Heater/AC blows thru defroster vents, not floor

I have a 96' Cherokee Sport. Whenever I switch the heater/AC to the "HEAT" position, it blows through the windshield defrost and not the floor or dash vents. Anyone know how to fix the problem?

JC

sounds like a vacuum leak or a bad diaphram.

Again, this seems too often an occurance. Look under your bumper, PS. there is a small ball mounted to your bumper, that is the vacuum resevor for your heater controls. Make sure the lines are connected to it and there are no leaks.