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For sheet metal supply/return plenum boxes, I noticed the builder didn't make holes in the top for any of the ductwork. I figure it's because it wouldn't make sense considering every duct works its way back down from that point. Question I have is for cases where I just have run out of room around the plenum box to make new duct, is it ok to make a new take off on the top of the box? Is there any gotchas with doing that? I figure not but just checking.

Where the insulation is piled on top is my proposed location for a new duct.

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The reason there are no supply ducts running from the top of the supply air plenum is that the air from the fan in the air handler hits the top of the plenum forcing air equally to the supply runs coming off of the sides of the plenum. If you were to have a supply run coming off of the top of the plenum it would get more air because of it having the path of less resistance. Return air plenums do not matter where the duct openings are as much.

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