I live in a 2 story condo with central heat/air. There is one 12x30 return at floor level on the first floor. When outdoor temps rise over 90, ac cycles normally, but upstairs is more than 10 degrees hotter than downstairs. I have r22 attic insulation and a thermostat controlled attic exhaust fan. Helps a lot, but I'm wondering if I should install another return in the ceiling of the upstairs hallway? Should there be a damper to shut it off in the winter for heating? Should it be the same size as the downstairs one? What size duct (about 25' long)?
Also, I'd like to replace the cover of the return. I've seen solid panels spaced away from the wall in some houses. How would I calculate the spacing? Should the square inch opening around the panel match the square inch opening I had before? Is it even all that important?
I had a similar situation and essentially extended the return to the top of the stairwell on the second floor. Now, the difference is about 2-degrees. Two things, I usually leave doors open, so it is a clear shot for air to circulate and, I usually leave the fan on continuous. This helps keep the air from stratifying. I basically close the return downstairs and leave it closed all the time now. If you close your bedroom doors, a single central return won't do much, regardless of where it is to keep things evened out..
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013