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If the window has FROST on it you will need warm air to melt the ice. If the inside of the window has vapor on it you need DRY air. When you turn the defroster on (in most vehicles) the air conditioning compressor starts to run as well, drying out the air. The temperature should also be as warm as is comfortable for the driver. When the dried air is blown across the window, the water vapor evaporates and you can see again.



So in reality, the "pump" or compressor has no actual drying affect on the air, the compressor itself is a closed system pump, that forces gas into a compressed tube, and then upon release of the gas the tubing freezes. The a/c fan then blows outside air over the frozen tubing, which looks like a tiny little radiator, thus cooling the outside air.

During the winter, people typically do no use their a/c system (unless they live in the southwest US or someplace HOT), which basically would mean that for many months the A/C pump, tubing, condenser and other components would suffer from what's basically called "Lot Rot". Rust would build up from trapped moisture, the lubrication that keeps the pump nice and smooth would pool up and dry off the moving parts. And when the summer time hit and someone turned their A/C on it would sputter and spit and probably seize the pump. Therefore, an ingenious engineer figured out to wire the A/C system to the Defrost button, since they use very different air intakes. So you can send warm mixed air to the defrost vent, and at the same time keep the A/C system running and lubricated through the long winter months. So your A/C runs when you push defrost not to dehumidify your air, but to keep you A/C system from taking a crap in the winter.

The reason you would see an affect on fogged windows from using the A/C switch, is because it's bringing in outside cooler air, which is probably far less humid than the air you've been breathing that fogged up the window in the first place. Dry external air, whisks away the water vapor on the windows. The cooler air also has an impact on the air that was fogging the window, in that it cools the air bringing it farther away from it's condensation point and causing it to reabsorb the moisture it shed on the glass.

The defrost system, is of course by name designed to get rid of "frost", but it also uses outside air mixed with heated air to give you the same affect of removing fog. However, not only does it bring in external dry air, it also brings the temperature of the glass up, which changes the condensation capability of the air on the glass.

Take your mind back to last summer, when you had a nice cold beer on a hot day, the can sweats, water drips off the cold can because of condensation of moisture in the air on the cool can surface. Think of your cold windshield, it's probably just as cold as that beer, and the air you are exhaling and sitting in is warm and juicy, so the glass fogs up. You can either heat the glass, cool the air in the car, introduce dry air to the car, or pull over and get a beer. Basically, I would suggest that the defrost feature offers two of the 3 viable options, it heats the glass, and it brings in external warmed dry air directly onto the glass to whisk the moisture up and prevent further condensation. The A/C option only brings in cooled dry air. Which works faster? maybe A/C does or maybe not, I think it depends on how warm your engine is, since A/C can be cold in seconds, but heat takes many minutes to manifest. I think the better question is which works longer and is more comfortable, I think that's definitely the defrost.



The above "Correction" piece is by far the best answer I've seen on the web that bests answers the question: how to defrost/defog your car window. For what it's worth, the Correction piece is spot on about the A/C being used in the winter months as a maintenance issue. However, there is a correction (or at least some additional piece of information) to make the above answer a bit more precise.


A/C operation:

The A/C operation described above is a bit unclear. Refrigerant is compressed to around 100psi as a gas (today its r-134a) by the compressor. Then, this gas is sent to the condenser which cools the high pressure, high temperature gas into a lower temperature gas. This gas turns into a liquid by the time it leaves the condenser. This liquid (highly pressurized) is sent through tubing until just before the evaporator it is sent through a fixed orifice that quickly depressurizes the liquid and quickly turns back into a gas. That is, it evaporates. When this refrigerant evaporates, it absorbs heat; thus, we get a cold sensation. Since the boiling point of r-134a is around 35 deg f, the air around the evaporator will be about 35 deg f. As the gas evaporates in the evaporator, a blower motor blows air across the evaporator and sends this cold air (about 35 deg f) into the passenger compartment. Thus, cold air.


A/C as a dehumidifier:

The A/C does dry the air, but only in warm temperatures (i.e. you will see the moisture dripping from underneath your car). This moisture comes from the evaporator drain (which is inside the passenger compartment). So inside air is actually being 'dried' in the warmer months. So, the above statement about the A/C not drying the air , however, is true only if it is cold out.


A/C bringing in outside air:

The A/C system doesn't necessarily introduce outside air. The blower motor is inside the vehicle and blows inside air or outside air (depending on the recirculation door position) across the evaporator, then the heater core. Thus, if the A/C is on and the recirculation door is closed (closing off outside air) then no external air is introduced, and defogging doesn't occur. BUT, if your car lets you turn on defrost (which always brings in outside air) and ac, then of course, external air is introduced. So, although technical points, I add these facts. Thanks to whoever posted the above "Correction" piece as I learned a lot from it. Thanks, and I hope my little piece shed some light.

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โˆ™ 2010-02-15 21:35:00
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Q: Does warm or cool air defrost car windows faster and why?
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