I'm not sure what you mean "setting for heater". I think you mean that little button or dial you see in cheap american cars that basically turns the heat on....
In the BMW The vents temperature are more precise than that which is why you get numbers for setting the cabin temperature you desire ....60,70,80 etc...I always assume that hot is 80 and cool is 60 and ice cold is when you press the AC button (and turn on the AC compressor).
The Big Vent above the radio is independent of the other vents in the cabin and has its own temperature dial right under it....it is the horizontal dial with the blue (cool air) and re (hot air) dots under it. In fact, last summer when some of my friends were in the backseat of my car during a hot summers day driving on the Palisades Highway they led me to realize that the Big Vent is aimed directly at the rear seats and any air coming out of there blows on the rear seat passenger. I had it set on warm and they were all hot whiles our vents in the front were on cool (and the AC was on as well)
Miss Bunny...I understand you have an IS and not a sedan so I can't say on how the performance of your air ventilation system should be or if it would be different than mine.