When I have the heater turned on in the front, it doesn't blow hot air. It still blows air and the fan speed control works and such but it isn't hot. The air feels the same as when it is set to cold (with A/C off). It doesn't matter if I have it set to recirculate or not and it blows the same out of the floor vents. The air sounds different (if that makes sense) when I switch between hot and cold.

However, the heat coming out of the rear vents which are controlled by separate controls blows hot. The antifreeze levels are fine. I'm not a "car guy" by any means but I have fixed some stuff in the past. I'm just not sure what to check to start things off. What should I be checking?