The Differences Between Spiral and Rectangular Ductwork

Ducting is used in many buildings as part of heating, ventilation and air conditioning systems to both deliver and remove air. But all ducts are not the same – in particular, there are both rectangular and spiral types.

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Proper ventilation is important for many reasons. If you need ducting to achieve it, what are the differences between them? And in what circumstances is it preferable to use one rather than the other? Read on to find out.

Spiral Ducts

Typically, spiral ducts are made of metal – usually aluminum or galvanized steel. They can, however, be made from fiberglass or from a composite of plastic supported by metal wire. Spirals ducts are the most commonly used type because they are easier to handle, transport and install.

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A wide selection of ductwork parts is available from suppliers like, and the ducts can be connected by various means. Because of their shape and design, circular ducts are less prone to leaks and are quieter when air is flowing through them because they reduce the ‘drum effect’. The pressure drop over distances is less with spiral ducts too, so you can run lower fan speeds, thus saving on energy consumption.

When it comes to insulation, spiral ducts are easier to work with too. However, spiral ducts have to be insulated on the outside, which can increase their bulk and makes retrofitting of insulation more difficult.

Rectangular Ducts

The design of rectangular ducts means that they use more material, making them more expensive. They are also more time-consuming to install. They are, though, often a better solution where space is restricted, as they can be adapted to the requirements of the building with different widths and heights. The flat-sided design also makes joins and branches easier to achieve.

Rectangular ducts can be insulated on the inside, though obviously this needs to be done during installation. This makes for more effective insulation with less chance of it becoming damaged over time. Generally speaking, rectangular ducts once installed will need less maintenance.

Choosing which type of duct to use involves a number of factors. If you are in doubt, then it’s best to talk to a specialist who will be able to advise you on the appropriate type of ductwork to use for your particular application.