Q: What is the proper size of return for a 3 ton 16 seer unit. Is having 2 return with 16×25 (16”) and 12×12 (9”) advisable? Also, we have 12 different size of vents in the house (3 rooms – master (2 vent), other room (1 vent each) , what should be the right size of duct supplies per each. I would appreciate if you could enlighten me. Thank you very much.   

One 16 x 25 return opening is good for approximately 2.5 tons. So, if you have the two 16 x 25 return grills, each on individual 16-inch flex ducts this alone is sufficient return air. The 12 x 12 helps. If the two 16 x 25 are twinned with only one 16 –inch flex duct, we recommend increasing  the 9-inch duct on the 12 x 12 to 12-inch to maximize total return air draw as it will only aid in circulation. Additional return air is not detrimental to the cooling system.

As far as the supply air registers:  The size and style of register depends on the size of the room, ceiling height, amount and type of glass and sun exposure to the outside wall. The key component is to have a grill that properly throws the air into a given room and to have a register that is not restrictive on the air delivery.   Depending on ceiling height ( 8-10 ceilings ) if the grill is located from the center of the room, we prefer a 3- directional grill. If the grill is located back further towards the inside wall we prefer a one way grill. This is designed to get as much coverage to the outside wall as possible.