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Discussion Starter · #1 ·
If I have a 3" dia. round duct of length L with a reducer at the end that goes from the 3" pipe to a 4" opening, does this count as some "negative length" on the 3" pipe. Let's say it vents a bathroom fan.

If I have a 4" dia. round duct of length L with a reducer at the end that goes from the 4" pipe to a 3" opening, how much does this fitting add to the equivalent length of the 4" pipe?

Are these formulas/tables for pipe this small in Manual D?
 
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