How to calculate efficiency of old Westinghouse furnace VS new variable speed Furnace
What is “old” a 20 year old system may have the same specs or close.
The serial # can help find the date but if it has an open combustion chamber your efficiency will improve.
As far as pay back that really never happens I say this because by the time even a 10% increase in efficiency you save $10, $20 per month by the time it pays back its time to replace it again ,,, how could that be? you don’t need heat half of the year watch the sales scams that throw in 12 months of savings.
I tell my friends the way to look at it if you have a 30-40 or 50 year old system is it may be getting close to the end of its life and proactive replacement makes sense. And is usually less expensive (most all furnace failures happen when it is cold that makes sense) having the system installed off peak heat or cold is when the contractors are hungry for business and if you shop around you get a better price. But prices go up in the peak heat and cold times because they can get it.
You would like to add AC /heat pump. To your system so if it is a truly old system that you would gain efficiency that makes sense but trying to justify based on payback in reality is a shell game , what you gain in comfort is the main thing because adding AC your annual cost of heating/cooling usually goes up but the home is more comfortable throughout.
If you do get a ac/heat pump that makes sense to me. a high end thermostat that can use the outside temp and select the cheapest heating method may increase your savings on heating when it’s not as cold outside. Again it takes a long time to pay for bells and whistles like that but it is also greener in the long run.