Energy is expensive and will probably continue to increase in cost.  As a result many builders and homeowners have sealed their home from uncontrolled outside air infiltration. 

Building codes require fresh outside air to be piped in from an outside air intake to the AC system.  The outside air is usually delivered to the return air duct.  This air is filtered, heated or cooled.  An offsetting amount of conditioned air is forced out of the building.  Opinions and building codes vary, but 15 to 20 CFM per person of fresh outside air is the range normally recommended. 

With a well sealed building we can control how much fresh air enters the building and we can recover energy from the air leaving the structure.  An ERV (Energy Recovery Ventilator) can be employed for energy recovery.