[miszindependant]

How do new furnaces differ from old ones ?

[miszindependant]

How do you calculate the pay back period for a high efficiency furnace using graphs?

[wmproop]

Do the math, with an 80% unit the operating cost is around 20 to 30 % more expensive than a 93 to 95% unit
Guessing, a $100.00 gas bill would drop to $70 or 80 $$,, probably even more
Say you save 120 bucks a year,, it might take you 3 .5 years to save the extra bucks you spent on the high efficiency unit

This my friend is an uneducated guess,,

[hvac1000]

The bad part is the fact that when the new high efficiency furnace has a problem you will spend more than your savings on your gas bill to fix it.