Industry Articles From Kaempf and Harris

The Difference Between Rectangular And Spiral Ductwork


Kaempf & Harris knows a lot about commercial ductwork, especially in Maryland. Between ductwork design and materials, our sheet metal experts offer the best information and perform the most quality work on commercial HVAC, including spiral and rectangular ducts. 

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What's a spiral duct?

In general, ducts are passages used in heating, ventilation, and air conditioning (HVAC) to deliver and remove air. They can be used in residential or commercial buildings. 

A spiral or round duct is typically made from galvanized steel and occasionally aluminium. If not from sheet metal, spiral ducts can be fiberglass board or a flexible plastic and wire composite.

Spiral ducts are being used more frequently and can be any length, but the most common cut is 20 feet in order to fit in a standard truck. The ducts can be connected using slip joints or flange-to-flange connection methods, depending on additional performance requirements. 

What's a rectangular duct?

A rectangular duct has approximately 32 percent more sheet metal in it (likely galvanized steel), costing slightly more with insulation, support, and labor. They can also be made of fiberglass board. However, the overall capital cost is worthwhile because rectangular ducts:

  • Can be adapted to any building height restrictions
  • Are easily shipped in a truck when broken down or nested
  • Are conveniently fabricated to provide flat surfaces for branch tap-ins in a trunk-and-branch design

How do these ductwork designs differ?

Leaks. According to PDH Online in Fairfax, Virginia, "Round spiral duct leaks less than rectangular duct due the lack of longitudinal joints and generally fewer transverse joints when run in long straight duct sections."

However, commercial building owners should avoid requesting or applying sealant to spiral ducts because it can result in poor seam closure and less-than-satisfactory control if done incorrectly.

Noise. Round duct, which has lower and better pressure drop characteristics that reduce fan speed, is also more rigid than rectangular duct. This reduces the drum effect from duct vibration in comparison to a rectangular duct, which can transmit excessive noise if not properly supported.

Insulation. In comparison to rectangular duct, spiral ductwork is stiffer and easier to insulate. However, it must be insulated on the outside, which is more difficult and costly. Because rectangular ductwork can be insulated inside, your commercial building will be warmer when you need it most. 

Which is best for your commercial building?

If your commercial building offers a lot of height for installation, Kaempf & Harris suggests a spiral ductwork design. This is also beneficial because spiral ducts: 

  • Create lower pressure drop, which means less noise
  • Are less expensive due to low ductwork installation costs and less sheet metal for the same airflow rate
  • Are easy to seal and prevent leaks 

If you're looking at rectangular ducts for your commercial building, they: 

  • Create a warmer indoor environment due to inside insulation
  • Are more flexible if your building has width-to-height restrictions
  • Require less long-term heating and cooling maintenance 

John Reints, president of, as reported by SNIPS Magazine in 2012, prefers the computerized spiral design. 

The enhanced computer engineering used to design can produce total material weight savings of 40 percent to 50 percent. Spiral ductwork design enhancements and the resulting savings are the next major market change for sheet metal contractors.

Class A sealing requirements are the final nail in the cost coffin for rectangular ductwork."

Overall, it largely depends on what you're looking for in an HVAC system. Factors include location, number of building occupants, square footage, and more. It's best to talk to a sheet metal fabricator or HVAC technician for insight on your specific commercial building. 

For more information, click on the button below to download Kaempf & Harris' Ultimate Guide To The Commercial Ductwork:

Download The Ultimate Guide to Commercial Ductwork

Topics: Tools & Techniques, Sheet Metal Fabrication

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