The efficiency of a furnace is described in terms of input and output expressed in British Thermal Units, also known as BTUs, and Annual Fuel Utilization Efficiency, also known and AFUE.
If you have a furnace with a 90 percent AFUE rating and an input of 100,000 BTUs per hour, you would have an output of 90,000 BTUs per hour that would heat your home. The remaining 10,000 BTU difference between the heat generated and the amount of usable heat produced by the furnace is lost up the chimney through venting. Many of today’s high-efficiency furnaces are equipped with secondary heat exchangers and other features that limit the amount of heat lost through venting. Furnaces are available that have AFUE ratings as high as 97 percent that extract more heat from each unit of fuel. Switching to a furnace with a high AFUE rating can lower your heating costs by as much as 30 percent.