If every gas furnace sold in the U.S. met the new ENERGY STAR requirements, energy cost savings would be about $171 million per year and yearly greenhouse gas emissions would be reduced by about 177,000 cars.  Your furnace that is 15 to 25 years old can use as much as 20% more energy than an energy star rated furnace.  To see if it’s worth spending the money on a new furnace.  j Just add up the fuel bills for last winter and multiply that sum by 20 percent. Divide that figure into the cost of buying and installing the new system.  Then you can see how many years it will take to recoup your investment.