Abstract: Apparatus is disclosed for automatically defrosting a heat exchange surface of the type that incorporates an evaporator with cooling fins, a refrigerator motor, and a fan to circulate air around the cooling surface of the evaporator to the area to be refrigerated and back again. The defrosting cycle is initiated by pneumatically sensing the air flow restriction caused by frost build up on the evaporators cooling surfaces. The defrosting cycle starts at the optimum time by means of a weight adjustment on a louver device and/or associated part. Then by a novel means, the evaporator area is automatically sealed off from the refrigerated compartment. Simultaneously, warm ambient air, or the like, outside of the refrigerated compartment is introduced into the sealed off evaporator area. This is a definitive, highly efficient circulation path, formed for rapid defrosting purposes, which utilizes the warm ambient outside air, or the like.