I am replacing my HVAC (forced air) system and I am getting different sizing requirements from contractors. Does anyone know where I can find out what tonnage air conditioner and BTU furnace that I need? I have a three bedroom ranch (1000 suqare feet upstairs with 7.5 foot ceilings, no insulation in the walls but insulation in the attic). I also have a finshed basement that is used frequently. It adds on another 700 square feet of space. Also in the basement there are three registers and one return. Do I need an additional return?