what can cause heat not to work? on 2005 Kia Sedona

My van has a separate heater control for the rear of the van, which works fine. The front however, blows out cold air at all speeds. I checked my fuses and they all appear to be fine, what else may cause this problem?

Asked by for the 2005 Kia Sedona
I would check the blend door to see if it is working