Renault Laguna 1.9dti yr 2000. Heating problem

I have a renault laguna it is a 2000 dti model. The problem is that the heating is very poor and the car is always misting up. It takes forever to clear in the mornings. It is not too bad when set to recirculate the air inside the cabin but whne you put it on the fan ie turn the power to the right no the left then it plays up. If I set it a 4 (full power for the fan) after about 30 seconds all you get is a loud humming and no hot air what so ever. Please help someone.