Pontiac Repair: heating system, pontiac firebird, vacuum line

Question
I have a 1986 Pontiac Firebird 2.8.  When I turn my heater on the air blows out of the defroster vents and comes out somewhat cool no matter what.  When I turn on the defroster it comes out right but only cool.  Im not sure what problem im looking for to fix or if i should even try to or bring it to a shop.  If you have any idea as to what it might be and how much please let me know.  thanks

Answer
Hi,. check the water in the radiator, low coolant can cause this. check for a loose vacuum line under the hood. there is a small vacuum line that runs through the wireing harness to the in side, make sure this line is conected. and has no breaks in it.If this is ok, then it would be one of the control valve under the dash. I don't no how much it would be.