As stated, check the blend doors. Here is what you should check:
1. make sure you can put it on feet only, and the air only escapes from the feet, if it also goes to the defroster at least one blend door is broken
2. Make sure you have heat on BOTH sides (drivers, and passenger), if it is single zone, and you have it on one side, there is a heater core problem, if dual zone, there could be a blend door or heater core issue
3. Make sure when you do recirculate air isn't coming in from the outside (you can stand outside and put a paper towel over the vents between the hood and windshield, if it is open you will 1) hear it, and 2) it will suck the paper towel. Make sure both outside and recirculate work when selected, if not, this is another blend door issue.
Make sure you get heat, and there is no sweet smell when you have the heat on (or starting) or the heater core is leaking. You can also tell if there is a film on the inside of the windshield.