Why should I replace my existing furnace?
Back in the day furnaces came in 2 sizes, big and bigger, so if you have an older furnace chances are it is oversized and extremely inefficient. An old standard efficiency furnace may be running anywhere from 55 to 65% efficient, which means anywhere from 35 to 45 cents of each dollar worth of gas burned in your furnace is effectively going up the chimney.
The old furnaces would end up sending flue gases up the chimney at temperatures of 400 degrees Fahrenheit plus. The new furnaces take the flue gasses and send them through a secondary heat exchanger and in turn they are venting the burnt gas fumes through PVC pipe out the side wall of the house with a temperature of about 80 degrees Fahrenheit. That extra 320 degrees or so are being used to heat your home instead of heating the great outdoors.
Also, besides replacing your furnace to save some money on energy costs, there are grant programs and rebates that are running at any given time, to help you make your home more energy efficient.