What Does the 'Auto' Button for My Air Conditioning Actually Do?

Even many low-cost cars have long come with automatic climate control as opposed to the manual-control systems of the past. These automatic climate-control systems typically include a button marked “Auto” that engages automatic mode when pressed, illuminating a corresponding light. But what does the Auto button actually do?

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The beauty of automatic climate control is that you simply set the temperature you want, and the system will do whatever’s best to get to and maintain that temperature in the cabin. Besides mixing warm and cool air to arrive at the desired temperature, “whatever’s best” can include adjusting fan speed, air direction and air recirculation.

For instance, a low fan speed might be best in the winter until the air blowing out is fairly warm, after which a higher fan speed would help quickly bring the cabin up to the desired temperature. Heat rises, so the system might direct most of the warm air out the floor vents, where it can warm your feet and then rise to heat the cabin. The Auto button can make all of this happen automatically.

The Recirculation Setting

Most tricky might be recirculation. Whether you have an automatic climate system or a manual one, there’s usually a button marked “Recirc,” which is short for “recirculation.” On some cars, that button may have just a circular arrow on it, which might be inside an outline of the car. Either way, there’s typically a small light that will illuminate when recirculation is on, too.

When recirculation is off — which is generally the default and usually preferred — fresh air is pulled from outside the car to pass over the elements that either heat or cool it before it enters the cabin; when recirculation is on, air is pulled from inside the cabin to pass over those elements. Generally speaking, if the air that’s already inside the car is closer to the desired temperature than what’s outside the car, it’s more efficient to turn on recirculation and start with inside air. Trouble is, you’re then rebreathing air, which can get stale, and your breath contains moisture that adds to the humidity of the air. Neither is ideal over the long term, so it’s usually best to have recirculation turned off.

In the summer, air conditioning pulls some water out of the air (producing that drip of water you might see under your car), but the humidity in your breath adds some back in. Drier is better here, so Recirc is best used only when it’s really hot out, the sun-baked cabin has already been cooled to a lower temperature than the outside air, and you want to get the cabin cooled further as fast as possible. (Sometimes, if it’s really hot out, Recirc might be needed even to keep the cabin at the desired temperature.)

If it’s really cold out and the cabin is warmer than the outside air (which is usually the case), it’s more efficient to heat up the already-warmer air from inside the car than to start with the colder air that’s outside the car. But once it’s warm inside, it’s best to turn Recirc off because of the “fresh air” advantage and the moisture that your breath is adding to the air, which can fog the windows.

To the latter point, whether with automatic climate control or with manual control, if your windows fog up in the wintertime, it may well be due to recirculation being turned on. That’s because the aforementioned moisture in your breath keeps accumulating in the cabin and condenses into fog when it hits the cold windshield. Turning Recirc off and cracking a window for a bit will usually help clear the fog up. (If it doesn’t, you may have a leak in the heater core under the dash that’s spraying coolant into the cabin. If this is the case, you’ll often find that the floor under the glove box is damp.) But having the Auto setting on usually takes care of this fairly well itself.

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When You Might Want to Override the Auto Setting

Automatic climate-control systems are smart, but they’re not all-knowing.

For instance, you might find the chosen fan speed to be too high — maybe when trying to make a phone call or talk to a fellow passenger — and some people just don’t like having air gusting around the car’s cabin. You can turn the fan speed down manually, then hit the Auto button to return to automatic control.

You also may want the air to come out of different vents than what the automatic climate system has chosen. Manually changing the mode setting will override and turn off the Auto setting, and you can just hit the Auto button again to turn it back on.

Since recirculation is usually turned off, you may want to turn it on when you pass a herd of cows, a construction crew laying tar, or a farmer plowing a dusty field on a windy day so that the smells and particles don’t enter the car’s cabin. Just hitting the Recirc button will do the trick, though you’ll have to hit it again once you’ve passed the offending area to turn it back off.

Advantage of the Auto Button

Particularly in temperature extremes or when the outside temperature is changing, it can take a lot of fidgeting with manual climate-control systems to get the inside temperature just right. But automatic climate control is typically “set-and-forget,” which is not only more convenient and more efficient, but also safer, as it relieves you from the distraction of having to make adjustments as you’re driving down the road.

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