Sorry to rehash this point, but I've spoken to over a dozen HVAC installers (some on the phone, some actual bids). They've all been in business a long time -- some since the 1960's. Only one of them considers a Manual-J important -- and that company costs $4K more than the others. The others all say they go by square footage; that with a house built in 1948, the Manual-J doesn't apply anyway. Since the one company that did the calc came up with 3.5 ton -- what's wrong with asking for that size system from another company and getting the job done for less $?