What size grill and duct do I need for my cold air return to fit between wall studs in the following scenario?
I have an open-air return in my basement mechanical room with no cold air return ducts or transfer grills. Thus I'd like to install a cold air return duct to the floor above.
I have a Rheem R96VA0602317MSA 3 ton 56,000 BTU furnace rated for HEATING 1025 CFM @ .2" [.049 kPa] conditioning a 1000 sqft single first floor apartment with 8 foot ceilings.
I'd like to punch up through the ceiling of the mechanical room above the furnace and add a cold air return.
How big of a grill and duct do I need for the cold air return? Looking online at some calculators it seems like I need 421 sq inch grill - but is that for a supply? Using another calculator, I have 8000 sqft of total air volume with 6 changes an hour which suggests a 2.5 ton system, but since I already have the spec saying 1025 cfm in a 3 ton system should I just go off the 3 ton 1025 cfm value I have? Another calculator says I need 360 sq inches of grille?
I couldn't find calculators on return duct size - just supply total duct area. What duct size do I need if it only travels 9 feet, from basement to ceiling? I'd like the duct and grill to be something that can fit between wall studs so that I can have it come up into an interior wall on the first floor.