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Defroster - inside or outside air?

This is a comment about the show aired on October 6th 2012. Tom & Ray said that the defrosting foggy windows only worked when sucking in outside air. Their reasoning was that the air you suck in is not moist and therefore able to pick up the moisture in your car. I always thought it was like this: Hot air can pick up more moisture than cold air. So when you defrost, you heat up the air in your car. If you recycle the (warmer) inside air, it will become hot more quickly and defrost more quickly. Once it is defrosted, only then should you let in outside air.

Years ago, we defogged the windshield with only inside air because the heaters only recirculated inside air. When the radiator core in the heater became warm, the air was directed through hoses to the windshield and the heated air would absorb the moisture on the windshield. For cars that didn’t have the defroster provision, one could purchase a fan that mounted on the dashboard or steering column and the airflow was directed to the windshield. I didn’t have a rear window defroster grid on my 1978 Oldsmobile Cutlass Salon. I mounted a couple of fans on the package shelf and connected them through a switch on the dashboard. When the car warmed up, the airflow from the fans would clear the rear window.
Fresh air from the outside is better, but recirculated inside air will work.

Try it both ways on your car.  Choose which ever method works best for you. 

Frankly from my experience, there is little difference.

Heating does’t neccessarily defog though.
Driving through a downpour in 80 degree weather caused the inside windows to start to fog up.
Turning the air to defrost mode with the a/c ON defogged in seconds.
The a/c compressor is the key here, not temp.

You can always tell who’s driving around with recirculated air. Their windows are fogged up. By the same token, the compressor has a harder time removing moisture when the air is reciculated.

Hokie, can you explain how the compressor removes moisture?
Also, could you explain how recirculated air creates fogged windows? Where does the fog actuallly come from?

Tom and Ray live in New England, and for that part of the country, they are right. Now you asked about defrost and I’m not sure whether you are asking about only when there is frost on the outside of the windshield or not.

For frost on the outside, it does not matter. You want to heat up the windshield enough to melt the frost and then the wipers can remove the liquids.

However, defogging is another issue. Even when you are only defrosting, you can start fogging up the interior, so then you need to defog as well. If you and others are inside the car, your breathing is adding a lot of moisture to the air. Even if the inside of the car is warmer, it is warm moist air. The heater core will further warm the air, increasing its moisture holding ability, but then it blows it on a cold windshield where that moisture will condense and fog up.

The outside air has much less moisture in it so when it is heated and blown against the cold windshield, its absolute humidity will not take it below the dew point, so no condensation, no fog. You can add AC if you are using the recirculating position and it will remove the excess moisture so that the windshield won’t fog up.

Now if you live in the south with hot muggy summers, and its raining, the outside air has way too much moisture in it. You are not defrosting, only defogging and in this case, you have to use the AC and it works best on recirculated air. The inside air has been through the AC, is now cooler and drier than the outside air.

If car designers lived in the south, they would have a vent selection for this condition that would use the dash vents and the defrost vents. You have floor, dash, floor/dash, defrost, and floor/defrost, but no dash/defrost.

Keith, you ruined my fun!

I’m from New England, about 50 miles north of Click & Clack. But it doesn’t matter. It’s about air at a specific temperature that’s carrying moisture beyond what the air at the surface of the cooler windshield (or other surface) can accommodate. Think “relative humidity”. At that barrier layer, the air with the moisture will deposit it on the cooler surface. You described it beautifully.

But I was waiting to hear the explanation of how the compressor removes moisture. And how recirculating dry air creates fog.

OK, you asked for it. To start with, it helps to have this table handy so open this in a separate tab. Since this web site is from the Transportation Information Service in Germany, the temperatures are in Celsius. 0°C is 32°F, 20°C is 68°F, 25°C is 77°F and 30°C is 86°F.

There are several terms here that you need to understand. First is Absolute Humidity (AH) which is measured in grams per cubic meter (g/m3). This is not temperature dependent, it is simply the amount of water that you could extract from a cubic meter of air.

What is temperature dependent is that maximum amount of water (g/m3) that one cubic meter of air can hold. For example, at 20°C (68°F), air can hold a maximum of 17.3 g/m3. At that level the air is holding 100% of the moisture it is capable of. The percentage of the actual amount of moisture (AH) to the maximum it is capable of holding is relative humidity (RH). So if the room temperature is 68°F, and the AH is 12 g/m3, then the RH is (12/17.3)*100=69.4%. If the room temperature goes down and the AH remains the same, the RH goes up. At around 14°C or 57°F, the RH becomes 100%, this is known as the dew point (DP).

When moist warm cabin air is sucked into the vent and across the AC evaporator, whose surface temp is around 40°F, the RH of the air passing through drops below the dew point and the excess water condenses against the evaporator’s coils. Now the drier air goes through the heater core and heats up, now with a much lower RH.

