Ducted or non ducted air systems

Designing High Performance Duct Systems


Ducted or non ducted air systemsFor the past 50+ years, engineers have been “designing” duct systems. Quite often, they were not actually designing the duct system, but were sizing duct the duct according to some rules of thumbs. The rules of thumbs varied by different engineering firms and often ducts were sized to be round by a predetermined friction rate that was chosen by the engineering firm. Then if the round duct did not fit and sometimes when it does, it’s often converted to rectangular, making it a lot less efficient system. Often, the whole ductwork system is “sized” that way using a Ductulator. Not much thought is given to the fittings used and often leakage of the system is not considered during the design phase.

Duct Design General Concepts

So how do we go about designing duct system to be green? There are many aspects to design, but focusing on the duct system design we want a system that minimizes the use of energy, time and material and make sure it meets the acoustical requirement of the application. To accomplish these goals we have to consider what the duct system will be used for (standard air conditioning, exhaust, or other), duct system layout, fitting selection, system leakage, acoustical properties and equipment selection.

There are three common methods of duct sizing or design for commercial and industrial duct systems: equal friction, static regain and constant velocity. Both the supply side (positive pressure) of the fan or air-handling unit (AHU) and the return side and makeup air side (negative pressure) of the fan have to be considered. The equal friction method and constant velocity systems can be used for either the supply or return side. Most often equal friction is used on the supply and the return systems while constant velocity is used for exhaust systems that have to convey particulate or fumes. Static regain however is strictly used for a positive pressure design.

This article will focus on the design of the duct system on the positive side of the fan or Air Handling Unit (AHU). Other important aspects that that should be looked at during the design phase may include fan or AHU selection, system effects, leakage, diversity, room air distribution, equipment layout and commissioning. Those are topics for other papers.

spiral duct


Choosing a Duct Design Method

When we design duct systems we want the design method that minimizes energy, material and time. But first to dispel a myth, static regain does not automatically design systems at a lower total pressure. Any design method can be used to design a duct system for almost any pressure. A 6-in wg system can be designed either by Equal Friction (just increase the design friction rate) or by Static Regain (just increase the initial velocity). In either case though the velocities should be kept within acceptable limits to avoid noise problems. But a static regain design goal is to produce a balanced system. That is, one in which all paths are design legs, or require the exact same amount of static pressure for the legs respective airflow. For final balancing of systems, smaller sizes in non-critical paths will use excess pressures. So if two designs of the same system are created that have the same operating pressure, one with equal friction and one with static regain, the static regain method should use smaller duct sizes, because it is balancing the system. When this is the case you will likely have the benefit of more round sizes and because smaller sizes in general are used for balancing the non-design paths, the benefit of lower duct and fitting cost as well. A benefit of round duct is it has lower breakout noise, resulting in a quieter design. Another benefit is that round duct and the resultant smaller sizes are easier to install and seal. Some pros and cons of the two design methods are shown below:



Total Pressure Design

Whether a system is designed with Equal Friction or with Static Regain, there is still likely to be imbalance. There should be less with the Static Regain design, but because in the real world we don’t have an infinite number of duct sizes, there is always some imbalance. Using the Static Regain design though helps to minimize this imbalance. Ideally we want all paths to be critical paths. That would mean the system is perfectly balanced. With imbalance means some paths will have more pressure (excess) available than they need, which means they likely have sections that can be made even smaller. Note to designers less efficient fittings will generate more noise, but that is generally not a problems till you get close to the final runouts. It’s best to design with high quality fittings that have lower pressure drop, then use smaller sizes. Using multiple runs of round rather than rectangular or flat oval duct could save even more money on the job. The process of taking a given design, determining the excess pressures available in the non-design legs, and making them smaller to use up the excess pressure, is often referred to as a total pressure design. It can be applied to any design method, but is most suited to be applied to the Static Regain Design method since it should already be fairly balanced. The good news is some of the duct design programs pinpoint the critical legs making it easy to identify where there will be excess pressure.

