For years I've drawn flexible duct the same way, basically a wavy line similar to the polyline used to draw insulation.
Years ago, a friend created a double line routine (unfortunately not for Autocad) that made a more realistic appearing flexible duct, BUT,,,,
I was working for a design build company, so it only had to appear correct to our company. Now, I work for a design company, and we ship out our drawings all over the country.
My question is this; what is a good typical flexible duct symbol, and is there a lisp routine or line style for it? I noticed that if you create a flexible duct OR pipe in Revit, you get a symbol like a one rail train track. Is this industry standard? What does ASHRAE use?