There has been a lot of buzz in the past several years surrounding the whole-house approach to analyzing energy efficiency and matching the correct system and components with each building envelope. Not only is it vital to choose the proper equipment for each individual application, but it is equally important to understand how this equipment acts as part of a total system and how the other components play an equally important role.

One of the components of a building’s HVAC system is the ductwork, which is essentially the veins and arteries that move air throughout a building. Without properly sized and installed ductwork, even the most efficient components cannot maximize comfort and save energy costs. That’s why it is vital to identify problems in existing ductwork and plan out the most efficient labyrinth of ductwork in a new installation.

The NEWSturned to several experts in the ductwork segment of the industry to learn about the proper ways to install and maintain ductwork, so that a whole-house or whole-building HVAC system performs the work it was intended to do and brings the desired results: efficiency and comfort.

Our experts include:

•  Jeff Elzinga, product manager for flexible duct systems, Hart & Cooley Inc.;

•  Rex Anderson, director of communications, Goodman Global Inc.;

•  Mark Terzigni, vice chair of TC 5.2 Duct Design, project manager for technical services, Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA);

•  Stevie Jones, Gemini Connections/North American Insulation Manufacturers Association (NAIMA).


What makes an inefficient duct system and what can be done to increase efficiency?

Elzinga: “Inefficiency in ductwork generally results from two things: thermal losses and airflow restrictions. Thermal losses can occur when ducts are not insulated properly, especially if the ducts are routed through unconditioned spaces such as attics and crawl spaces. Through the use of insulation with the appropriate R-value for a given application, conduction of heat or cold from the duct wall can be avoided.

“Air flow restriction is a major cause of inefficiency in ductwork. There are many causes of restriction. Generally, restriction results in lower system efficiency by making the system work harder and longer to condition the space. The shape and diameter of the duct, the design of the system, and proper installation are all factors in the efficiency of the duct.

“Proper installation is critical to maximizing system efficiency. Flexible ducts must be fully extended, and only the minimum length should be used to make connections. Bends and sagging can have a dramatic effect on the efficiency of flexible ducts.”

Anderson:“Installing ‘runs’ too long without reducing the size of the duct are also culprits of an inefficient duct system. In addition, improperly sealing the joints between two connecting pieces of flex duct can create leaks that can hinder the efficiency of a flex duct system. A properly installed flex duct system is easy to install, provides outstanding air flow and delivery, plus offers long-lasting performance.

“Following the procedures outlined by the flex duct manufacturer coupled with the guidelines established for hard duct systems can increase the efficiency of flex duct systems. Using higher R-value flexible duct can increase the efficiency of the system, too. Using R-8 value insulated flexduct provides enhanced efficiency compared to lower R-value rated flexduct.”

Terzigni:“There are many things that can be done to make a duct system more efficient. Most of this opportunity occurs at the design phase. Like most decisions regarding design, there are tradeoffs that must be considered. For instance, round duct is the more efficient shape in terms of material use, resistance to airflow, and air leakage.

“The tradeoff is that round duct will not always fit in the space provided. In these cases the duct is converted to rectangular or flat-oval so that it can fit the available space. The closer to round or square, the more efficient the duct will be, and the less it will cost to fabricate and install. As space for duct becomes constrained, the duct is typically downsized, the operating pressure is increased and the velocity is increased - all of these ‘actions’ lead to the design of a duct system that will require more fan horsepower over the life of the building.”

Jones: “The best way to improve the efficiency of any duct system is to make sure it is insulated. Rigid fiberglass ductboard systems are one of the cost effective and thermally efficient duct systems available for today’s residential and light commercial HVAC systems.

“A fibrous glass ductboard system comes in flat board form and is fabricated into the duct itself. The ducts are made up of 4-foot modules which are fabricated from 1-, 1.5- and 2-inch flat fibrous glass ductboard stock having a factory-applied reinforced aluminum exterior air barrier finish. Duct system modules are fabricated using either specially designed hand tools or grooving and closure machines.”


What are the most common mistakes when installing ductwork?

Elzinga: “The most common mistakes when installing ductwork are undersizing the duct, poor connections that come apart, compressed ductwork, improperly supported bends, and excessive sagging. Undersized ductwork can cause excessive static pressures, poor balancing, and noise at the registers.

