How does my car "warm up" cold air-conditioned air?

Dear Car Talk

Dear Car Talk | Jan 01, 2003

Dear Tom and Ray:

Here is a question that I'm sure the dealer will not have a correct answer for. When I run my car's air conditioner (2003 Toyota Corolla), the air is too cold. I've been instructed to turn the temperature dial to a warmer setting. What does this do? Does it just air-condition the air and then heat it up again? In which case, is this a really inefficient way to cool a car? Or does it run the compressor less, and therefore use less power from the engine? -- Jim

RAY: It just mixes in warm air, Jim. The air-conditioning compressor has only one setting (well, two on some cars with an "economy" option). And when it's on and cycling, the only way to regulate the temperature is to mix in heat to make it "less cool."

TOM: It is somewhat inefficient, but at least the heat is free -- it's taken from the heat that's naturally created by the engine. So it's not costing you any MORE in gas consumption than just running the air conditioner would.

RAY: By the way, this is exactly the way lots of climate-control systems work. They run the air conditioner and then regulate the temperature by turning up the heat.

TOM: Turning up the heat. That's what I've been trying to do to my brother to get back the 200 bucks he owes me.

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