How does AC really work? Today: Tom and Ray resolve a marital dispute.

Dear Car Talk

Dear Car Talk | Aug 01, 2006

Dear Tom and Ray:

Please settle an argument between me and my wife. We have a 2005 Toyota Highlander with an "automatic air conditioner." You set the temperature and it maintains it. My wife contends that the AC works like a house AC, so that the temperature of the cold air is constant and the compressor cycles on and off. I contend that the AC (just like the older "manual" ACs) simply mixes hot and cold air to obtain the desired temperature. The dispute comes to a head when it's really hot in the car. I claim that we should set the temperature as low as possible. My wife counters that it will (like a house AC) reach a given temperature in the same amount of time, no matter what temperature you set the AC to. Please settle this issue for us! -- Roger

TOM: Isn't it wonderful when a marriage is so good that all you have to argue about is trivial stuff like this, Roger?

RAY: Yeah. My brother and his wife usually argue about what size frying pan she's going to hit him with next.

TOM: The great news is, you're both at least partially correct. As your wife says, all the cold air that's produced comes out at a constant temperature, and the compressor does cycle on and off. And, like YOU say, Roger, the car's climate-control system also blends warm and cold air to maintain the temperature you set.

RAY: Now, will cranking down the temperature all the way cool off a hot car faster? Oftentimes, yes. It depends on the particular system. But when you crank most systems down past a certain point (it doesn't have to be all the way), three things happen.

TOM: First, you ensure that no warm air is getting blended in. Second, the system puts itself into "recirculate" mode.

RAY: "Recirculate" limits the amount of fresh, outside air the system takes in. Essentially, in recirculate mode, the air conditioner is recycling the air it has already cooled, and cooling it more.

TOM: The third thing that happens when you set the climate control on "arctic" is that the fan speed gets kicked up to its highest setting.

RAY: That doesn't produce any more cold air, but it moves the air around more and might make you feel cool faster.

TOM: But as you've probably noticed, having the fan set on high also makes it sound like you're in Dorothy's backyard in Kansas just before the big twister hits. And all that noise makes it harder to hear you two lovebirds cooing at each other.

RAY: So, our advice would be to do whatever pleases your wife the most, Roger. Doing it your way -- even if it cools off the car faster -- is going to make it seem a lot hotter in there. Trust us.

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