The amount of heat a boiler can produce is measured in British Thermal Units (BTUs). All boilers have a BTU input rating (the rate at which the burner consumes fuel) and a BTU output rating. Output BTUs heat the living space, while some input BTUs may be wasted (through venting, etc.). The closer the input and output ratings are to one another, the more efficient the boiler is.
Boiler manufacturers provide two separate output ratings: the DOE and the net I-B-R. DOE ratings assume that the boiler and its piping occupy a heated area. The heat lost through them, therefore, heats the space and helps offset the heating load. The I-B-R net output rating places the boiler in a unheated space, such as an unfinished basement. It assumes heat lost from the boiler and boiler piping is wasted. I-B-R ratings will always be lower than DOE ratings. Depending on the location of a boiler, either method may be more relevant when sizing and installing a boiler.
