Windsheild defrost dehumidifying the air in winter?

There seems to be a consensus that you should dehumidify air that you are going to use for windshield defrost. In the summer, this is accomplished by turning on the air conditioner which takes a lot of moisture out of the air as it cools it. No problem there.

However, people also seem to think that this is necessary in winter too. Not only people I’ve met, but I’ve also heard Click and Clack say that you should turn on the AC in winter when you’re heating the air and defrosting your windows. First of all, cold air in the winter has barely any moisture in it anyway, and the idea that you could remove moisture from the air by heating it up is absurd; warmer air can hold more moisture. But I just figured they knew more about cars than weather and ignored them.

Now, on my new car, when you turn on the defrost, it automatically turns on the AC. In the owner’s manual it states that it does this, even in winter, to remove moisture from the air. It would seem to me that Infiniti engineers know what they are doing and have run tests on the best method of defrosting windshields. So am I missing something here? Does turning on the AC in winter really dehumidify the air and help defrosting? Or is there another reason altogether for turning it on?

:smack: Could a mod please fix the title?

Note to self: Firefox 2.0’s spell check does not work in the thread title field.

The air conditioner isn’t just an air cooler, it’s a dehumidifier because the water in the air condenses in the AC. Just because the AC’s on doesn’t mean you get cold air – try turning on your AC and setting the temperature to high. You get hot AC.

Per why it should matter in winter, well, you breathe and exhale a lot of moisture that will immediately freeze to your windshield. The dry air from the AC system sublimates this rapidly.

If you blow just hot air on it, then the inside of the windshield just gets kind of wet. Back when I had cars without AC, I used to have to carry a rag or paper towels to dry off the inside of the windshield after the defrost got working really well. That sucked and my windows were always streaked.

But you have not explained how the AC takes moisture out of the air in the winter. In the summer the AC takes moisture out by cooling the air below its dew point which cases moisture to condense out of the air. How could the AC, by heating the air, possibly take moisture out? Warm air holds more moisture than cold air. That is why the AC dehumidifies in the summer, and why I say it does not dehumidify in the winter. Besides, in the winter, the air is already extremely dry (because cold air does not hold much moisture.) The air it is pulling in from outside will be dryer than the air you are exhaling.

Unless you can explain to me a how the AC removes moisture from the air while heating it?

The AC itself does not actually heat the air. It takes the moisture out of the air by cooling it below its dew point. The air then passes over the heating core and is warmed, just like when you run the heater without the AC. Hence, you get warm, dry air.

Even though the A/C is commanded on by the climate control system, if it is very cold the A/C compressor may not be running as there will not be enough pressure inside the A/C system to trigger the safety switch.
R-134 (and R-12 before that) have a well known temperature / pressure curve. At 70 F the pressure inside the A/C system is 70PSI at 40 F it is about 40 PSI etc. There is a low pressure cut out to prevent the evaporator from freezing. If the ambient is below that temp, the A/C will not kick on even though the light on the dash says it is on.
So why do they do command the A/C on?
What if it is raining when the temp is say 60 degrees? The incoming air is very humid under those conditions. The A/C system pulls the water from the air, and the heater core warms it before blowing it into the cabin.
The engineers did not know when they designed the system just what your weather was going to be today, so they assumed the worst (warm humid rain). It doesn’t cost anything to command the compressor on when the air is very cold and dry. And it really does help when the air is warmer and humid.

As was said, your breath has moisture in it, and your boots and clothing often have snow on them. I end up with puddles in my car in the winter.

The “AC” is the device that cools the air. Even in winter. As the air is cooled, water condenses in the AC and drips out of the drain.

In winter, when you turn on the defroster, the air goes through the AC and becomes cold, dry air. It then goes through the heater and becomes warm dry air, which is then blown into the passenger compartment.

As a side benefit, it’s a good idea to “exercise” the AC compressor at least every couple of months so the internal lubricating oil can circulate and keep the seals from drying out, thus prolonging the life of a fairly expensive item.

Linking the AC to the defroster will all but guarantee this will happen automatically for pretty much everyone.

Really? You sure about this? So it takes air that is already below freezing, cools it down, and water condenses out and “drips down the drain?” I don’t think you’ve thought that one through.

When the dew point is below freezing it is called the frost point, and water condenses into frost. Someone else has already pointed out that the AC will not turn on if it’s too cold to keep the evaporator from freezing.

Air in the winter is already dry. Cold air can hold very little moisture.

Yes, but as the warm air circulates through the car it will pick up moisture ( from you, the melted snow, etc) and then the a/c condenser will dehumidify that air.

In the circumstance where you recirculate the air in the car, you can be dealing with moist warm air.

I, myself like to be able to choose whether or not to have the AC come on with the defrost. I can do this in my new Pathfinder. Not so in all cars.

Ahhh, now this makes sense. I’m pretty sure that when I turn the defrost on, it shuts off the recirculate button automatically.

Also, while the air may be ‘dry’, very often (at least for me) it will be carrying snow. Even if it’s not snowing, roads may be wet, or snow covered and other cars will kick up the snow.

“Winter” does not necessarily mean below freezing temperature. The defrost is often needed on cold rainy days.

Rick’s explanation is on target. First, the air passes through the A/C evaporator. With the A/C switched on, it is dehumidified to the extent possible under the conditions. Some conditions will require/enable dehumidification, some won’t, but it’s a lot easier to just have the A/C on whenever defrosting than to measure the ambient temperature and humidity and calculate whether or not to turn it on at that time.

After going through the evaporator (which might cool the air somewhat, again depending on the conditions), the air goes through the heater core. There’s more than enough heat available there, and the air will come out hot. So the air is first dried (as possible), then heated.

On cars that automatically select when to recirculate the air, recirculation is done only on the “MAX A/C” setting. On cars that let you choose recirculation*, you should never use it for heating or defrosting. The moisture you’re exhaling will build up in the car and make fogging worse. The fresh air position not only allows outside air in, it also helps purge the inside air out, getting the moisture out of the car.

*Use recirc are for the coldest A/C. It allows further cooling of the air. If the car has been parked in the sun and it’s hotter inside than out, start with fresh air until there’s some cooling, then switch to recirc. The only other time to choose it, and then only briefly, is when you’re driving through an especially dusty (dirt road, car ahead) or smelly (skunk) area where you don’t want that stuff getting inside.

Gary T Yeah, I’ll switch to re-circ when I end up stuck behind a particularly smokey deisel.

Also, when it’s just colder than hell. Like 15-20 below zero F.

I have an automated climate control in my car. It’s pretty smart. For instance, I start the car in the morning, set the temp, and the blower fan won’t switch on util the engine warms up a bit, and for the first few minutes, it uses re-circ then switches over to outside air. Pretty swift, though I also have the option to turn its brain off :D.