Weighing the advantages of spiral vs. rectangular duct
Many contractors have a comfort zone when it comes to the type of ductwork they prefer to use.
In many cases, this comfort zone can be outlined by various factors, such as product cost, availability, engineered specifications, habit or simple personal preference.The questions we must ask are: “What are we gaining by making these choices, and how are we potentially limiting ourselves and our customers?”
Both spiral duct and rectangular duct have their obvious and known advantages. Rectangular duct for instance, can be easily fabricated by most sheet metal works shops with a pair of snips and a hand brake. It also has the ability to be shipped collapsed to reduce storage and freight costs. Spiral duct and flat-oval duct require significantly less time to install and seal. Most spiral applications are more cost effective to manufacture, whether in a production facility or by a contractor using their own machine. However, it’s important to not only look at simple labor and materials expenses. There are various other factors that may incur costs.
The factors that can affect the installed cost of an HVAC system include labor cost, raw material costs and aesthetic requirements. On average, installation costs of a job using round or flat-oval spiral duct may be up to 50 percent less overall when compared with rectangular. This is a result of reduced installation labor, more economical duct and fitting costs, and reduced sealing requirements. It should be noted that when comparing the overall weight of a spiral system to a rectangular one, the round system could be up to 40 percent less in weight.
Considering labor and material is often estimated by the pound, this can pay off quickly.
It could be argued that in situations with ceiling-depth restrictions, rectangular duct has an advantage. An alternative to both rectangular duct and round spiral is flat-oval duct. When it comes down to the details, flat-oval duct is truly just round spiral in disguise. Flat-oval duct provides the same benefits of round spiral, such as low leakage rates and less static-pressure loss when compared with rectangular duct.
When comparing the static-pressure loss between spiral and rectangular, spiral systems result in lower losses in pressure. One of the main reasons for this is due to less surface area required to move the same amount of air. For example, a 12-by-12 rectangular duct has a perimeter of 48 inches. In comparison, the equivalent diameter round duct is 13.1 inches, which has a perimeter of only 41 inches — about a 15 percent reduction. This reduction in surface area results in less energy lost due to friction. Furthermore, decreased duct surface area means minimized heat loss through the wall of the duct.
Rectangular duct is comprised of two longitudinal lock seams for its entire length, as well as a transverse slip connection every 4 to 5 feet. Traditionally, it has been difficult to obtain less than 10 percent leakage in rectangular duct. With improved options for connections, a system with 3 to 4 percent leakage is achievable. Spiral-seamed duct on the other hand is practically airtight, and there are countless options for coupling and sealing the transverse joints, which together can obtain only 1 to 2 percent overall leakage.
What do static-pressure loss, heat loss and gain, and duct leakage have in common? They are all forms of lost energy or, in some cases, energy delivered to the wrong place. In order to make up for the lost energy, the air handler or fan size must be increased to make up for these losses. These losses will be incurred for the service life of the system. In conclusion, not only does a spiral system deliver the best performance while saving time and money, it also looks better. At least we at SPIDA think so.
If you would like more information on this topic, SPIDA welcomes you to register for our January general membership meeting in Las Vegas. John Reints, P.E., of StaticRegain.net will go into detail about the static regain method and why duct selection is important. For more info on this presentation, please email firstname.lastname@example.org.
Peeter Vesik is a product engineer at British Columbia-based Ecco Manufacturing. He is also chairman of SPIDA’s publications committee. Contact SPIDA at www.SPIDA.org or call (803) 732-5818.