Example of Using Equal Friction vs Static Regain vs Total Pressure Design

In an article published by the Air Conditioning, Heating and Refrigeration News in their June 18 1990 edition, a system was designed with the Equal Friction sizing method with three different friction rates (0.05 in wg/100 ft., 0.10 in wg/100 ft. and 0.50 in wg/100 ft.). Then each of these three systems was designed with Static Regain, then the Total Pressure design. The system design airflow is 26,800 cfm and the designs were such that the Static Regain and Total Pressure methods had about the same operating pressure as the respective Equal Friction designs. The first section was the same size in all design methods. The results of the three designs are given in the Table 2:


The system had a 12-inch height restriction. For the 0.05 in wg/100 ft Equal Friction design, the percent of sections that became round went from 32% to 61% by using the Static Regain design method and 71% for the Total Pressure design. Similar results can be seen for the two higher pressure designs as well.

Round ductwork cost much less than rectangular or flat oval, saves installation time, is easier to seal and the Static Regain and Total Pressure design methods are much more balanced!

The other advantage of total pressure design is that since it is a balanced system using smaller sections of duct which have higher attenuation and insertion losses. So using that knowledge, and keeping the velocities reasonable, total pressure designs should not require as much noise control.


A high performance duct design should minimizing leakage. ASHRAE recommends the system leakage be no more than 5 percent of the total airflow volume. Why is that so important? If a system leaks and airflow requirements are not met in the locations they were intended, the leaks either have to be sealed or the fan speed must be increased to generate more volume. If the leaks are not sealed, a practice which is no longer allowed by many codes, that additional volume will need to be pushed through with a higher static, because remember that the system will continue to leak and that the airflow volume, plus the additional leakage airflow volume caused by higher static pressures will need to be overcome as well. Essentially you are changing the system curve from what was designed to what is actually happening.

The design point for a fan is where the system curve crosses the fan curve as shown in Figure 1. It assumes no leakage is occurring. If there is 10% system leakage, the duct system is essentially relieved and the system curve moves down. That is because the same volume of air does not have to move through the entire system as some has leaked out. The airflow increases while the Fan TP decreases as shown in Figure 2. We can’t just speed up the fan as shown in Figure 3 so it produces the same original Fan TP as leakage is still occurring. We will actually have to move to a higher RPM and increase both the Fan TP and the airflow so we get the correct airflow to the outlets as shown in Figure 4.

ASHRAE studies have shown the cost of leakage could be $0.00050 per cfm-hr. So a 50,000 cfm system operating 2600 hrs per year with 10% leakage of 5000 cfm, could cost an additional $6500 per year.



To design high performance duct systems that do not have acoustic sound problems we need to

  • minimizes the use of energy
  • minimizes the use of construction/manufacturing labor and material
  • make sure it does not contribute noise to the environment
  • design balanced systems

To best meet these objects, the Static Regain/Total Pressure design should be used to determine the duct sizes in a system. To use the Static Regain method, you have to choose an initial velocity. It is recommended you use the follow chart published in the ASHRAE Applications Handbook 2011, Table 8, page 48.14.



Then use Static Regain to size the duct sections, utilizing the most efficient fittings. When finished designing, use the Total Pressure design to further balance the system using smaller sizes and less efficient fittings in non-critical paths.

The final result will be a well-balanced system using the smallest sizes possible for the initial velocity. Smaller duct sizes mean:

  • more of the duct sizes will be round
  • round spiral duct is much easier to install and has fewer joints
  • many sizes will be smaller than those in other design methods, making even them easier to install and use less materials
  • smaller sizes will be easier and less costly to seal making very low leakage duct systems possible
  • the duct system will be balanced assuring everyone gets enough air and the Testing and Balancing peoples time will be minimized
  • done right, round ductwork results in quieter system with less risk of noise problems

You will likely need a computer to do the proper designs, but if you do; less time, material, and energy will be expended. With the proper controls and other components, the Static Regain/Total Pressure duct system designs are used in true High Performance Air Systems. What more could you ask for in a duct design.

