I have forced heating only. I want to get an a/c - heater unit installed in my attic. 1950 sq ft - 2 story built in 1965. I lived in southern calif with hardly any humidity. I want to do my homework before I start getting estimates. Original heater is built by Carrier in 1975. It says input is 100,000 btu /hr and bonnet capacity is 80,000 btu /hr. Maybe I can get some pointers on what to do 1st to get started and keep posting here as I walk through the process. I have metal ducts but I obviously dont know if they will be the right size for the next unit. Any input would be appreciated. I have dual pane windows,lots of insulation in attic,carpet,composition roof,3 turbine vents on roof,all ridges have 1" gaps at top and that gap is covered with honeycomb type plastic piggyback roofing vent caps that allow heat to rise out of attic,stucco,raised foundation,3' eaves all the way around both stories. House faces the west. Any thoughts?