I live in N. Calif, and am about to have a new furnace installed and I'm really confused. I have had sales people from Trane, Carrier and Lennox show me units. Is there a comparison chart or something like that anywhere? I'm looking at 80% efficency Central Heating units, are the 90%+ that much better or is the 80 good? Is a seperate air cleaning unit necessary? Do I need to have the duct system cleaned? Help!