BTU Defined
The letters BTU are an abbreviation for British Thermal Unit. This is a unit of heat energy used to rate furnaces, boilers, fireplaces and other heating appliances. A lit match puts off roughly a single BTU of heat while most furnaces are rated at several thousand BTUs per hour.
Input vs. Output
A furnace's BTU input rating is a measurement of how much heat energy is generated by its fuel. This number can be differ widely from the furnace's output BTU, namely the amount of heat it supplies your house. The difference is made up of heat that dissipates, escaping up the chimney or elsewhere without being used to heat the home.
Annual Fuel Utilization Efficiency
Most furnaces are labeled only with the input BTU rating, leaving the output unsaid. Calculate the BTU output by dividing the furnace's input by its annual fuel utilization efficiency percentage (AFUE). This percentage is the amount of the furnace's heat energy that is used for heating. Newer furnaces are required by law to display this number, making it easier to determine the efficiency of the furnace.
The Ideal
The best furnace will have a BTU input and BTU output that are nearly identical, producing an AFUE close to 100 percent. A perfectly efficient furnace is physically impossible, but new furnaces are increasing in efficiency all the time. If you are shopping for a furnace, pay greater attention to its output BTU than the input, as this is the amount used to heat the home.