annual fuel utilization efficiency
(20) The term “annual fuel utilization efficiency” means the efficiency descriptor for furnaces and boilers, determined using test procedures prescribed under section 6293 of this title and based on the assumption that all— (A) weatherized warm air furnaces or boilers are located out-of-doors; (B) warm air furnaces which are not weatherized are located indoors and all combustion and ventilation air is admitted through grills or ducts from the outdoors and does not communicate with air in the conditioned space; and (C) boilers which are not weatherized are located within the heated space.