Now if the heater is not on and you are traveling in the south in the summer, the cold dry air coming from the AC vents hits the warm moist air in the cabin of the vehicle, and under the right conditions, you can see a fog form at the boundary between the two. This is the same principle as when your warm moist breath hits the cold winter air up there in Boston in January.

All I know is that when your windsheild fogs up in damp weather, running the defrost on outside air takes it off.

I think we’re missing an important point and an important distinction, whether you have working air conditioning.

If you don’t have working air conditioning, you should never recirculate the interior air, but most importantly when you are trying to remove moisture from the inside of your windows. Outside air is always dryer than the air that is fogging up your windows, mainly due to the amount of moisture that people produce when we breathe.

In a car with no working air conditioning, getting outside air into the car is imperative to keep moisture from building. Blowing outside air on the inside of the windshield works just fine as long as you are moving. It’s when you stop at a red light moisture might begin to build inside the car. When I drive my car in the rain, I have the fan on full blast, and I keep my window cracked so the moist air can be pushed out. Getting a little wet is better than wrecking your car because you can’t see.

If you do have working air conditioning, it won’t matter whether you are recirculating the air or using outside air because the system removes moisture from the air in either setting. Your windows will be dry just as fast either way. The only important distinction here is what the temperature is outside. If it’s hot out (like in a sun shower), you might want to recirculate the air to keep the inside of the car a little cooler, but in most rain storms, you’ll get so cold you won’t want to recirculate the air. In fact, if you’re driving in rain for a long time, keeping the air on will make you so cold, you might consider turning on a little heat. If you’re driving in winter temperatures the outside air is usually so dry you don’t need the air conditioner, but if you’re running both the heat and the air conditioner at the same time to defog the windows and stay warm, outside air is probably the better choice, but only because at some point you will probably be turning off the air conditioner, and you don’t want to recirculate the air with the air conditioner off.

Keith, great link. That;s exactly what we bboth said, just said differently. Re: the tables…unnecessary understand the concept.

I was trying originally to get Hokie to discuss why he believes that the compressor directly removes moisture and why he believes that recirculated air fogs the windows. From that we could have entered into a discussion about how the system works. It’s a teaching technique. When someone states something erroneous, ask for an explanation and work from there. I suspect Hokie has left the room, unfortunately.

Whitey, I should point out that on those cool days you can run the AC and the heater at the same time. That’ll still remove moisture but allow the air cominng into the cabin to not freeze you out. The heater core is in the airstream post-evaporator, and will heat the air without adding moisture. The moisture in the air gets removed, and then the now-drier air gets warmed.

Yeah, I realize you can run the heat and the air conditioner at the same time and get warm air, but that’s only after the car has warmed up. However, on a cold morning, you’re probably not going to need the air conditioner once your car is blowing hot air from the heater vents. On a cold morning, when you really need your windows un-fogged is before the engine warms up, and if you run the air conditioner before you get warm air from the heater vents, you will feel the extra cold air.

Yup, agreed. And I feel it a lot more than I did when I was a younger man.

I remember my older son driving home from college at Christmas. His windows were dangerously fogged over. He asked why. He had been running the defroster on recirculate. This was the first car he’d owned, or driven in the winter. I told him to open up the outside air inlet. No more problem.

So I live in Phoenix, Arizona. Here the air is naturally pretty dry, they say “It’s a dry heat” for a reason. Normally there’s no need to defrost, even when it’s 40° out (that’s cold for us). The ONLY time I ever have to use defrost is when it’s raining or pre/post rain. Then the air is more humid outside than it is inside. My Mazda has a hard switch to swap between recirculation & outside air, & I’ve forgotten to swap it a few times. Here, with just me in the car, it definitely defrost near to the same speed. I want to know if there is anything that using recirculation can do that hurts my car. If not, in AZ, I’ll just use the recirculation unless it doesn’t work

Sorry but I’m just not going to read all your comments. I’m quite sure though on my cars, when you hit the defrost button, it only uses recycled air. Like why would you want to introduce 20 below outside air when you are trying to defrost the windshield. Makes no sense.

I grew up without recirculate or ac as an option, I would open a window a little to help get rid of the humid air if needed. These days I prefer to keep recirculate off just for the notion of getting fresh air, no problem getting warm air no matter the outside temp, but in reality it seems the ac component will do fine removing moisture whether in recycle mode or not.

Maybe nobody cares anymore but I do. When I’m wrong, I’m wrong. Sitting in line waiting for the car wash today, I tried my defrosters in the recirculate mode. Nope nothing doing, they would only use the fresh air mode. Makes no sense to me but it must to the folks that design the stuff. So outside air it is. Sorry.

Take it from me. I’ve been driving a car without functioning air conditioning for more than a decade, and the dryer air comes from outside.

The humidity is coming from the driver and passengers as they breathe.