Hot air does not come out on 1993 Nissan Altima

Why when i turn on the hot air to defrost the windshield the air does not come out from that vent but it keeps coming out from other vents , the heat works but does not defrost the windshield please send answer to haideecalderon@hotmail..com

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2 answers
This is caused by a vacuum leak in a detached or damaged vacuum hose under the dash or hood.
When you select to blow air on the windshield by selecting the specific windshield vent, it will blow cold air automatically, unless the A/C is turned off!
This is because, during cold/rainy weather conditions, the cold (dry A/C cooled) air dries the windshield more effectively than by blowing hot air.