There is a caseunder the dash that holds the heater core and ac evaporator core. In that case there are what are called blend doors. They are controlled by vacuum from the engine that runs to the doors through a diaphragm. The defrost/floor position is the default position that the blend doors automatically go to if there is a malfunction. Usually when that happens it is caused loss of vacuum. Another sign of a vacuum problem is when there is an apparent loss of air from the dash/side vent position upon acceleration which returns to normal when not accelerating. Annoying but harmless.