Spiral vs Rectangular Duct:
What’s the Difference?

Both spiral and rectangular ducts are the passageways commonly used in heating, ventilation, and air conditioning (HVAC) systems to deliver and remove air throughout residential or commercial buildings. Each offers several unique advantages, but which is the better solution? Let’s compare the two.

SPIRAL DUCT

Spiral, or round, duct is typically fabricated from stainless steel, galvanized steel or aluminum rolls, and in some cases fiberglass board. The duct is commonly cut to 20 feet for shipping standards but can be cut to different sizes upon request. Depending on the performance requirements, the ducts can be connected by using slip joints or flanges which helps to eliminate leaks.

Some key benefits of spiral duct include:

  • Less noise and vibrations due to the curved shape and lack of sharp corners and angles
  • Lower installation costs and less sheet metal required for the same airflow rate
  • Easily sealable which prevents air leakage and energy waste
  • Lightweight, but very structurally strong and airtight
  • No height restrictions
  • Exceptional appearance

RECTANGULAR DUCT

Rectangular duct is also fabricated from steel, aluminum, or fiberglass, but uses up to 46% more material in its fabrication than spiral duct. Due to the higher amount of sheet metal required for rectangular duct, installation, labor, and maintenance costs are generally higher.

Though rectangular duct can be more costly overall, it does have a few key benefits:

  • Can be customized to fit any building height restrictions
  • Ease of shipping due to its ability to be broken down or nested
  • Provides a flat surface for branch tap-ins in a trunk-and-branch design
  • Create warmer environments
  • Flexible for width-to-height restrictions

So, bottom line, which is the better option?

Spiral duct holds many significant benefits over rectangular duct.

Using less material for fabrication and ease of installation, spiral duct can save up to 37% of installation and operating costs over the life of the HVAC system. In many cases, only one worker is needed to install the spiral duct due to its lightweight, but strong, characteristics. Spiral duct also requires 20% fewer hangers and supports over rectangular duct, saving even more labor and installation costs.

Spiral duct is also a more sustainable option for the environment, reducing fan energy by up to 30%. The American Society of Heating, Refrigeration and Air-Conditioning Engineers recommends the installation of circular duct whenever possible.

Spiral Duct

Because of the round shape of spiral duct, there are no longitudinal seams, which accounts for approximately 15% of rectangular duct leakage. There are also fewer joints with connectors that are easier to seal, further reducing leakage and waste. The shape and smoother turns of spiral duct also reduce pressure drops throughout the system and provides a quieter operation over rectangular duct. Less surface area throughout the system reduces unintended heat transfer when air is passed through unconditioned areas of the building. Also, because there are no flat surfaces or sharp corners, dust, and contaminant accumulation within the system is significantly reduced and can be easily maintained and cleaned using spinning brushes.

Lastly, spiral duct boasts an exceptional appearance over rectangular duct. In many cases, it can be possible to eliminate drop ceilings, exposing the spiral duct and providing a unique, attractive look and feel to the building.

Overall, spiral duct significantly reduces material, installation, and operating costs, saves energy, is easier to maintain, provides quieter operation, and offers a greater aesthetic appearance over other duct types.

Stainless Steel & Aluminum Slit Coil for Spiral Duct Applications

Rolled Metal Products stocks a full range of stainless steel and aluminum slit coil for all your spiral duct needs. Our experienced team members are ready to assist you in choosing the right material for your spiral duct requirements and can help you find ways to reduce your overall costs. Get in touch with us today!