Older furnaces have an AFUE range of 56% to 70%, which means that between 35% and 50% of the natural gas goes to waste along with 25-35% of the electricity. New high-efficiency furnace systems typically have AFUE ratings of well over 90% and can be as high as 97%. Because of this great difference in fuel efficiency, these new high-efficiency furnaces can pay for themselves in only a few years.
outdated by TSSA, 18% current AFUE
outdated by TSSA, 64% current AFUE
TSSA mandated, 95% current AFUE