If you have the center button depressed, even with the knob to full heat, the a/c is always running, even though you are not aware of it.
MB designed it this way to remove humidity from the air.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".