I Replaced a 1989 Rheem furnace 120k btu input with a Luxaire LP9c100c16mp11. I kept the Rheem racc042jas condensing unit. The new furnace has selectable settings for the blower fan to run. 1st choice is 1655 cfm but when the fan runs at this speed it sounds much louder than the original furnace did. The next setting would be 1345 cfm but that may be too slow for a 3.5 ton a/c. Not sure what I should run the blower at? Also anyway to fiqure out what that old Rheem furnace would of been running at? Of course I don't have the model # but it was a plain Jane 120k btu unit.