Natural gas or LP gas furnaces tend to be the most popular option for homeowners due to their relatively inexpensive costs and limited production of waste byproducts. Furnaces work by heating the air in your home, and distributing warmer air throughout your home via your air duct system. A furnace’s operational efficiency is assessed by annual fuel utilization efficiency (AFUE), which measures how efficient the furnace is in converting the energy in its accompanying fuel source to heat over the course of a year. The Federal Trade Commission requires all furnaces to display their AFUE, allowing consumers to compare the heating efficiencies of a wide variety of models. An AFUE of 80% indicates that 80% of a furnace’s energy is converted into heat for your home, while 20% escapes out of your home through the chimney or somewhere else.