When trying to cool a typical house, the average central AC will only decrease the temperature by about 20 degrees. For example, when it is 100 degrees outside, the inside will typically cool to about 80 at best. However, in order to achieve the 80 degrees inside, the air coming out of the vent should be 45-55 degrees. The temperature coming directly out of the vent is different than the ambient temperature of the house. The air coming directly from the vent should be 45-55, however this will not be the temperature in your home. If using a thermometer stuck in the vent, and the blower fan on a medium speed, in the full cold position with AC on vent, temperatures should be between 40-50 degrees F.