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To elaborate a bit more. If you are running an 8" duct off the supply, would you do a quick 90 degree towards the destination or do a gradual 90 degree?

I would guess a quick would be better because you have less cfm loss per foot. I could be wrong.

The 8" will hit a wye to two 6" ducts which run into the register boxes.

The registers are 5 foot from each other. The more important register (deeper in the room to be air conditioned) will be straight off the wye. Maybe 10 foot straight from the wye versus ~12 - 15 foot off the other side of the wye.

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Sharp corners in the duct work are not preferred. They create eddy currents in the air flow that results in greater pressure drop than a straight run of pipe. In some situations the eddy currents can also create increased air turbulence noise as well.

Instead of using elbows for ducts you should consider using the newer style of non-metallic duct material. This has many advantages including:

  1. It is much easier to work with.
  2. There is far less thermal drop across the duct material than metal duct.
  3. It is lightweight.
  4. You can easily snake it in any direction during installation.
  5. The insulation material is built right into the duct itself as opposed to having to be installed separately.
  6. Duct tape sticks to it like a champ.
  7. Tools needed for installation are very simple ones that most DIYers will already have.

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In terms of airflow, I would think this is worse than solid metal ducting that has less friction with the sides of the duct. –  BMitch Jul 27 '13 at 20:49
    
@BMitch - The materials that I've used get rather smooth inside when they are stretched out to length. My take is that if the pressure drop is too much just use a larger diameter duct. The insulative benefits of this stuff cannot be ignored. –  Michael Karas Jul 28 '13 at 0:04
    
Yeah, I am a big fan of flexible duct. I will do a gradual 90 degree bend then and make sure I stretch it out before deploying. Thx for the help all! –  ThaKidd Jul 28 '13 at 1:06
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