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veesubotee 01-24-2009 03:28 PM

Ductwork question
What is the minimum distance from the end of a trunk, when adding a takeoff?


sgthvac 01-24-2009 07:07 PM

I've always been told 12-18" but, I'm not an installer.

tk03 01-24-2009 07:38 PM

For proper static at the end of the duct it is 18"

veesubotee 01-25-2009 09:08 AM


Originally Posted by tk03 (Post 218523)
For proper static at the end of the duct it is 18"

Wow, a little more than I had expected (hoped) for as there are some clearance problems.

My situation: My original ductwork is in need of some modification. Main trunk butt ends with a 5" & 6" takeoff on the butt. Prior to the end, there is a curved transition (smaller trunk) which runs perpendicular to the first section. At the butt end of that section is a 6" takeoff.

Both of the 6" runs are fed to the second floor and the airflow is anemic. So, I want to relocate those (3) runs off of the ends. I had planned to have both the 5" and 6" come off the underside close to the end (I have 10" of clearance until the transition).

18" would take me to the same center point as the transition, I mentioned. I could go farther back but would have to come off the bottom of the trunk (all other takeoffs are on top: problem?). The adjacent run could be fitted in.

Would there be a problem with 2 takeoffs being located at the same center point on different surfaces of the trunk? Probably not, as I just remembered that at the plenum, there is a 20" section of trunk which runs in the opposite direction. There are (3) 5" takeoffs on the underside about an inch from the end. There is also a 6" on the upper surface. It is only a few inches back from the end. The airflow from the 6" (upstairs) is very good; the 5's are dampered back.

The 3rd (last) takeoff would not be a problem.

I plan to use flexible metal duct to make gentle curves and eliminate some of the elbows while connecting to the existing ducts, and cap the abandoned ends.

Your comments and suggestions please.


tk03 01-27-2009 08:24 AM

I would avoid flexible metal duct for this change as there is far more Resistance to it than rigid duct. Even the insulated flex duct is far better than the metal flex duct. It still adds resistance so the flow would be reduced slightly. A complete system laid out with insulated flex must have some special attention paid to it.

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