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Rectangular and spiral ductwork: the differences

What are ducts?

Ducts are metal passageways which are used to transport air within HVAC or heating, ventilation and air conditioning units. They come in two varieties: spiral and rectangular. Most are manufactured out of galvanised steel, although fiberglass is also an option. Spiral ducts can also be made out of a wire composite and flexible plastic.

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Rectangular ducts use around 35 percent more metal than their spiral duct counterparts meaning that they are the slightly more expensive option. This is countered by the fact that they are easy to transport thanks to their flat surface and can be adapted to all building heights and restrictions, as shown in

How do rectangular and spiral ducts differ?


Longitudinal and transverse joints mean that rectangular ducts suffer more frequently from leaks than the spiral version. This means less efficiency as leakage impacts the costs of heating and cooling, resulting in higher bills at the end of the month. Spiral ducts are self-sealing and are more efficient as a result.

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The turns and pressure differential within rectangular ductwork can create heavy reverberation and, as a result, create unwanted noises within offices and home settings. The air pressure within spiral ducts is equalised throughout the system thanks to their rigid setup, which results in less duct vibration and reduced noise.

Energy loss

The surface area of rectangular ductwork is much higher than spiral ducts like those found at This greater surface area results in more heat loss through the duct walls. Spiral ducts have less surface area, so more energy is conserved as a result.

Cleaning and maintenance

Spiral ducts are easier to clean and maintain than rectangular ones, thanks to their smooth interior, which traps less dust. Equipment specially designed to clean spiral ducts also makes the task that little bit easier.

Which ductwork should you choose?

Your decision will depend on a number of factors, as follows.

Spiral pros

– They are quieter due to a reduced pressure drop.
– Initial installation is cheaper as less metal is required.
– They seal more tightly resulting in fewer leaks.

Rectangular pros

– If you have restrictions on the height to width installation, rectangular is a more flexible choice.
– They create an indoor environment that is warmer, thanks to their interior insulation.


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