said by Kenbass: Second of all, 3.5 tons for 2800SF? Wow grossly undersized. Rule of thumb if you want to be comfortable is 4-500 SF per ton. You do the math.
All the old rules of thumb are dead. Yes in the past many installers went by that, but today it's impossible to have any simple formula to determine what the correct size system should be.
You build a 2800SF house with 2x4 walls, 3.5" of fiberglass in the wall, large windows with a low R value, and vinyl siding. I build a 2800SF house with 2x6 walls, 5.5" of spray foam in the walls, small windows with a high R value, and a brick exterior. Do you think our two houses are going to need anywhere near the same size system installed?
The only way to determine what size system you need is to take all the variables into account and then determine the correct size (manual J calculation). 3.5 tons could be grossly oversized for 2800sf under the right circumstances.