“Leaky ducts or connections that have come apart can cause negative pressures in the living space and can pull in outside air, or contaminants into the airstream, such as fiberglass strands, dirt, and even radon gas. Ductwork that is not stretched tight, and that has extreme bends and kinks can have the same effect as undersizing the duct, causing poor system performance.”

Anderson:“Improper support of a flex duct run is most likely the most common mistake. Without proper support, conditioned air does not properly flow in a flex duct installation. Flex duct offers a much faster installation than hard duct, but during the installation, common practices are required to both systems. Flexible duct that sags between supports or does not have the correct number of supports hinders the optimum performance of the entire HVAC system.”

Terzigni:“Using more efficient fittings will allow for a more efficient duct system. But there are tradeoffs involved. Sweeping elbows are more efficient than mitered elbows, and the larger the radius of a sweeping elbow, the more efficient. But having adequate space to use a large sweeping elbow is rare. In many cases mitered elbows are used.

“Adding properly installed turning vanes to mitered elbows will make the elbow more efficient but improperly installed vanes can actually make the elbow less efficient than omitting the vanes. 

“Volume dampers are likely to generate noise, provide a path for air to leak, and they work by adding resistance to that particular duct path, but they are adjustable. An ideal duct system would be so designed that no ‘artificial’ resistance would be required in the way of dampers. Using a less efficient tap or even downsizing a portion of the duct run will generate less noise and provide less opportunity for leakage, but if it isn’t sized correctly there is no easy way to adjust it.”

Jones:“Fiberglass ductboard systems can deliver virtually leak free performance when fabricated and installed properly, and, by doing so, reduce the HVAC system operating costs by controlling heat loss or gain through the joints and duct walls.

“It is a single-component system - fiberglass ductboard is a thermal and acoustical insulation. Plus, essentially air-tight performance is provided with one product installed by a single contractor. Factory-controlled insulation thick- ness ensures that specified R-values are met.

“Among other benefits, they are essentially air-tight. Closures made with UL 181A listed pressure-sensitive tape, heat-activated tape, or glass fabric and mastic, save energy by virtually eliminating air leakage. And they are code compliant.”


What should contractors tell customers about duct maintenance?

Elzinga:“A duct system that has been properly designed and installed will provide a lifetime of reliable service. The most important tip for the long term cleanliness of flexible ductwork is to make sure the connections are properly sealed at time of installation, and the use of a good quality filter that is changed regularly. Sheet metal ducts can be cleaned if the system is not properly maintained, but the strong brushes used in the cleaning of sheet metal ducts cannot be used on flexible ducts. A damaged or dirty section of flexible duct must be replaced.”

Anderson: “Proper air filtration is a key to maintaining both the flexible ductwork system and the entire HVAC system. Whether mechanical or electronic filters are used, the media must be changed on a frequent basis to avoid increased static pressure and to avoid added particulates to flow through the duct system. Periodic checks are suggested to make certain that the ductwork isn’t torn or damaged which helps to maintain its efficiency level.”

Jones: “Inspect and maintain. And begin with the equipment itself. Check the heating and cooling units regularly. Make sure the unit is sized correctly for the system. Change the filters with each season. Make sure the dampers are working. Check the burners to make sure they are adjusted correctly.

“If the ductwork is specified, fabricated and installed correctly, there should be no maintenance, and perhaps no cleaning necessary.”


Is there any special training that ductwork installers need?

Elzinga:“Ductwork installers should be trained in the fundamentals of duct sizing and installation to develop the skills required to install efficient systems. North American Technician Excellence (NATE) certification is one program that covers all aspects of HVAC installation and repair. Achieving certification requires that a technician pass a core install exam, which covers many areas including ductwork installation. In preparation for this exam, technicians will have studied ADC flexible duct installation requirements and the ANSI/ACCA Manual D, which cover this area extensively.”

Anderson:“Quietflex® suggests that installers of flex duct follow the guidelines provided by the Air Diffusion Council (ADC). These easy-to-follow tips provide great training for contractors and technicians that wish to properly install flexible ductwork systems.”

Jones:“Like anything, care in craftsmanship is needed to assure a thermally sealed duct system. The North American Insulation Manufacturers Association (NAIMA) and their members have been working with vocational schools and contractor groups all over the country to provide the information and training needed to fabricate and install fiberglass ductboard systems correctly.

“In addition, NAIMA has a variety of information pieces, guidelines, and standards that are used and approved by the codes bodies and standards organizations across the country.”

Publication date:10/04/2010