Using your AC when defrosting your windows


#1

Recently you recommended using your AC and your defroster when defrosting the windows. The reason being the AC acts as a dehumidifier removing excess moisture from the air.



This doesn’t work for two reasons. First: the small amount of moisture that cold air can hold. Example – if you heat saturated air at 5 degrees F (average cold temp in January here) to 68 degrees (my heater doesn’t work well), it’s relative humidity is only 9%, dryer than any desert. Even air at 39 degrees when heated it is still less then 40% relative humidity. So when it’s cold, the amount of moisture in the air is too small to worry about.



Second: The second reason it doesn’t work is the way an AC unit removes moisture from the air. It dehumidifies by reducing the air temperature below the dew point and the excess moisture for that temperature precipitates out. If the temperature of your air is below the lowest temp your AC can cool air to, your AC cannot work as a dehumidifier. Since I never got ice from my vents, the AC will not help defrost windows in the winter.


#2

It dehumidifies the warm air in your car. The humidity comes mainly from your breath.


#3

In modern cars you have no choice. Switching the airflow to defrost, to direct it to the windshield, automatically turns on the A/C. This combination is highly effective in warm, humid weather. It may be true that A/C is of no help in cold weather but it doesn’t hurt.

Some car owners have rewired their systems to control the airflow and A/C separately. You can do the same if it is important to you. I could do it to my cars but never felt it was worth the effort.


#4

Also, it’s important to run the A/C a little bit year round to prevent the system from drying out-- having it automatically come on in defrost is a convenient way of forcing you to do this.


#5

Trust me on this, at least on the cars I have owned, it works. That includes cars from 1965 though current (well maybe not current since few of any allow you not to use the A/C when defrost/fogging.)


#6

Second: The second reason it doesn’t work is the way an AC unit removes moisture from the air. It dehumidifies by reducing the air temperature below the dew point and the excess moisture for that temperature precipitates out. If the temperature of your air is below the lowest temp your AC can cool air to, your AC cannot work as a dehumidifier. Since I never got ice from my vents, the AC will not help defrost windows in the winter.

AC to defrost a window works GREAT in the winter. You’re missing ONE MAJOR POINT. A car AC doesn’t work like a house AC. In a house AC you either have the AC on or the Heat on. In a car you can have both on AT THE SAME TIME. You don’t have to put the AC on at the lowest possible temp to defrost a windshield.


#7

In modern cars you have no choice. Switching the airflow to defrost, to direct it to the windshield, automatically turns on the A/C.

Is this an American Manufacturer thing?? My 2005 4runner and wifes 2007 Lexus es-350…the AC doesn’t turn on automatically when on defrost.


#8

One last point: the compressor WON’T run when the ambient temperature gets too low, regardless of the climate control settings.


#9

I have successfuly used the A/C ONLY to de-fog when it was too warm to want heat. It works and thats that.


#10

The HVAC air handler in the dash takes the cold return air through the cold evaporator to condense our the water to a dew point that is about 35-40 degree Farentheit. Then the air goes through the heater core and brought up to 80-100 degrees. The relative humidity is very low at this point so the dry warm air will absorb the evaporation from the windows. The humidified air sinks to the floor of the cabin and is drawn back in the air handler where the moisture is condensed out. So using the recirculate function of the HVAC unit is also critical to defogging.

The reason the dehumidifing function of the A/C is used is that as the cabin warms up moisture on the windows, the carpet, shoes, clothing, etc. raises the humidity inside the cabin to the point where moisture will condense out on the cooler surfaces of the glass. Even warmer air will not help except to raise the temperature of the air even though it allows the air to hold more moisture it still has a relative humidity close to 100% so it cannot defog glass.

JMHO


#11

I understand what the OP is trying to say, but at the end of the day its not worth trying to explain. The fact is, car manufactures have dumbed down the defrost setting so it automatically does what it needs to do to best defrost the windows.


#12

The very reason the air gets dehumidified is because of the small amount of moisture that cold air can hold. When the warm air is cooled passing through the coils it deposits its moisture on the cold coils. The moisture is then drained off.

As you said, relative humidity is the key. If the air coming out of the vents is warmer than the glass and its relative humidity is low, it’ll pick up the moisture from the glass. The relative humidity will have been lowered becaue it was brought to the coolest point in its processing at the coils, where excess humidity was removed at that point. From there it warmed as it traveled through the vents and since its relative humidity dropped. It’s then able to remove the humidity from the glass as it contacts it.

I personally would rather not have the AC come on automatically like it does in new cars. I’d rather control it myself. When I’m trying to clear ice off the windshield I’d rather have the air as hot as possible. Then, if I decide to remove moisture, I could turn it on myself.


#13

Nice chart.


#14

If there is ice on the windshield, the compressor won’t be running because the air will be too cold.