Originally Posted by firebane

Then you circulate cold air :P If you notice in the cooler climates people have a tendency to use this mode and you end up with foggy windows. The best way is to pull the air from outside and set your temp just to the warm souch and then point it to the floors and vents and your car should almost never fog up.

Originally Posted by Civic2Scooby

I have always been told you shouldnt run heat on recirc only ac...:dunno: mite be a wives tale.

Originally Posted by MrFatbooty

Your car has a heater core. Engine coolant circulates through it, and the ventilation system puts air through it to pick up heat from the engine coolant. It works like an invese liquid-to-air intercooler. Instead of the air going through it putting heat into the coolant, the air takes heat out of the coolant and then is pumped into the cabin. Turning on the heat always passes air through the heater core. If you take in fresh air, you suck outside air through the heater core then into the car, and the recirculate sucks air from inside the car, passes it through the heater core, then puts it back into the car. Skipping the heater core wouldn't accomplish much of anything.