Will dumping cold water on your car cut down on the need for AC?

Dear Car Talk

Dear Car Talk | May 01, 2010

Dear Tom and Ray:

There is a teacher in our community who regularly can be found pouring buckets of cool water over his vehicle before starting it on warm days. His claim is that the water cuts down on the time needed for the air conditioner to lower the temperature in his car to a comfortable level. Is there any truth to this claim? He drives a full-size car from the '90s era. I would think that the time and water needed to cool the car significantly would negate the environmental and fiscal advantages of this unorthodox practice. What do you think? If this is a good idea, the local hardware store will need to stock up on buckets.

-- J.D.

TOM: I don't think the country has to worry about a bucket shortage, J.D.

RAY: Does the cool water do anything? Yes. The metal roof and doors of the car do absorb heat. And if you cool them off, you will reduce the amount of heat they transfer into the passenger compartment.

TOM: But as you say, J.D., the amount of water you'd need to make a significant difference would be absurd. You'd really need a garden hose, and you'd need to play it on the car for several minutes to have any effect at all. And in many parts of the country, we can't afford to waste water by pouring it over our cars.

RAY: Besides, in that amount of time, or less, you could open the windows, turn on the fan or the AC, or -- even more effective -- just start moving with the windows open, and create just as much heat reduction, or more.

TOM: Here are some even better, and more environmentally friendly, ideas to help the air conditioner cool your car more quickly in the summer. You can share these with your teacher friend:

RAY: Use a sunshade. Or two. A cardboard or microfiber sunshade in back of the windshield will keep the sun's rays from beaming right into your passenger compartment. Side benefit: You won't sear the shift pattern into your palm when you go to put the car in gear.

TOM: Better yet, use a sunscreen on the rear window, too. That window often is even more steeply raked than the front windshield, and can let even more heat in.

RAY: You also can install rain guards on your windows. Those are little metal awnings that allow you to leave your windows open an inch or two to let hot air out without worrying about letting rain in.

TOM: And the easiest and most effective solution of all: Park in the shade.

Get the Car Talk Newsletter

Got a question about your car?

Ask Someone Who Owns One