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Should be a button or dial that says "re circulate".
This will close the air intake vents and prevent the outside air from coming in to the cabin.
If the selector is set to "fresh" or non circulate, the force of the air when driving, will push it through the vents.
If the air seems unusually warm when it's set to cold, the heater tap may be faulty.
Its got a little pic of the car with a 90deg arrow inside (on Climate) or arrows in a circle shape on the manual dials.
May be the dial is faulty or you have a vacuum leak some where.
Because on re circulate, you should only get air through the face vents when the fan is switched on.