Why do defrosters work better with the AC turned on?

Dear Car Talk

Dear Car Talk | Jul 01, 1993

Dear Tom and Ray:

I own an '88 Hyundai Excel. During the rainy season, I experience only minimal success with defogging my front and side windows with the defroster going full blast (and it really heats up in there!). My wife told me she had great success defrosting the windows by--now get this--turning on the air conditioner while the defroster was on! Of course I laughed at the notion, explaining in my best "Oh, you women" tone that you can't POSSIBLY aid the defogging of a car window by turning on the air conditioner. Imagine my surprise and embarrassment when, during a foggy-windowed drive home, she reached over and hit the air conditioner button while the defroster and heater were on, and presto! The windows were crystal clear in moments! How can you explain this phenomenon? Can it damage the air conditioner to run it while using the heater? Please help me get my foot out of my mouth and recover a small bit of my dignity.

TOM: Sorry, Ron. It's going to take major surgery to get your foot out of your mouth this time.

RAY: What your wife is doing is absolutely correct. What you may not realize is that the air conditioner does two things. It lowers the temperature of the air. But more importantly, it also removes the moisture. That's the "conditioning" part.

TOM: And when you're trying to remove condensed moisture (a.k.a. fog) from the inside of the car's windows, air "conditioning" is exactly what you want. And in fact, many cars automatically turn on the air conditioner when you turn on the defroster. Careful, Ron, I can hear you gagging on that foot from here!

RAY: It doesn't hurt the air conditioner when you run the heater at the same time. They're two completely separate systems. They come into the passenger compartment through the same set of ducts, but they're completely independent. What you're doing when you run them together is pumping cool, conditioned air out of the air conditioner, and mixing it with hot air from heater. The result is warm, dry air, which is perfect for defrosting.

TOM: The other defrosting trick we've learned is that some cars--particularly Asian cars--have a selector for "ouside air" or "recirculated air." If you don't have air conditioning, and you're trying to defrost the windows, you should always use the "fresh air" setting. Otherwise, you'll just be recirculating the same, moist air, and the windows will never get clear.

RAY: So there's no point in trying to recover any of your dignity here, Ron. Your wife is completely and utterly right in this case. So just pull your trousers up to your chest, bow your head, look sheepish, and start mumbling "yes, dear... whatever you say, dear."

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