A BTU or British Thermal Unit is the amount of heat required to raise 1 pound of water 1 degree Fahrenheit. All gas appliances are rated in BTU's/hr Input which is an approximation of the amount of gas consumed on full fire in one hour as expressed in terms of the heat potential of the gas. Depending on the design and efficiency of the appliance, a certain proportion of the heat generated from burning ends up as useful heat in the room (Heat Output) while some portion is lost to vents and chimneys (Flue Loss).