I worked on the AC on 2 mobile homes recently. Both had electric furnaces with A coil AC evaporators added. Coming into the top of the unit in the area where the evaporator is located is a duct about the size of a dryer vent. This duct is connected to a vent on the roof of the mobile home. This duct allows outside air to be blended with the recirculated inside air which is then drawn through the A coil and conditioned to be dispersed throughout the home. Obviously this outside air is hot and that heat increases the load to the AC. What is the main function of this small duct on an electric furnace?