I have the climate control set to OFF when this happens. It's very hot air! It can totally overide the airconditioning when it's on. (sorry to those with snow etc.) I close all the dash vents to keep it from burning me up.
On a similar note, When my sunroof is popped open for a little air the dash vents allow fresh air in while the climate control is set to off.
1989 560 SEC