A furnace is rated for efficiency using a AFUE (Annualized Fuel Utilization Efficiency) measurement. Essentially the AFUE number is a percentage which relates to the amount of energy that is used to heat your home. In other words - how much heat do you get to enjoy versus heat that escapes out of a chimney or vent pipe.
A 15-20 year old furnace can have a AFUE score as low as 50%, meaning you are only utilizing 50% of the heat created. Modern furnaces have dramatically better AFUE scores ranging between 80-95%. With the recent price increases in heating fuels (Natural Gas, Oil, etc..) this can mean dramatic savings over the course of just a few years. In addition to savings on your heat bill there is a positive effect on the environment Reductions in the amount of carbon dioxide that get released into the air by a new furnace are significant particularly when you consider the life of the furnace will likely be 15 years or more.
Interested in a high efficiency furnace? Request 2-3 competing estimates from local contractors. The HVAC technicians are pre-screened (licensing, insurance, etc..) allowing you to focus on service and price.