Recirculate versus fresh-air: Tom and Ray settle one family's bet. On the line? A steak dinner.

Dear Car Talk

Dear Car Talk | Mar 01, 2007

Dear Tom and Ray:

I was driving with my dad on a road trip to see my nephew graduate from boot camp at Paris Island. The air conditioning was on, and I was cold. I noticed that my dad had the air conditioner set on recirculate, and I suggested that he change it to the fresh-air setting. He and I got into a friendly discussion about this, but there is a steak dinner on the line for the correct answer. I heard that it is best to only use the recirculate or max AC for a short time to initially cool off the car, and then it should be switched to fresh air to keep the air inside from getting stale and polluted. My dad insists that it should stay on recirculate, as this is most efficient. My dad believes wholeheartedly in efficiency. I do, too, but I value health also. Who is right, and who has to give the other the steak dinner? -- Susan

TOM: I have one recommendation for you, Susan. Do not -- I repeat, do not -- marry a man like dear old Dad.

RAY: Not that he's a bad guy. It's just that you don't want to be having these kinds of discussions for the rest of your days. You don't want to be sitting there freezing, while he says: "That's too bad. You have to freeze, because it's more efficient!"

TOM: Air-conditioning decisions really should be made based on the comfort of the passengers. Neither recirculate nor fresh air is more efficient in terms of how much fuel they use. On most cars, the compressor does the same amount of work on both settings.

RAY: It's just that when you put it on recirculate, it recirculates the same cabin air (actually, it always lets in some fresh air). It takes the air it's already cooled down from inside the cabin and runs it through the evaporator again, making it even cooler. So in that sense, recirculate DOES cool the car more quickly.

TOM: On the fresh-air setting, the ventilation system brings in much more fresh, outside air, and cools that down. So it takes longer to cool the car that way. But, as you say, it does freshen the air, which might be important, especially if Daddy had one of Mom's bean-onion-garlic burritos for lunch.

RAY: So, recirculate does cool the car more quickly, but once it's cool enough, there's no good reason to leave it on recirculate. You can switch it to fresh air or keep it on recirculate -- whichever keeps you more comfortable.

TOM: So I'd say that, in terms of the bet, we should declare Daddy the winner, because, in recirculate mode, the air conditioner does cool the car more quickly -- and we could interpret that to mean more efficiently. But we're also going to rule that he has to buy you a steak for making you sit there and freeze in the name of efficiency. So you should both enjoy your steaks. And during dinner, talk about something else.

Get the Car Talk Newsletter

Got a question about your car?

Ask Someone Who Owns One