BTU is the abbreviation for British Thermal Unit. It measures the heating output of a furnace or any other typical heat source. The higher the BTU output, the more heat. Most homes will have a furnace with an output of 60,000 – 100,000 BTUs. When shopping for a furnace, several factors will determine the ideal size. House size and geographic location are the two primary factors. Others include ceiling height, window quantity and sizes, and how insulated the house is.