Does AC system act as dehumidifier when heat is turned on?

[MR_Loyale]

I have heard from numerous sources that the air conditioning system is needed even when you are not actually cooling the car. The reason given is that supposedly it removes humidity from the air and makes clearing a fogged windshield quicker. I had heard and read this in several places but not really given it much credibility until recently.

 

Saturday I went on a long trip through the nasty wet weather we have been experiencing in Washington. Now for those that know me, also know that when my AC compressor kicks in, I know it because the belt squeals. Call it my "AC Compressor Activation Indicator" if you will or just call me too damn lazy to tighten the belt to proper tension - either is probably an accurate description.

 

Anyway as we were driving Saturday, even though my AC was not activated, the darn compressor kept kicking in. I hit the window defrost to clear the windshield, top button on the HVAC contol panel, slide the temp bar all the way to the top and then turn on the fan and I get that squeal. It was very annoying and so at the first rest stop I pulled the fuse to the circuit so the relay would not engage the compressor clutch. I thought the thing was malfunctioning, but then over the course of the next 8 hours it did appear to take longer to defog my windows. Fellow member jj421 has always commented how my windows were faster at defogging than his were and the only real difference had been he removed his AC completely and mine was functional, until Saturday when I disabled it by removing the fuse.

 

This experience leads me to believe that the AC does act as a dehumidifier even when the system is in heating mode and not actively cooling the car as it would during a hot summer day. I know that during a hot summer day, moisture collects and is drained throug a tube to the outside. But how does this work in our cold raining days when it isn't even turned on? Dehumidification may not seem all that important, unless you live in Wet-stern Washington where it is always so damp that even the concrete turns green. Of course on a below freezing day this is not relevant as the air is dry anyway. But on super humid days like this weekend, it appears to make a big difference.

 

Question: Does the AC really act to dehumifiy the air in any meaningful way in wet damp weather when only the heat is selected?  Please explain this process as I am baffled.

 

Bonus question - running the AC compressor steals about 3 to 6mpg (ask me how I know), how can I get the dehumification benefit without the MPG robbing drag on the engine? (and throw in the plans for the perpetual motion device while your'e at it ) And telling me to buy a box of Dri-Z-Air is not an answer.

Edited September 30, 2013 by MR_Loyale

[presslab]

Yes, air conditioning de-humidifies the air.  The moist, warm air passes over the evaporator coils inside the cabin.  This cools the air rapidly, and the now-colder air can't hold the same moisture as when it was warmer.  That's why there is a drain below your car for this water to drain out of.  A side benefit is that a lot of pollen and pollutants also get pulled out of the air with water. 

 

http://en.wikipedia.org/wiki/Air_conditioning#Humidity_control

 

The air conditioning compressor will be engaged when defrost is selected for exactly this reason.

[MR_Loyale]

Yes, air conditioning de-humidifies the air.  The moist, warm air passes over the evaporator coils inside the cabin.  This cools the air rapidly, and the now-colder air can't hold the same moisture as when it was warmer.  That's why there is a drain below your car for this water to drain out of.  A side benefit is that a lot of pollen and pollutants also get pulled out of the air with water. 

 

http://en.wikipedia.org/wiki/Air_conditioning#Humidity_control

 

The air conditioning compressor will be engaged when defrost is selected for exactly this reason.

 

Wouldn't warm air passing over the evaporator make it cold air coming out of the vents? I understand why it does that in the summer , condensation and the drain tube and all when you want cold air coming out. Just don't understand how it works when it is cold and humid and somehow hot air does come out.

Edited September 30, 2013 by MR_Loyale

[presslab]

The cold air from the evaporator then passes over the heater core, heating it if the heat is turned up.  So now you have warm, dry air that is very willing to absorb moisture.

[MR_Loyale]

The cold air from the evaporator then passes over the heater core, heating it if the heat is turned up.  So now you have warm, dry air that is very willing to absorb moisture.

 

Excellent. Thanks for the explanation.

[jj421]

This is useful information. Like he said, I deleted my A/C entirely, and it takes some time before my windows defog. Not to mention that the rear side windows (it's a wagon) are almost always fogged up in the cold, especially if my clothes are wet from walking around outside.

 

Doesn't bother me too much; my cold idle problem irks me more. I guess without A/C, you just gotta drive with the window down and/or the heat on full. No matter how cold/hot you get. I'm the kind of guy that'll roll his window down whilst driving in a snow storm, so that doesn't bother me. I'll need to make a pamphlet for any passengers that get in my car: "This car has no air conditioning! What does this mean for you? Windows down when it's hot out, and windows down when it's cold out! More details inside!"

 

Looking online, I read that cleaning the inside of your windshield and putting shaving cream on it (wiping it off afterwards, of course) helps keep the fog away. Rain-X defog stuff also seems to be popular. I'll probably try at least one method.

 

Time to buy new gloves for my drive to school in the morning!

[bratman2]

I had to take a business trip to Canada a few years back with an associate from our head office in Chicago. A snow storm blew through so he did all our driving. I noticed he always cracked a back window a half inch or so when we started out. When I asked why he told me it defrosted the windows faster. Seemed to work and I have done the same with my Brat, cracked a rear glass just a hair or so. I also have removed my compressor many years ago and the cracked back window does help defrost and remove condensation.

[jj421]

Yeah, I might start driving with the driver side rear window open a hair. The problem is I have manual windows and manual locks. If I want to open the right rear window, well, I'm sure you can imagine the time it would take to do so.

 

I don't mind having the window down. I just hate when it's pouring rain and the rain spray from the semi truck passing you gives you an unwanted bath, haha.