Take a route that will save you winter and summer on utility bills for
maybe less than a new unit. Upgrade insulation and windows to modern
high efficiency. Are ducts insulated, Do you run the unit 24x7 or are
you just bitchin all the time.
If you had a 2.5 ton furnace it isnt the installers fault.
You dont even know if it is running anywhere near 100% correctly , yet
you blame everyone else . Get it checked out, and run it 24 hrs a day