Do anyone use the round pipe for a/c ducts in the attic? Easier to use than rect. duct?
Rectangular duct is used to deliver air to the round ducts,
round ducts deliver the air to the registers.
The only time I have seen rect duct used to supply the air to the register is in commercial work.
Rect duct costs more.
But anything is possible in this line of work,
If you try to fail, and succeed.
Which have you done ?
round pipe is used directly off the plenum. Then round flex to the registers off of the round duct.
Round trunk isn't common in my area, Lanc, PA.
But I can't see why there would be any thing wrong with doing it that way.
Sizing is more important then shape.
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How many times must one fix something before it is fixed?
A section of straight smooth round duct has less resistance than a straight section of rectangular duct of the same cross sectional area.
An excellent and inexpensive book on duct systems and airflow is "Airflow In Ducts" by Leo Meyer available at http://www.lamabooks.com
Easy to understand and right to the point. One of the most clearly written books on the topic I have ever seen in my 34 years in this business.