The exact size(s) are beyond scope of less than onsite calculation but maybe part of your questions can be answered here. Bigger is usualy better for single returns except where return duct is extra long and in a hot attic. With two returns,oversizing one will cause the other to return less air and possiably make that area not change out air often enough. Much also depends on return locations and if you presently have a dead space when deciding between 3rd return and larger return(s). The science of ducting is far less understood by most than the mechanical side is.

You shouldn't need a 90,000BTU furnace in Dallas. 4 Tons sounds like a lot for a downstairs unit.

The two equipments are in the attic. Each unit has 2 returns, one 16 and one 10'. Three return are upstairs on the walkway, and one, the 10' in the MBR downstairs.
A new return could b easily added upstairs (above the main door (at 21 ft high) and also with more work and costs downstairs in the master bathroom, in the closet or in the studio.
Our MBR is cold in winter since furthest away from unit and (probably) short on supply. One new supply duct of 8' (with new grill) will be added to the other 2. The problem is that from the furnace one 14' delivers air to the east side of the house to 8 grills...apparently this is too much.
The downstairs unit has on the supply side a 8, a 10 and a 14' duct; a 16' and a 10' on the return side.
What do you thing, is it a valid and correct option to increase the 16' return to a 18' and to add a new 8' supply to the plenum for the MBR or what would you do?
I would like to do it the right way and to fix what can be fixed only one time.
None of the 9 contractors I got could convince me with a solid knowledge, unfortunately!
Thank you!

Angel,we realy want to help but there is not an engineer on earth that can give you optimum duct sizes and locations without data they or thier repersentive gathered first hand on site. I reccomend you have somone assist you in writeing "a scope of work" that amoung other things,makes it clear you are buying comfort,efficeny,reliability and satisfaction not a bunch of equipment and installation methods. Then let the contractor do thier job. Were you a diy,there would be a wealth of info and guidance from the folks here but us telling you details that you tell a contractor is byeond a mistake,it is setting up possible disaster. I'm not trying to be difficult,the same is true for hireing most anything out. A famous quote regarding this is "a person acting as thier own attorney has a fool for a client".

Thank you for your help.
I just would like to know if you see something completely wrong or bad in the quote and work that will be done.
Unfortunately nobody could provide a complete and accurate analysis and quote, the resulting idea to solve the problem in the MBR is mine according to the infos I got the the different contractors.
All of them told me to install the new unit and see if this will solve the issue as they expect.
I didn't trust this idea since I will get more airflow due to the increased return everywhere, not only in the MBR.
I'll probably get the desired temperature in a shorter time, but proportionally not more air in the MBR.
Now I don't know if the increased airflow in the MBR with the same return in the MBR will have side effects and if a 18' and a 10 return duct are enough for a 4ton unit.
I hope you can help me since tomorrow the company will install the new equipment.

Does anyone have suggestions these folks might find helpful? I just don't know any thing 9 contractors could miss. On going complaints with MB and no other downstairs rooms seems unusual since the only return downstairs is in the MB. If I were forced to make a shot in the dark,I would say the MB door is closed while occupied resulting in the t-stat is being tricked.

Thank you for your help.
I just would like to know if you see something completely wrong or bad in the quote and work that will be done.
Unfortunately nobody could provide a complete and accurate analysis and quote, the resulting idea to solve the problem in the MBR is mine according to the infos I got the the different contractors.
All of them told me to install the new unit and see if this will solve the issue as they expect.
I didn't trust this idea since I will get more airflow due to the increased return everywhere, not only in the MBR.
I'll probably get the desired temperature in a shorter time, but proportionally not more air in the MBR.
Now I don't know if the increased airflow in the MBR with the same return in the MBR will have side effects and if a 18' and a 10 return duct are enough for a 4ton unit.
I hope you can help me since tomorrow the company will install the new equipment.

Klem1, All,
thanks again for the help!
The thermostat is at the entrance of the livingroom and we close the MBR door when we sleep.
Unfortunately with all 9 contractors I had the impression they just wanted to sell a new unit without worrying about load calc. nor the MBR problem.
The 3rd was the first that told me I was short on return. He also did a 15 min load calc... The 8th took his time for the new unit, took more time for the load calc but didn't check the supply duct in the attic. The only one that checked from the attic down to the east side of the house the supply duct was the 9th company (but no load calc.).
I don't know if I'm just too precise or just not lucky with the contractors....
Do you see something wrong with the idea of adding a new supply to the MBR? Is a 18' and a 10' supply enough for a 4 ton? Thanks

Want to experiment with a whole house; or separate 1st floor 2nd floor load-calc; do the floors separate.

Click on the calculator & figure the infiltration CFM; put it in the appropriate blank or latent load will be wrong.

Print & follow all the simple instructions.

Here is a link that might be useful: online whole house load-calc