because I have a large bedroom, I’m adding another return duct which I will put on the ceiling against the wall. It's a 15’ ceiling and I want to get the trapped hot air out.
Now, I was thinking of putting either a 12x12x8 or a 10x10x8, and then realized would it matter, because ultimately no matter what size it is, since they both have an 8” flexible hose, won't that determine how much air is drawn in anyway. Seems like an excellent physics problem. I'm going nuts trying to figure this one out. Thank you in advance for the help. EdE
Size of a return duct
ducts
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Best Answer
A 10"x 10" register box would be fine with an 8" flex duct.
I think you need to do a bit more research into your AC system though. 8" flex duct has a lot of resistance to air flow, much more than rigid duct. Depending on how well your existing system is balanced, supply and return, you might find out that adding that register won't help get rid of that trapped hot air. If you connect a length of 8" flex to a return plenum or another rigid return duct, the resistance of the flex might be too much to make any difference. Have you considered just adding a ceiling fan to move the trapped air around so the existing system can remove and cool that air down?