A. Input BTU refers to the amount of energy that a heating appliance consumes or requires to operate, while output BTU refers to the amount of heat energy that the appliance produces or delivers into a space. In other words, the input BTU is the total energy that the appliance consumes to produce heat, which includes losses due to inefficiencies such as combustion, venting, or heat transfer through the appliance's walls. Output BTU, on the other hand, represents the actual usable heat that is delivered to the room or space being heated.