The total fuel savings data shows it pays to upgrade your furnace. However, the ENERGY STAR qualified furnace provides a far superior savings in your energy cost than the mid-efficient furnace.
After this "pay back" period, the operating cost of a less efficient furnace will cost you more on your fuel bill and during the life of the furnace, produces much more greenhouse emission which is damaging to the environment.
When making your final choice, keep in mind the future energy cost is a significant operating cost of the furnace. Buy the most efficient unit is the rule of thumb, for your bottom line and for the environment.
Using the following automated calculator, you can compare the true savings of various furnaces by inserting applicable values: