Four rules when installing flexible ducts

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The basic idea when installing HVAC ducts is to design their route for the most efficient airflow. However, in the real world – especially on large projects when sub-contractors find themselves competing for service channel access – the final branches of ductwork trees, from plenum to outlet, are often determined on site.

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By eliminating joints, elbows and offsets, a flexible duct is quicker to install and provides elbow room if obstructions are encountered, but these are not without dangers. The flexible ducts may be made from a number of different materials and may involve processes such as Plastic Moulding, like that you can see from Because air is invisible, few people, including some contractors, understand how it flows. Careless installations cause performance problems that are difficult to identify, locate and rectify after they’ve been drywalled in. Following four rules will help you avoid those pitfalls.

Airtight seals and insulation

Tight and accurate tie wraps are needed at all connection points, followed by the use of proper duct mastic to ensure perfect, long-lasting seals.

Insulating sheathing is often appropriate, especially for runs through areas beyond the air-conditioned space. Such transits should be avoided because they risk significant energy loss to the environment (loss of warming or cooling capacity), but even if the chase of a flexible duct is entirely within the conditioned space, delivering maximum energy to the intended outlet is still desirable.

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Frugal duct runs

The longer your duct, the greater the friction presented to moving air.“Erring on the side of generosity is a bad idea: slack means the air is travelling further and being obstructed by its concertina-like ribs. Short, straight runs stretched smooth are best.

Avoid tight turns

Both excessively slack ducting and poor routing can introduce constrictions that severely resist airflow. A catalogue of routing errors can be seen at

Thorough initial planning of the duct run and its framing should minimise the length of the ductwork and avoid any tight turns. An efficient project manager should then prevent contractors treading on each others’ toes by obstructing planned service routes. When turns cannot be avoided, the radius of the turn should never be less than the duct diameter.

Adequate support

Being flexible means ducting can stretch, sag, twist or kink, so in any place where this could happen, prevent it with supporting saddles or strapping. Note, however, that if you let supports interrupt insulation, cold spots could encourage mould growth in the duct.

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