Guest Blog Writer:

Pat Brooks, General Manager



High Performance Air Systems – History of Ducted and Ductless Systems

Ducted or non ducted air systems

Ducted air systems are used in the majority of comfort cooling applications in North  America. In contrast, ductless refrigerant systems in Asia, and ductless water systems in Europe dominate their respective HVAC markets. These practices reflect the historical evolution of air conditioning in each market.

Use of the coal-fired basement furnace evolved in the in the late 19th and early 20th century in North America to keep coal dust and storage away from occupied spaces.  Coal was used for coal-fired boilers  or coal-fired furnaces.  Older buildings in the Northeastern part of the US used hydronic heat distributed via radiators.  District steam in New York City and other major cities gave rise to ‘cast-iron’ and the pipefitting trade.  With the invention of air conditioning, radiators were replaced with two to four pipe fan coil systems and existing pipe chases were upgraded to handle chilled water as well as hydronic heat.  This expertise and influence for water-based systems still exists today in the Northeast US.

Different factors influenced construction practices with the western expansion of the US.   Both wood and land was plentiful.   Demand for detached multi-room single-family homes grew at a rapid pace.   It became common practice to duct heated air from the basement furnace to all the rooms – first by gravity and then forced air.  Wood construction was common and the space between the wall studs and floor/ceiling joists became the ductwork.  After WWII, contractors invested heavily in duct-fabrication to satisfy the new construction demand for housing and quick-build commercial applications. When the market for central air conditioning began during the 1960’s and 70’s, it was easy to add an “A” coil (evaporator) to the top of the furnace and add a condensing unit outside. Because residential and commercial practices influence each other, the use of a common duct for both heating and cooling carried over to all commercial building types as well. DX (direct expansion) Rooftop VAV (variable air volume) exploded in the 80’s as the demand for cost-efficient and fast installation increased dramatically.

Air Conditioning CondensersEurope and Asia’s HVAC evolution was driven by different construction practices and constraints.  Europe’s path was/is similar to that of the Northeast US.  Older buildings (and there are a lot of them in Europe) allowed no provision for ductwork – masonry construction had no hollow walls.  These buildings were heated using piped radiation systems.    Contractors developed a strong pipe-fitting culture and engineers developed tremendous knowledge of designing water-based systems.   Hence, non-ducted water-based systems are still preferred today.

Asian apartments were/are predominantly one or two room.  Through the 1950’s, many were heated by a single kerosene heater. Again, construction was/is poured concrete or concrete blocks with no hollow walls. Central heating using hydronic radiators were rare. The window unit boom in the 1960’s and 70’s provided cooling and safer electric-strip heating in one device to satisfy the needs of these small living quarters that had no provision for piping or ductwork.  In the late 1970’s and 1980’s, the evaporator/heating and compressor/condenser were separated into indoor and outdoor components– hence, mini-split.  This was not only a quieter alternative to the ‘window-shaker’, but gave the owner back his/her outdoor view.  The next logical evolution was multiple DX fan coils from one condensing unit for commercial and multi-family applications.   VRF split systems with multiple indoor units were introduced for multi room applications.

Ducted or Non-ducted?

Ducted SystemAs the above history lesson shows, the driver for ducted or non-ducted has nothing to do with efficiency.  The driver has to do with simple evolution of HVAC in relation to cultural building market drivers.

When deciding which system to use, each has advantages and disadvantages dependent upon application, climate, building construction, and other factors.  Multi-family housing, dormitories, and hotel applications all are construction-types where a common air return is not permitted.  This  is the logical market for non-ducted products.  VRF provides an alternative to hydronic fan coil systems, water-source heat pumps, and PTAC’s.  Existing masonry buildings – where ducting may be too expensive to add – also is a sensible solution for non-ducted products. Several building types require a mix of system types. Health care applications are an excellent example of a mixed-use system approach (e.g., fan coils for patient rooms, ducted for common areas).

For other commercial applications – e.g., schools, education, offices, retail/restaurants, theaters, casinos, factories – a high performance ducted system makes sense from a total life-cycle cost standpoint.  Ducted systems offer lower installed cost, better efficiency (e.g., cooling with compressor-free outside air) and code compliant ventilation within the same system.

