Rectangular v spiral ductwork: the differences

Ductwork is essential to an HVAC system, as it is used to deliver air to rooms from the central heating unit, as well as removing air both to the outside and back to be recycled.

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An example of a simple duct is the fireplace chimney, which conveys smoke to the outside, whilst also providing ventilation. The airflow which is allowed from the room to the outside also helps to keep fires burning.

The two types of ducting we will look at here are the spiral duct and the rectangular duct. Each has its own specialist applications within construction projects.

Spiral ductwork

Spiral, or round, ducts are generally crafted from galvanised steel, and sometimes aluminium. As well as being made from sheet metal, these ducts can sometimes be made from flexible plastic and wire, or fibreglass board.

This type of ducting can be constructed to be any length, with a standard length of 20 feet common. The reason for this is the fact that it will fit in a standard delivery truck. Spiral ducting can be joined using flange-to-flange connecting methods, or slip joints. The method chosen will depend on the performance required. For more information on spiral ducts, click here:

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Rectangular ducting

A rectangular duct is constructed from around a third more steel than spiral ductwork of a similar size. This is likely to mean that they cost more in terms of insulation, support and labour. As with spiral ducting, rectangular ducting can be constructed with sheet metal or fibreglass board.

Many suggest that the additional expense is justified due to the fact that rectangular ducting is able to be adapted to any building height necessary. The way the ducting is constructed means it can be nested when it is transported, allowing more to be shipped in a truck. The flat surfaces of rectangular ducts make tapping branches into a trunk and branch design easy to achieve.

Round ducting, generally speaking, is more rigid than its rectangular counterpart, and transfers less noise throughout the system. On the other hand, rectangular ducts tend to have fewer leaks due to the lack of seams and longitudinal joints.

No matter the building, an HVAC system will require ducting of some description. Round and rectangular ductwork both have their own pros and cons, which will have to be weighed up.