#2 is, of course, more expensive. Suppose
- I know how much gas I have used every month for the last year or two.
- I assume that I will use the same exact amount every year.
- I assume today's gas price

Are there formulas that would let me compute when I will pay off the difference between the two furnaces?

Are they putting in the same size furnace as is there now. If so, you could use as much or more gas with a new furnace. Newer furnaces need to move more air then older ones did.

Have a load calc done to see what size furnace you need.
A 90,000 BTU input 80% efficient furnace has an output of 72,000 BTUs.

A 80,000 BTU input 95% efficient furnace has an output of 76,000 BTUs.

Here is a site that you can use to compare operating cost.

The load calc can mean more on increasing the fuel savings than the efficiency. Need to proper size the unit first than look at efficiency.
See link below for FAQ http://www.comfort-calc.net/faq.html

We were offered two different replacement furnaces.
[B]#1 (regular)
[B]#2 (efficient)
#2 is, of course, more expensive. Suppose
- I know how much gas I have used every month for the last year or two.
- I assume that I will use the same exact amount every year.
- I assume today's gas price

If you assume no noticeable difference in utility bills with either the old furnace or #1 or #2, and I think that is a reasonable assumption, the difference in price between #2 and #1 will never be paid off unless #2 is much more reliable than #1 and the replacement parts cost is much less.

The control board price is a good place to price-check; they are costly and fragile.