When switching 5" round duct to 3 1/4x10" rectangular duct, is it okay that the area of the rectangular duct is greater than the area of the round duct or does that present a problem? I get that they should be equal at minimum but is it okay to go bigger? I only ask because that size rectangular duct is the most available at local stores. Will have it fabricated if it is an issue. Thanks.

 

If you're just moving air going from a smaller to larger duct is not an issue. It's only a concern, usually in industrial applications, when you're transporting something in the air like grain or dust and you want to keep the velocity up. But in something like a HVAC system the lower velocity in a larger duct can be more efficient.

 

If it is on a supply register it will decrees the velocity going bigger

 

Another question re this run: I want to remove the elbow and connect the rectangular duct to the 5" duct using a collar. Is that okay or would that complicate the flow as no 90* turn would be available.

 

keep the 90! it will help turn the air

 

Airman.1944 What if I can't: Is there an alternative?

I need to lower that run because I need to brace between those 2 joists as the last joist is a rim joist and the wall is sitting on that joist not fully supported.