Abstract: An internal space air conditioning system includes supply and return ducts communicating with the internal space, a blower circulating air through the ducts and space and a refrigeration unit having evaporator cooling coils located in the air path proximate the blower suction end. The return duct communicates with the ambient atmosphere by way of a louver type damper and thermostats are disposed in the internal space and the atmosphere, and are each actuated and deactuated respectively when the corresponding temperature exceeds and is below a preset value, the external set value being at least 10 degrees F. less than the internal set value.