We put in a new 2.5 ton Trane HVAC unit, and when it hits 90 outside, it won’t hold 74 degrees inside. (1400-1500 sq. Ft). Will it help to put in a 3 ton unit, or will it do more harm than good?
With a 1500 sq ft home a 2.5 ton should get it nice and cool. Things that may affect your ability to cool, ducting not insulated, home not insulated, extreme heat (I don’t consider 90 extreme)
You did not state what the unit did cool to. Without this we cannot calculate the delta to tell if a larger unit would work for your home. The tonnage of a unit is how much it will cool the Seer is the efficiency higher seer less monthly electricity demands.
I have installed larger evaporators in the past when I knew the owner was going too small. The evaporators can handle a range of cooling where the compressor and condenser are usually 1 unit (the outside unit).
I would check with your installer is this unit have a dual speed or variable speed fan that is not being turned on? I have seen ac units air handlers on low speed and they did not cool as well. For a friend I installed a new controller and my friend thought I put a new system in it worked so much better (~150$ took care of him)
So there are possibly things that could improve the performance of your existing system insulation and possibly different fan speeds both of these would be much cheaper than a new compressor / condenser and possibly a evaporator if yours is not large enough to handle a bigger compressor.