Smoke when demister on full

This prob I have every winter when I have demist on full or heater after about 5 mins smoke begins to bellow out from under bonnet and under car on passenger side and surrounds the car and comes in through vents and would fill the car if left all I do is turn the engine off and back on and it stops ive been a couple garages they tell me it a common prob with these and galaxys something to do with a oil heater but they dont have a solution I have also never come across anyone else with this prob in 4 years I had the car ive tried looking where it coming from buy there is that much smoke when it starts its impossible I also dont seem to be able to locate this oil heater im told is the cause any help gratefully demister thanks