Round to rectangular duct change

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  #1  
Old 04-12-18, 11:30 AM
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Round to rectangular duct change

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.
 
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Old 04-12-18, 12:01 PM
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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.
 
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Old 04-12-18, 12:49 PM
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If it is on a supply register it will decrees the velocity going bigger
 
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Old 04-12-18, 01:40 PM
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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.
 
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Old 04-13-18, 10:03 AM
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keep the 90! it will help turn the air
 
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Old 04-14-18, 06:01 AM
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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.
 
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