Rectangular and Round Spiral Ductwork: What’s the Difference?

In HVAC systems, ducts are used as air passages to carry and remove air. They’re employed in commercial and residential buildings alike. Round spiral ductwork, rectangular ducting, or both may be employed in these systems. 

Spiral Ductwork is a type of ductwork that is spiral in shape

Spiral duct, also known as round duct, is frequently made of galvanized steel and, on rare occasions, aluminum. If sheet metal isn’t an option, fiberglass board or a flexible wire and plastic composite material can be used instead. It can be manufactured to any length, with a normal cut of 20 feet fitting on a conventional truck. These ducts can be connected using flange-to-flange connectors or slip joints, depending on the performance requirements.

Rectangular Ductwork is a type of ductwork that is rectangular in shape

Spiral ducts are made up of around 1/3 more sheet metal (or fiberglass board) than rectangular ducts. Because of the additional insulation support and effort required, they are slightly more expensive. Rectangular ductwork, on the other hand, is favorable since it is:

  • For a trunk-and-branch design or branch tap-ins, it’s simple to construct into flat surfaces
  • When nested or broken down, it’s simple to ship by truck
  • Easily adjustable to any building height restrictions

What are some of the application differences between spiral and rectangular ducts?


When compared to rectangular duct, round duct has a reduced pressure drop. In comparison to a rectangular duct, it is also more stiff, which reduces the drum effect caused by duct vibration. If rectangular ducts are not properly supported, the noise level can be severe.


Round spiral duct leaks less than rectangular duct because it has fewer longitudinal and transverse joints in long straight stretches of duct. If sealant is applied to spiral ducts incorrectly, it may result in inefficient seam closure and insufficient control; hence, commercial building owners may want to prevent this.

Selecting Spiral vs. Rectangular Ductwork

When the building has enough height available for installation, spiral ducting is frequently the best option. Spiral ducts are a type of duct that has a spiral shape.

  • Are simple to close and prevent leakage
  • Reduce pressure decreases, resulting in reduced noise
  • Because they use less sheet metal for the same flow rate, they are less expensive than rectangular ducts
  • Reduce the cost of installation

If your construction has width-to-height constraints, rectangular ducts for commercial buildings allow you more freedom. They also require less long-term cooling and heating maintenance.

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