The heater is set on recirculate the air of the cabin. Set it on normal (from outside the car)
You have to move a button on the console to take air from outside the car or else the air will be too moist from all the humidity that accumulates in the cabin.
Also the engine temperature must be hot enough. I usually run my car at least 5 minutes driving it downtown before it is hot, during cold winter days.
The most likely reason would be because you also have the defroster turned on with your heater. The defroster causes the A/C compressor to kick on which helps remove extra moisture in the air keeping your windows from fogging. The A/C compressor is linked to a "kick up" that brings the RPMs up to keep your engine from stalling due to the extra load possibly causing a surge.
You didn't mention what vehicle you have but in most vehicles the heater core is located under the dash on the passenger side. The heater core is known to spring a leak and show up on the passengers floor. Turn on the defroster and see if your nose senses the sweet smell of radiator coolant. The inside of the windshield my fog up when the defroster is turned on, this would also indicate a leaky heater core.
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