Air/Heat will blow out of the top vents(defrost) and middle vents, but air will not blow out of the floor vents. The cables are connected from the control knob to the side of something located on the wall between the engine and inside of the car and they move when the knob is turned. (sorry I am new to this site and didn't put what kind of car it was and couldn't figure out how to add that info.)
2005 Toyota Corolla - Heat will not blow out of the floor vents on 2005 Toyota Corolla
by leealyon in Madison, MS on January 12, 2013
ANSWER by Tomiko , January 12, 2013
maybe the mode door is stuck from the floor vents
ANSWER by johns10 , January 12, 2013
You probably have a vacuum leak somewhere. When that happens the heater is built to default to your defrost vents. Look under the hood by the firewall for small rubber tubes coming out of the firewall. Check all their connections.