Most furnace manufacturers offer two types of furnaces: Mid-efficiency Category I fan assist, High-efficiency Category IV condensing-type furnace. The minimum energy efficiency for mid-efficiency furnaces is 80 percent annual fuel utilization efficiency (AFUE). 80 percent furnaces have a flue gas temperature at least 140F above the dew point of the flue gas.

This article wraps up our information on furnace efficiency ratings and categories. This should give homeowners a better understanding of their heating system. You have insight into the limitations that are placed on installation, furnace location, vent piping, condensate drains, and integration with your cooling system.