Q. Is flexible duct a good choice for a duct system?
A. It doesn’t matter what materials a duct system is made of if the design is bad. Duct board and flexible duct are acceptable choices and work well when designed correctly for the materials. The choice does not need to be a trade-off between the higher material and labor costs of metal ducts and the performance characteristics of other systems.
Flexible ducting is inexpensive and easy to install, but because of its spiral wire helix construction, flex duct has a higher friction loss when compared to sheet metal duct or fiberglass duct board. The inner core of flex duct changes shape with compression and bending, which increases turbulence and friction loss. Unfortunately compression and bending are the very attributes that make flex duct so popular and easy to use.
When installed and designed correctly flexible spiral duct can and does work well. The biggest problem is not the material, but how it is installed. It is true that when it comes to air conditioning and heating ducts … “no duct job is so poorly done … that someone can’t do it cheaper”.