What to do with a wet dog, defroster, and A/C.

Dear Car Talk

Dear Car Talk | Jun 01, 1991

Dear Tom and Ray:

When my husband and I are riding home with the dog after a hike in the rains, we experience a severe moisture problem in the car. The defroster barely copes with the windshield and the other windows remain covered with condensation. My husband has advised me to turn on both the air conditioner and the heater-defroster on high. I tried it. Even with the dripping wet dog in the back seat, it worked! All the windows cleared within a few blocks. Is there any reason to avoid running these two antagonistic systems together?

RAY: Your husband is right, Bonnie. You should use the air conditioner to defrost the windows. In fact, many cars automatically turn on the A/C when you choose the defroster setting.

TOM: The air conditioner does more than just lower the temperature of the air. It also removes moisture. And moisture-removal is exactly what you need when when you have condensation (i.e. moisture) all over the windows.

RAY: Of course, in the winter, you don't want to lower the temperature of the air. If you do, the poor dog will have icicles hanging from his ears before you pass the first mile marker. So you negate the cooling effect of the air conditioner by also turning on the heat.

TOM: And you don't put any "strain" on the A/C by turning on the heat. These are two separate systems, and operate completely independently. All you're doing is mixing hot air with "air conditioned" air before it comes into the passenger compartment through the vents.

RAY: One note, Bonnie. When you're trying to clear the windows, be sure your vent setting is on "fresh air" rather than "recirculate." Otherwise, you'll just be recirculating the same old moisture, and neither the windows nor the dog will ever dry out.

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