RSKN_Go_Green_LogoFor more information about Ruskin’s complete product line, application and design support, and our state-of-the-art manufacturing capabilities, contact your local Ruskin representative  nearest you or Contact Ruskin directly at (816) 761-7476.

VRF has it’s place, but certainly not advantageous vs VAV

Letter from Thomas R. Edwards, Ruskin Group, Grandview, Mo.:
I just completed reading your two-part article on DOAS and VRF (“Combining DOAS and VRF, Part 1 of 2,” March 2014, http://bit.ly/Bowers_0314, and “Combining DOAS and VRF, Part 2 of 2” April 2014). Of course, VRF certainly makes sense in a variety of applications—most specifically, where ductwork is not available or jobs where it is not practical to install. However, the author is assuming benefits vs. ducted VAV systems, and in my experience, it doesn’t add up in both first cost and life-cycle costs in North America climates for the following reasons:
• These systems have been sold in the U.S. since 1983. The reason they have not had a lot of success is because they not only cost more, but typically are not more efficient than ducted VAV systems.
• VAV systems sometimes get a bum rap by not including the latest technology available on modern VAV and air-distribution systems—they, too, are available with modulating/variable-speed compressors and fans, along with energy-recovery wheels.

Duct systems in commercial buildings now are being installed virtually leak-tight using today’s sealing methods, providing multiple points of air distribution in each zone.
• Ducted systems can use outside-air economizers for compressor-free cooling and distribute it throughout a building. VRFs do not have economizer capability. Today’s tight commercial buildings need cooling year-round in the interior. VRF systems must run their compressors on a call for cooling—even at outside-air temperatures below 60°F. In the article, the author mentioned reheat can occur on VAV systems. However, please note that during the winter and transition months, this is compressor-free cooled air! Additionally, many VAV systems use separate perimeter heating systems, thereby eliminating any box reheat.
• Most climates in the U.S. can use economizers six months or more during the year—these climates certainly don’t make sense for VRF, if VAV with economizers can be used.
• VRF systems need DOAS to handle minimum ventilation requirements required by code in the U.S. All DOAS are sized to handle only 100 percent of the ventilation load at design conditions, whereas VAV with enthalpy comparative economizers can handle 100 percent of the total system load with a greater capacity for free cooling (no compressor or pumps necessary). On VAV AHUs, you can have both economizers and ERVsERVS (with bypass)—you can’t have this with a DOAS.
• VRF advocates imply the flexibility of their systems. Ask a contractor how easy it is to move DX fan coils and refrigerant lines in a building. Moving diffusers and VAV boxes is much easier, offering greater tenant flexibility.
• Hundreds of pounds of refrigerant running above a plenum space requires compliance with ANSI/ASHRAE Standard 15, Safety Standard for Refrigeration Systems, in a number of applications.
• As stated in the article, the industry is moving beyond minimum code-compliant systems in order to meet more aggressive targets. If we really look at optimizing the energy-saving controls and features of modern VAV systems, energy studies and real-life building applications show that VAV knocks the socks off these perceived “new VRF” technologies.
I encourage the author to do energy studies comparing modern VAV systems with enthalpy economizer to VRF systems that require year-round compressor operation. He will hear what I continually hear, “Why doesn’t this VRF system beat the energy savings of a VAV?” The answer: Because they simply don’t in most U.S. Climates!

Spiral Duct – The Energy Efficient Way to Go!

With energy costs going through the roof and no relief in sight, HVAC system designers and specifiers are taking a much harder look at the many choices in ductwork available to them today. One question that is being asked more and more is whether to choose rectangular or spiral duct for an HVAC system. If the healthy state of the spiral duct manufacturing industry is any indication, the trend in the United States is moving towards spiral duct, as specifiers become more enlightened to the advantages of circular duct.

The inherent energy- and labor-savings features of the duct’s design are what are causing this shift. This changeover has already happened in Western Europe, where the cost of energy has always been higher than that in the U.S. In the region of Scandinavia, for example, the percentage of the spiral duct market share has gone from 5 percent 40 years ago to 85 percent currently.

