The heater isn't blowing hot air.
The heater is connected to the air-conditioner by a blend door that moves inside the dash. Depending on where it is located determines whether the air is hot or cold. This blend door is a mechanical component that can break or jam up. If it were to get stuck in the cold position the vents would never blow truly warm or hot air. Also, if the coolant is low in the cooling system or the vehicle never has time to heat up the vehicle wont blow hot air either. A diagnostic inspection should be performed on the heating/air conditioning system by a qualified technician in order to get to the true source of your concern.
Have a car question? Get free advice from our top-rated mechanics.