Oh my goodness.Originally posted by chaddawson
I'm sitting here trying to visualize the air currents inside this contraption. I'm thinking the air enters from the oval, strikes the bottom of the box, flows out primarily in several directions along the bottom, curls up along the sides, circles back along the top, and then strikes the additional air trying to enter. And it's not doing this in a nice neat fashion, since even in good ductwork, airflow is seldom laminar. The turbulence going on inside this thing is unreal. Friction loss sky high.
I guess some folks think all you have to do with ductwork like this is take a sheetmetal box and charge it with air pressure, and something will flow out the holes. Yeah, you'll get flow, but at a severe penalty to velocity. Here I was obsessing over the oval vs. the 12" round, only to see what kind of box it's flowing into! I'd be curious to run some numbers on that puppy!
At a bare absolute minimum, if the 12" round entered the upper end of the box (not the top as pictured) and the take-offs were cut into the far side of the box, it might flow a LITTLE better. Not much, but an improvement. But apparently since this 12" round is draping down from the attic and then being faced with the small amount of space between the drywall and the underside of the top floor, WTF are you supposed to do with that?
Me? Being a two story house I'd not consider trying to cool two floors with one unit. I'd also never put any equipment in an attic. Not even ducts. Air handlers in closet or drop ceiling, ducts in drop ceiling, below attic insulation, IN CONDITIONED SPACE!
But...you're in Phoenix. And...you're not alone. Do a search on this site for a pro member named PaysonHVAC. He's a Phoenix tech and he's filled the Wall of Shame section of this site with infamous Phoenix installs.
As for how to realistically solve the problem you have, looks like it ain't gonna be fun. That box, IMO, has to go. Absolutely. I don't know how the ducts run out from that box...maybe you could snap some pics for us and post them. Pics...as your drawing has...go a long way.
Meanwhile, when I hit the books and calculator again, I'm gonna play around with your scenario more to teach me some really good object lessons. I'm glad I've helped, in whatever way that was.