Some of this changeover resulted from specialist manufacturers intensifying their level of automation, standardization, and R&D, but the bottom line has been that growth was spurred along by the competitive edge created by spiral duct.

The design, installation, and performance benefits of spiral duct are many, with some appealing to HVAC contractors and others to HVAC system designers. Since energy efficiency is the biggest advantage.

Quality spiral duct, which includes round, oval, and flat oval configurations, exhibit a very high level of air tightness. Many leading manufacturers of sheet metal spiral duct can guarantee a line of spiral duct that meets or exceeds the highest air leakage standard, Class 3, recognized by the Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA). Since some duct is manufactured with self-sealing double-gaskets, the connection is guaranteed to prevent duct leakage.

Duct air leaks can waste energy and cause IAQ problems, such as condensation. Loose-fitting joints or blow outs of improperly maintained seams can cause problems in supply ductwork, while air leakage can allow soil gases and molds in crawl spaces and below slabs to enter return ductwork. When leakage is virtually eliminated, the cost for filtering, heating, cooling, and distributing the air is kept to a minimum. In addition, studies have shown that round duct has a lower airflow resistance, which significantly contributes to the energy efficiency of the system.

One industry study demonstrated that a shift in the U.S. to more airtight systems would mean an annual energy savings of approximately 10 TWh (terawatt hours, each equivalent to one billion kilowatt hours), which is comparable to the annual energy product of three nuclear power plants.

So that the correct flow of heated or cooled air can reach the areas to be conditioned, fans must transport the total airflow. A poorly designed or leaking duct system needs larger, and often more expensive fans to achieve the desired effect, as well as more space in the building’s design, where extra space is often at a premium.

With the many flow measurement units specially designed for circular duct that are now available, HVAC engineers also find that it is easier to measure the airflow passing through circular ductwork. With these relatively inexpensive devices, the ventilations systems can include fixed measuring units as a low-cost method of enabling regular check ups or continuous monitoring.

Another benefit of spiral duct that contributes to the overall IAQ of buildings with spiral ductwork is the ease of duct cleaning, using industry-recognized duct cleaning equipment outlined in the North American Insulation Manufacturers Association (NAIMA) Duct Cleaning Guide.

Aesthetics plays a very important role today as mechanical engineers, architects, and other decision makers are specifying ductwork. Innovative design elements often include exposed ductwork, to give a property a contemporary look. Spiral duct with gasketed joints requires no unsightly sealant at the connections, which enhances the clean look of the ductwork. Contributing to the attractive look of the duct is the fact that this type of duct requires fewer joints and flange connections, and requires less supports and hangers. In addition, the tight, gasketed seal allows the system to remain free of dirt streaking, common with manually sealed systems, ensuring that it will look good for many years. And those systems with RGS-3 registers do not require register taps, since they are mounted directly to the ductwork, again giving it a progressive, streamlined appearance.

Finally – of concern to the HVAC system designer – is the fact that round duct that includes a double wall system with a non-woven poly fabric liner can deliver a level of acoustical performance competitive with other types of ducts.

How do the HVAC contractors who install ductwork view the choice of spiral ductwork? The overall cost of spiral ductwork can be relatively low because installation moves more quickly than with other forms of duct. Correct fit at each joint can be counted on with premium spiral duct because standardized components are manufactured to tolerances of -0.25 percent. And elimination of sealants speeds installation along, as well.

Likewise, the lighter weight of spiral ductwork enables one worker to install some sections of round duct into place, rather than two, which also adds to labor savings. Manufacturers say that the complete weight for a typical system comprising a normal combination of straight ducts, bends, and diffusers, can typically be 30 to 40 percent lower for a circular system.

Other advantages to the contractor are the ability of circular duct to be nested, with various sizes resting together, which helps with storage and transportation, and the duct’s inherent strength, created by the strong seams made during the fabrication process, which gives each section an increased rigidity. This also reduces the need for additional stiffeners and hangers during installation.

Specifying the best duct system for each project requires a careful study of the many choices of duct products and their inherent benefits. With energy costs on the mind of everyone much more in recent years, it always pays to study and compare ductwork design before selecting the best type to be used for a specific application.