I presently have a 120,000 Rheem gas hot air furnace which must be replaced. I was told on one of three estimates that the unit is much too big for my home and that 80,000 BTU's is the right size. What size unit would be correct for my 1200 square foot ranch? If you include basement it would be 2000 square feet. The other two estimates I got claimed 120,000 is correct. Who should I believe?