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Considering The Benefits Of Spiral Vs. Rectangle Ducting

Oct 21

When it comes to the type of ductwork they prefer to utilize, many contractors have a comfort zone.

Various factors, such as product pricing, availability, engineering requirements, habit, or simply personal taste, can often define this comfort zone.

"What are we gaining by making these decisions, and how are we potentially limiting ourselves and our customers?" we must ask.

Both spiral and rectangular ducts offer apparent and well-known benefits. Most sheet metal factories, for example, can simply construct rectangular duct using a pair of snips and a hand brake. It can also be transported collapsed to save space and money on transportation. Installing and sealing spiral duct and flat-oval duct takes much less time. The majority of spiral applications are more cost effective to produce, whether in a factory or by a contractor using their own machine. However, it's critical to consider more than just labor and materials costs. There are a number of other things that could result in expenses.

Labor expenses, raw material costs, and aesthetic criteria are all elements that might affect the installation cost of an HVAC system. When compared to rectangular spiral duct, round or flat-oval spiral duct installation costs can be up to 50% less on average. This is due to lower installation labor costs, more cost-effective duct and fitting expenses, and fewer sealing needs. When comparing the overall weight of a spiral system to that of a rectangular system, it should be remembered that the round system could be up to 40% lighter.

The Return

Given that labor and materials are frequently estimated by the pound, this can quickly pay off.

It might be argued that rectangular duct offers an advantage in instances when ceiling depth is limited. Flat-oval duct is an alternative to both rectangular and round spiral duct. When it comes down to it, flat-oval duct is nothing more than a disguised round spiral. When compared to rectangular duct, flat-oval duct delivers the same benefits as circular spiral, such as reduced leakage rates and minimal static-pressure loss.

When comparing the static-pressure loss of spiral and rectangular systems, spiral systems have smaller pressure losses. One of the key reasons for this is that moving the same amount of air requires less surface area. The perimeter of a 12-by-12 rectangular duct, for example, is 48 inches. The equal diameter circular duct is 13.1 inches, with a circumference of only 41 inches - a 15 percent reduction in size. Because of the smaller surface area, less energy is lost owing to friction. Furthermore, a smaller duct surface area means less heat loss through the duct's wall.

The entire length of a rectangular duct is made up of two longitudinal lock seams and a transverse slip connection every 4 to 5 feet. In the past, getting less than 10% leakage in a rectangular duct has been problematic. A system with 3 to 4% leakage can be achieved with enhanced connecting options. Spiral-seamed duct, on the other hand, is nearly airtight, and there are a plethora of alternatives for coupling and sealing the transverse joints, which when combined can achieve only 1 to 2% overall leakage.

What are the similarities between static-pressure loss, heat loss and gain, and duct leakage? They're all examples of energy that has been wasted or, in some situations, energy that has been given to the wrong location. To compensate for the lost energy, the air handler or fan size must be raised. These losses will be accrued throughout the duration of the system's service life. Finally, a spiral system not only provides the finest performance while saving time and money, but it also looks nicer. At least, that's what we at SPIDA believe.

SPIDA invites you to attend our January general membership meeting in Las Vegas if you want to learn more about this issue.'s John Reints, P.E., will go over the static regain method and why duct selection is so crucial.