Forgot about the flap!
As far as I am aware, the flap is controlled by the intake sensor. The flap is adjusted automatically (if it's working properly), depending on the air temperature and controls the hot/cool air mix. For example, it will be fully open when the engine is cold to allow maximum hot air (from the hot air feed) through for starting/warming up and will gradually adjust to a cooler mix as the engine warms up.