I'm in SoCal and planning to replace my really old Central AC sometime this year and doing all the background research so I hope I'm somewhat prepared when I start getting proposals.
One of my questions has to do with return grille size and location. I currently have a 3 ton system (I think, it's from the mid 60's and the model # has no tonnage info that I can see) that I assume would be designed to run at ~1200cfm. The return is basically just a grille below the furnace closet door and is 24"x10". From what I can tell, this seems undersized to me (it does make a fair amount of noise in cooling mode) and figured based on grille data I've seen it's probably running around 900 fpm face velocity. Given the fact that my Man J calc showed I probably need 3.5 ton, I would think it might be a good idea to add at least another return somehow to reduce noise and pressure drop across the grille. There is a wall around the corner in the master bedroom that has direct access to the bottom of the furnace closet and might even be able to use a larger grille size. Would that be a acceptable location or would it be much better to try and duct a return or two in other locations of the house?