This is the thirty-second article inspired by a DOE report covering energy-saving HVAC&R technologies. Central air-conditioning (AC) units account for most residential cooling energy consumption. Two recent studies of split central AC units in California found that more than half suffered from faults that can result in ineffi cient operation, primarily improper refrigerant charge and insuffi cient evaporator airfl ow due to evaporator fouling or clogged air filters.