Which cold air returns do you close in the winter?

If you have both high and low returns, close the top ones in the heating season, and the bottom ones in the cooling season.

Close the high ones in winter to pull the heat down to the floor. And open the high ones in cooling to pull the heat out of the room as heat rises and cool air falls.

Lets discuss weather or not you should close any of these returns. Here are some formulas to follow. A/C's need 400 cfm per ton. So a 3 ton A/C needs 1200 CFM (cubic feet per minute. 1 ton equals 12000 BTU. Your condenser model number will have in it a pair of numbers ex. 24 or 024=2 ton 30 or 030=2.5 ton 36 or 036=3 ton and so on. Now at the furnace disconnect the power, locate the fan speed terminals and record what color wires are on heat and cool, Usually cooling speed will be high and heat speed will be med/low. Change the blower speeds so that the heat speed is the old cooling speed.
Start the furnace and let it run up to a constant temperature. Record the cold air return temperature (H2) and the supply temperature (H1). Find the BTU rating of the furnace lets say 75,000 BTU (BTUH) (output) "not input"

... BTUH
CFM=(T1-T2)x1.08

... 75000
EX. (142-68)x1.08=938.43 cfm

enough air flow to support 2 tons of cooling. If your airflow is to low with your return closed you can freeze up the evaporator coil, or slug the compressor with liquid refrigerant greatly reducing its life. Change fan speeds back when you are done.

I know this seems like allot to know, but HVAC can be done right or you can keep buying new equipment and complain about what a piece of junk you have. In my experience must of the time it's not the equipment, even though there is allot of junk out there.