I had some work done on my heater and the tech checked the air flow. He found that one of the 2 air return ducts was smaller than it should have been. So I had a man come out to give me an estimate on putting in a larger duct. His proposal was to replace the 10" metal duct with a 14" flex duct. I looked at it and the very idea of flex duct seemed to be kind of iffy. So I did some research and found that the general consensus is that flex duct is ok, if it is installed right. And hat there is a lot of flex duct that is not installed right. That involves mostly short straight sections. Well, my installation involves a right angle bend and a 1 foot offset near the end. Not exactly straight. It took some doing but I finally got the salesman to understand that it is going to be hard to get the duct 'fully extended' with that much play in it. So you would get a lot of turbulence and reduced flow. He thought that by oversizing it from 12 to 14 inches it would make up for it. But he did come around after a while. So they will put in
12 inch metal duct.It costs more, but I figure you can do it cheap or you can do it right.
In my search the site that I found to be about the best was
Bill