The center vents will usually only blow cooled air. Where do you have the temperature wheel set to? In auto mode the blower speed will increase if the temperature inside the car changes or is already considerably higher or lower than what you have it set for. This is to get the cabin to the optimum temperature as quickly as possible and to maintain the optimum temperature. Also if I'm not mistaken when in auto mode the car may not start blowing heated air until the coolant has warmed up sufficiently or engage the compressor until the oil pressure has built up.

__________________
1989 300CE