What could be wrong with my car's heater?

When I have the temp switch on my car set to cold and the fan set to high I get a lot of air flow out of my dash vents but as soon as I turn the temp switch to hot the air barely comes out of the vents. I don't hear any change in the blower motor. I can switch it back and forth from hot to cold with the same results. Same thing when I have it set to defrost or floor. The air that comes out when it is set to hot is warm but if you move your hand more than 2 inches from the vent you feel nothing. Same thing if I run the fan on other settings - more air flow when set to cold than when set to hot.