AC not affected by car’s speed

07/28/2012 5:00 AM

07/28/2012 6:28 AM

Dear Tom and Ray:

Nearly 40 years ago, I was told that it was a bad idea to turn on the air conditioning when driving at highway speeds. I was advised that the AC should be turned on only when the car was idling or moving very slowly. I guess the idea was that starting the AC when the engine was operating at high RPMs would jolt it and put a lot of sudden strain on the belt-driven AC parts. I have always followed that old advice, and I get funny looks when I slow to 20 mph to turn on the AC. Is it a bad idea to start the AC when not idling or moving slowly? Was it ever a bad idea? Am I the victim of a 40-year-old joke? — David

Tom: Bad news, David. For at least the past 40 years, while you were being so careful, your AC was cycling itself on and off at highway speeds.

Ray: That’s how the air conditioner works. It has a clutch, and it engages based on demand, and then disengages itself when the demand or other conditions have been met. This happens at all speeds, from idle to highway.

Tom: For example, the AC also will cut out when you start the engine so that all the battery power can go to the starter. And it’ll cut out if you put a large demand on the engine for power. So if you’re entering a highway, or climbing a hill on a highway, the AC may shut off during that hard acceleration to allow all available engine power to go to the wheels.

Ray: So the air conditioner is designed to be started and stopped at all kinds of speeds. And it’s doing that whether or not you turn it on and off.

Tom: So you can forget all about this advice. And — let me do some calculations — it’s been 40 years, so hopefully your father is still around. He’s obviously the one who gave you this lousy advice, so give him the good news, too. Tell him he can now enjoy his golden years by turning on the AC at any speed he wants to.

Dear Tom and Ray:

As I write this, I am still fuming. This past week, the temperature has been in the high 80s to low 90s. I hate the heat and get super grumpy. So, I get into my wife’s VW, which, of course, has been baking in the sun all day, and she’s waiting for the engine to warm up before she turns on the AC so the AC will be colder! After what felt like a four-mile drive in hell, she finally turned on the AC, but she would only set the fan speed to No. 1, so there was barely any air blowing. I asked if we could put the fan on the highest setting to, you know, get some cold air into this hot box. She said no, and kept it on the lowest possible setting. I wanted to scream! Her reasoning was that since the air is recycled, the AC works better, and the air is therefore colder, if the fan is not turned on full blast. Huh? Is she right? Why on earth are there other fan speed settings, then? I’m thinking so one can actually feel the cold air. Has all this heat fried my thinking capabilities?

— Skip

Tom: Oh, boy. She’s 100-percent wrong, Skip. But even worse, the way you guys are relating to each other reminds me of some of my earliest marriages.

Ray: With one exception: In those marriages, Tom was always the one who was 100-percent wrong.

Tom: There’s no reason to “warm up” the air conditioner before using it, or to warm up the engine before turning on the AC. The engine is more than capable of powering the air conditioner right away without any adverse effects, and the AC is ready to work within seconds.

Ray: And because the air conditioner recirculates the air inside the passenger compartment, it cools that air faster if the air is recirculated more times per minute through the evaporator. So, having the fan on the highest setting not only would cool the air faster, but it has the added benefit of creating an evaporative effect on your skin, making you feel cooler, too.

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