Here is a 'rough' drawing showing the bottom floor of my 2 story house. It should be easy enough to interpret.
The total downstairs area is a little over 2000 sq ft. The 2.5 ton system was NEVER able to maintain a temp lower than around 75 and the unit ran almost 24 hours a day from late May to early October here in good ole N Texas. The Manual J calc called for a 3.5 ton, but since I wanted 2 stage compressor / furnace and variable fan, I had to go to a 4 ton Amana ASX16 system.
The 2 main (shared) return airs are at floor level (in Red) and go straight up to a huge plenum in the attic. I'd say 24 x 16 at least, maybe larger. The new 16 x 16 (in a hallway -ceiling mount) return connects via a 10 inch flex duct cut into the plenum.
One new supply vent (12x6 w 8 in flex duct) was added on the S. side of the house in the main living room. It blows air all along that south wall as the sun beats on it from sun up till around 1 or 2 in the afternoon, then slowly sets off to the west/south side of the house.
The remainder is all pre-existing nicely insullated metal ductwork of various sizes, but off the plenum they are all quite large. The vent in the living room is 16 x 6 as is the main vent in the game room. Huge registers! There is also a 14 x 6 in the kitchen, breakfast nook, and dining room. There is a 12 x 6 in the entry hall by the front door (on the north side), a 10 x 6 in the bedroom and a smaller 8x4 in the bathroom and laundry room. A total of 9 ducts.
It has been suggested by some that I have a major over-capacity system. When I was getting bids for a new a/c system, one of the contractors did a Manual J calculation on his laptop computer and it came up suggesting a 3.5 ton unit. The contractor has been in biz in this area for over 23 years and has all the accreditations and affilliations one would expect.
Another inference here is that the existing ductwork is too small to go from a 2.5 ton to a 4 ton unit, even though another large and separate return air was added as was another supply vent. Would add more, but literally no way to get between the 1st and second floor to add more. Could add one more in the gameroom ceiling by the s side wall between the 2 windows. That would make a total of 10 ducts,
I am pleased with the system. The A/C comes on high, runs for 5 min or so, then goes to low speed for another 10 to 15 min, then off. Repeats this cycle now 3 or 4 times an hour and can easily hold 71 (nicely cold) air that is pretty much uniform in all rooms. The kitchen area is a little low on airflow so they are coming back next week to adjust some dampers. At that time, I will also request they do a static test as well. Teh contractor
I am no expert on this whatsoever. All I know is for the first time in 12 years that I've been in this house, it is FINALLY cool and can be made as cool as I want it regardless of how many people are in the house, doors opening and closing etc. It will knock 77 degrees down to 72 in about 30 minutes. Humidity is roughly 45% give or take a little.
What I am trying to figure out, is why some here are saying I went WAY over capacity??