The metric used to measure furnace efficiency is called the AFUE rating . AFUE is an acronym for Annual Fuel Utilization Efficiency and it measures the amount of fuel converted to heat in the space in proportion to the amount of fuel which enters the furnace.
The higher the AFUE the more efficient the furnace. Homes today are required to have an AFUE rating of at least 78% but furnaces of this low AFUE are typically found in manufactured homes. For a furnace to meet the DOE's Energy Star program, it must be a high efficiency furnace with an AFUE of 90% or higher.