What model Benz do you own?

The symptoms that you listed suggest that the air system has switched into "default mode". Mercedes, always safety minded, sets the A/C to blow hot defrost air if a problem arises in the sir conditioning system (so you can at least see out of your scalding car).

You may need a new electronic a/c circuit board. Not a huge repair cost. It could be worse, your evaporator could be leaking - that's a $2,000 repair or worse.

Please let us know more about your car and symptoms (where is the air coming from?) and we'll help as best we can.

Troy
1995 E420 82k