Hi Gotoman, I live in an old town home that is multi level with a garage. Built in 1974. I just removed the original avocado green Rheem air handler that was still running, but the compressor finally failed. I live in Marietta, GA so it's hot right now! All metal ducts running through the house and attic. Original insulation on ducts. I know the optimal solution would be to install a split system but that is not in the budget so a single air handler is going in. The original configuration was the closet was the supply plenum and it was not sealed. So the garage, inside the walls and who knows what other air was making it into the air handler. I'm replacing the old 3 ton with a new 3 ton air handler which is a Rheem RH1V3621MTANJA. The original air intake is 14.5" x 20"=290 sq in. cut so the old 3 ton was getting about half of the air it needed. Maybe less than half because of how inefficient it was compared to today's units. Well the new air handler needs 600 sq in of space to flow the air it needs. So now you know what I have. Here's what I plan to do and would like your opinion. I am cutting two additional spaces in the wall of the closet in addition to keeping the original cut. On opposite sides of the closet (it sticks out like a peninsula with 3 sides) will be 20" x 20" newly cut vents with an 18" circular flex duct and the original front will be 14" x 14" with a 14" flex duct. I plan on putting the air handler on a sheet metal plenum box measuring 25" H by 22" L X 22" W. The two 18" ducts will join on opposite sides of the box and the 14" duct will attach to the front. Basically a "T". Space is really tight in this closet so the flex line will only be about 8" for the 18" ducts and 4" for the 14" duct in the front. I know this is not ideal. I would love to run intake vents in each room or in the upstairs hallway, but the construction just doesn't make it easy or cost effective. I really need to send you picture. Let me know if I've explained it well enough. Thanks for your time.
P.S. Three story town home, 4 bdrm, 2000sqft.