Is the misty air coming out of my A/C dangerous?

June 21, 2006 10:06 AM

My car's A/C unit is sending out sort of misty air? Is this safe? Or is it just moisture? The A/C unit still cools down the air.
posted by sandmanwv to Travel & Transportation (10 answers total)

I am not very competent in these matters, but it may be a leak in the heat exchanger and the "mist" is actually atomized engine coolant/antifreeze. My car had a similar problem and I don't think it went away until the heat exchange was replaced. (Though my memory is fuzzy - we had the mist, we had the heat exchange replaced, but I'm not 100% sure those were correlated).

Anyway, an outside possibility. It might just be moisture. Does it happen continually or just when the AC starts up?
posted by GuyZero at 10:14 AM on June 21, 2006

Response by poster: It happens when I start the A/C, but if I increase the temp (put the slider in the middle) it goes away.
posted by sandmanwv at 10:16 AM on June 21, 2006

This happened to me before and I remember being told (by the mechanic?) that it was harmless -- I think it was something like frozen condensation. (Braces self for embarrassment from being way wrong.) Has it been very humid?
posted by theredpen at 10:29 AM on June 21, 2006

If it's not already been done, switch to the recirculating air. I think that the vapor could be the result of constantly cooling particularly humid air from outside. I've heard that, as it's only recooling the air that's already in the car, the recirc is usually much more efficient, too (but that, when using the heater, you should use the outside air function, otherwise your windows will get foggy).
posted by penchant at 10:30 AM on June 21, 2006

I think theredpen is right, especially if the effect is pronounced with colder air. I think you're seeing condensation from cooling humid air. Should be fine.
posted by futility closet at 10:40 AM on June 21, 2006

If it were an antifreeze leak, you would probably smell it. Vaporized antifreeze has a distinct sweet smell. I bet penchant is right.
posted by Kirth Gerson at 10:40 AM on June 21, 2006

I believe most auto AC systems have something called a dryer. The dewpoint in southern climates is 70 degrees during the summer, and the air coming out of the vents is 40, so somehow the system is removing moisture (unless it's already been dried somehow, such as by recirculating). Otherwise the air coming out would be foggy and the ducts would be soaking wet. So I wonder if this is a dryer issue. I'm not so sure it's harmless... if moisture is condensing inside the ducts, mildew and mold are going to have a field day.

If it makes you feel any better, I sometimes flew on C-5 cargo jets, and when the plane was on the ground in hot, humid weather, the heat exchangers would be blowing out cold, foggy air. It was nice and cool, but it was like London fog where you couldn't see more than 30 feet.
posted by rolypolyman at 10:46 AM on June 21, 2006

This used to happen to me all the time in muggy weather. I'm still alive. I always assumed it was condensation.
posted by CunningLinguist at 11:47 AM on June 21, 2006

rolypolyman writes "I believe most auto AC systems have something called a dryer."

True but the dryer is part of the internal refrigerant system and has no effect on the cabin air.

rolypolyman writes "The dewpoint in southern climates is 70 degrees during the summer, and the air coming out of the vents is 40, so somehow the system is removing moisture"

The evaporator (the cold part that cools the air) will have moisture condense on it there by dehumidifying the cabin air¹. This can sometimes cause a mist to be blown into the cabin but it shouldn't be an every time thing.

Most likely you have either 1) a leak in your heater core (which as Kirth says you should be able to smell/taste) or 2) your A/C drain vent is plugged causing liquid water condensed by the evaporator to be blown by the fan into the cabin.

This excess moisture can cause a musty odour from your A/C system and if the water level over flows the catch tray it can wet your carpets and at in the extreme rust out the floors from the carpet being wet all the time. The latter is more of a concern with older cars that didn't have multi layer paint and anti corrosion coatings.

The A/C drains are fairly easy to unstop with a piece of wire if you can find where the tube exits the car.

[1] This is why modern cars run the A/C when the HVAC is set to Recirculating Defrost.
posted by Mitheral at 2:33 PM on June 21, 2006

Mitheral: ...your A/C drain vent is plugged causing liquid water condensed by the evaporator to be blown by the fan into the cabin. and This excess moisture can cause a musty odour from your A/C system and if the water level over flows the catch tray it can wet your carpets and at in the extreme rust out the floors from the carpet being wet all the time.

This problem happened with my '96 Accord; the A/C was blowing misty air into the cabin, but after I had convinced myself that the vapor wasn't anything toxic, I ignored the problem for a few months. Eventually enough water collected somewhere to soak through to the carpet, stinking up the car with mildew. While I don't remember the explanation given by the mechanic, Mitheral's sounds right to me.
posted by cobra libre at 5:26 PM on June 21, 2006