AFUE Math
As we stated earlier, when you have an 80% efficient furnace, 20% of your heat goes out of your exhaust (your flue pipe). The higher AFUE rated furnaces have a secondary heat exchanger that allows the furnace to absorb more of the heat generated into the air being circulated into your home or business. Because of that, it’s much more energy efficient to go with a 90% or better efficiency furnace. 90% or better efficient furnaces are also vented with PVC rather than a galvanized metal flue pipe like what you would need on an 80% efficient furnace. As with SEER ratings, the older the equipment, the lower the efficiency due to deferred maintenance. That means that the older your equipment, the efficiency is often lower than the original AFUE rating. Many older furnaces have efficiencies as low as 56-70%. The table below shows an estimated comparison between the AFUE ratings based on $0.954 per CCF (average price of natural gas in West Virginia). It is also based on 494.9 yearly heating hours average for Southern West Virginia. The savings are not guaranteed and depend on various factors (such as the insulation in your home, how many people live in your home, variations in weather, etc.)
