The cost of the energy source is also a key factor in determining payback. Energy prices vary widely from region to region, and from season to season. Other factors, such as the rate of production and inventories of fuels nationwide, can also affect local energy prices. Also, the weather from year to year varies, so your energy costs from year to year will vary as well. Typical AFUE values are 0.6 to 0.88 for oil or propane furnaces, and 0.7 to 0.95 for natural gas furnaces.
Older systems are usually less efficient.
This information can usually be obtained from your local weather station, utility, or oil dealer. You have a gas furnace with an AFUE of 0.88. Let's also suppose that you supply the following values for the variables in the formula. By plugging the numbers into the formula, you obtain the years to payback. 12.936 6.98 years This brief was updated in December 2000.