Older furnaces are unsurprisingly less energy efficient. The average lifespan of a furnace is about 20 years.  You can calculate how much money you’re unnecessarily spending every month and determine how much you could be saving with a replacement new furnace. Manufactures use an AFUE number to describe the percentage of fuel consumed and how much is wasted.   If you don’t know your furnace’s AFUE rating, you can call the manufacturer to find out. Lower energy bills that come with a newer replacement furnace can help you recoup the expense of replacement costs.