How to successfully use flexible ducting

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Using flexible ducting avoids a lot of the work associated with its rigid counterpart, so it can speed up installation of HVAC ductwork. But the same physical characteristics that are so advantageous for timely installations can be the source of poor performance if there is too much haste and not enough proper care.

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Bad Installation, Weak Airflow

The aim is to create directness for strong airflow, so an installation of flexible ducting should be designed for short straight runs. Increased friction that works against airflow is undesirable, of course, but it is too often the unintended consequence of a poor installation. Problems can occur by having too much duct for getting from point A to B – it’s easy when working by eye to cut several feet longer than needed or to create a safety margin. However, if there is slack in the duct, then there will be performance issues. The air shouldn’t have to travel further than it needs to, as otherwise it is exposed to more interior surface area. Because the duct isn’t stretched, the ravelling together of the ribs creates more friction than usual.

Support for Gentle Turns

Bends and sags can occur if obstructions in the way of the ducting are not bridged properly. When routing flexible duct up and over an obstacle, saddles or straps should be used at frequent intervals so that a gradual bend is created. As a very basic rule of thumb, a duct support should occur no less than every four feet.

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Different duct diameter sizes require variations in support frequency, but product-specific information should be available from a good supplier such as Moreover, manuals are available from FETA for guidance on installation and maintenance of flexible duct systems.

Unfortunately, despite all good intentions, a lack of planning – or poor coordination with other sub-trades on a project – can adversely affect ducting efficiency. If ductwork isn’t in a designated chase, it is often competing with other “traffic”. Avoiding the clutter of pipes and cabling can lead to kinks, compressions and even unwanted turns. But the necessity to “iron out” all the kinks couldn’t be more crucial in flexible ductwork, because when it is sealed behind walls and ceilings, any performance problems that could have been avoided become very costly mistakes.

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