Ventilation door is nothing but thermostat of Tandoor. Leaving it open will allow air flow and burn the charcoal quickly. Once the desired temprature is reached, close the ventilation door which will maintain the temprature inside.
TIP: put your hand in the Tandoor (half way through) and if you can count to 5, your tandoor isn't hot enough. Put some more charcoal
BE EXTRA CAREFUL AND DO NOT BURN YOUR HAND