 known species of mold," said officials of Armacell. "We can't avoid them, but we can minimize their potential for growth in our air systems."

Proper installation is the first step. But even before that, the company suggested that contractors explore options including the use of closed-cell, elastomeric foam. This can be especially important, the company said, in areas where humidity is likely to exceed 70 percent. "Closed-cell elastomeric foam won't absorb moisture. Its smooth surface also inhibits the accumulation of dirt which serves as food for mold."

Closed-cell polymeric duct liner is also getting attention, said Bill Ronca, a technical advisor for Nomaco K-Flex. The brand name for the company's version of that technology is K-Flex Gray™ duct liner. "This is fiber free, nondusting, and mold-resistant," Ronca said. ‘It is cleanable using mechanical brushes or cleaning devices in accordance with National Air Duct Cleaning Association (NADCA) guidelines."

Vigilant duct cleaning is the final part of the equation. Many in the duct manufacturing sector believe it is something HVAC contractors should be doing themselves rather than subcontracting the work.

"There is probably no one more qualified than the HVAC contractor to perform air duct cleaning services," said Peter Haugen, Vac Systems International president. He said an initial investment could run anywhere from $8,000 to $20,000. "The service has good, long term potential, compliments a contractor's existing services, is not difficult to enter, and offers good profitability."

Publication date: 10/02/2006