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Old 03-01-2010, 09:36 AM   #1
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Plenum to duct branch off placement question


Does it matter where you place the duct connecto to the supply plenum? I am working out a remodel plan, which includes zoning, but for it to work I need to run 2 sets of supply plenum pretty close to each other... so on the one I can run out the side to the right and the other out the side to the left... but can I also go out the top? the joists are going to be perpendicular to the supply runs so I can go up and out that way... is it ok to go out the top? My existing supply plenum has some runs out the top, and that was installed with the house being built.. or does this mess with static pressure?

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Old 03-01-2010, 11:07 AM   #2
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Plenum to duct branch off placement question


Not usually a good idea to come off the top because that is where the airflow makes it's turn to go out the trunk. To simplify; you lose a lot of duct pressure and that can cause the other branches to under feed. If you have two runs off the top now Your duct work was more than likely designed to compensate for it.

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Old 03-01-2010, 11:35 AM   #3
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Plenum to duct branch off placement question


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Originally Posted by hvaclover View Post
Not usually a good idea to come off the top because that is where the airflow makes it's turn to go out the trunk. To simplify; you lose a lot of duct pressure and that can cause the other branches to under feed. If you have two runs off the top now Your duct work was more than likely designed to compensate for it.
I think I may of worded that wrong, I don't mean the top of the supply plenum that sits on the furannce, I mean to say off a trunk line
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Old 03-01-2010, 12:16 PM   #4
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Plenum to duct branch off placement question


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I think I may of worded that wrong, I don't mean the top of the supply plenum that sits on the furannce, I mean to say off a trunk line

You may ending up stealing airflow form other take offs.

I have added additional runs off problem duct systems to poorly conditioned areas of an addition to the home. But I added a four foot extension to the existing trunk to make my connections (the trunk was in the attic and had room for the extra trunk I added).A new over sized furnace was installed
with a larger than necessary blower. So I had the extra capacity I needed to better condition those part of the house that not getting enough air flow.

I also have also seen guys run ducts too small additions from the the existing trunk and got little to no improvement.
builders are notorious for for building homes with undersized duct work and they keep the house warm at the expense of longer than necessary running times and the furnace blower working harder.

Sorry for the long explanation, it was to illustrate that this is not a black and white scenario when it comes to placement of new runs.

The zoning may be an issue. I would draw a layout of the existing system with dimensions of the supply and return ducts and post it here. There are zoning experts who can give you better input once that is done.
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Old 03-03-2010, 02:13 PM   #5
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Plenum to duct branch off placement question


where I work, we are currently redoing all the HVAC on the upper floors, and the guys are the same ones that would do work at my house if I called them out, and they are running ducts right off the air handlers with 90 degree elbows only off the top... asked them why they are doing that and not out the side and they told me thats all they do anymore is out the top... but they couldn't give me a reason why...
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Old 03-03-2010, 03:37 PM   #6
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Plenum to duct branch off placement question


If they are supply take offs, then they have a lot of turbulence in them. And are making the system static higher then needed on each supply.

They do it like that because its faster and cheaper. Its what they boss wants.

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