Heat from AC vents while AC is on
 

I have a 1994 s320 and the air conditioning was working fine until the other day. Heat started comming out of the driver and passenger side vents. No air is comming out of the center vent unless I push the blue knob attached to it then cold air comes out of this vent but the hot air still comes out of the side vents no matter what the temp setting is. Any ideas to fix this issue?